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Miami Pedestrian Bridge, Part XI
31

Miami Pedestrian Bridge, Part XI

Miami Pedestrian Bridge, Part XI

(OP)

RE: Miami Pedestrian Bridge, Part XI

There are 3#11 vertical bars that were suppose to be on the inside face of member #12 missing from the deck. It looks like they were not well embeded into the deck (assuming they were pulled out and stayed with member #12). The drawings show the centre #11 vertical bar offset to clear the drain pipe. Although the offset shown on the drawings does not look like it is sufficient to clear the pipe. This could be one of the contributing reasons why the shear plane dips down below member #12. It looks like the bars should have been better embeded than they were. I was just going over the latest photos and notices this

RE: Miami Pedestrian Bridge, Part XI

I prepared a cropped, enlarged version of the truck-cam video that is as free of artifacts as I can make it. A section of the original video was cropped to 240x135, then I resized it by a factor of 400% to 960x540 using nearest neighbor interpolation, both operations done with native filters built into VirtualDub2. From what I've read, nearest neighbor interpolation doesn't introduce many artifacts because it simply moves pixels apart and inserts the value of the nearest neighbor. It enlarges an image while preserving the "chunky" nature of the original pixelated image.

I also created individual images for each frame in the cropped video, and they are named after the frame number in the original video - Frame0072.jpeg through Frame0086.jpeg. The JPEGs were saved at 100% quality.

I zipped the video and JPEGs into a single ZIP file that can be downloaded with this Link. The video format is the ancient AVI windows format because that was all I could coax out of VD2. If someone asks I will convert it to MP4, but no guarantees on the quality.

My observations about the frames:
  • 72-73-74 Nothing apparent
  • 75 - If you squint really hard and rub a magic nickle, you can just barely see the bottom of the deck move ever so slightly with respect to the traffic light beneath it. Collapse has begun. Or not. See the next frame.
  • 76 - The deck and canopy are starting to drop, but member 11 does not appear to have moved much.
  • 77 - Member 11 still does not seem to have dropped or rotated much in comparison with the dropping of member 10 and the canopy.
  • 78 - Member 11 has started to move significantly. The canopy above member 12 has dropped slightly, so member 12 has been blown off the pier.
  • 79 - Member 12 and its attached canopy are in free-fall. The area around the 11-12 junction seems to be filled with debris, and the lower 20% of section 11 seems to have disintegrated.
  • 80 - The bottom of member 12 is plainly visible to the left of the pier, and the canopy attached to 12 has now dropped 3-4 feet, using the "falling man" for scale.
  • 81-82-83-84-85 - As the entire canopy continues to fall, the section attached firmly to member 12 causes it to rotate, and the bottom of 12 is pulled up and dragged onto the top of the pier.
  • ADD: 82 - The north end of the deck is falling off the north pier.
  • EDIT: 86 - The north end of the deck and the bottom of member 12 have reached their final resting places.
  • 86 - The bottom of member 12 has reached it's final resting place.
ADD: Here are links to the Whirled Gnus Playlists
And here is a link to all the individual videos sorted by age, with newest first.

RE: Miami Pedestrian Bridge, Part XI

Quote (Epoxybot)

If a PT Rod did shear, it sheared when the canopy fell and pinned/sliced the PT Rod.

The lower PT rod certainly did break.

Remember it was passing thru the crack (which was nearly an inch wide), so it was likely acting as a dowel across the crack and would have been helping to hold the crack together. It failed when they tensioned it back up, which was probably what triggered the collapse, given that this rod was no longer contributing.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

The lower PT rod certainly did break.

Remember it was passing thru the crack (which was nearly an inch wide), so it was likely acting as a dowel across the crack and would have been helping to hold the crack together. It failed when they tensioned it back up, which was probably what triggered the collapse, given that this rod was no longer contributing.

The rod could not have broken/sheared as part of the shear friction failure. It had to stay intact since it ripped through the bottom of #11 and ripped out all the ties as the deck fell to the ground. It essentially ripped out the back of #11 rather than shearing.

RE: Miami Pedestrian Bridge, Part XI

It is worth to examine how the span failed to distinguish the root cause from the consequential damages.

NTSB and OSHA photos indicate a gap of about 5mm or 1/4” in the cast-in duct in 11/12 before March 15. It would be safe to suggest 11/12 has deflected outward relative to the deck.

On March 15 Member 11 was re-stressed to nearly the original level, upper PT 280kips and lower PT 230kips, when the span failed.

The geometry of Member 11 tells us any additional outward deflection, from the re-stress, will flatter the angle resulting a higher horizontal force pushing outward.

(1) While the angle in member 11 progressively flattened and 11/12 outward movement increased the bridge reached a point that it was no longer able to support itself. It therefore quickly deflected downward. With increased curvature the deck would tilt at both bearing supports.

(2) When the bridge finally broken the deck profile changed from a curve to a “V”. This is important because horizontal distance between the two top points of the “V” is shorter than original deck length. This could cause the bridge to depart from the bearing supports.

(3) When the bridge was about to snap into a V the tilted deck could slide downward to clear the pier. This happened to the 11/12 while the south end, still with everything attached, was resting on top of the pier.

(4) The 11/12 joint was partially damaged prior to re-stress. The re-stress would inflict additional damage. The collapse mechanism flattened the Member 11 beyond the point of no return. Thus during collapse 11/12 was progressively moved outward while the broken deck pulled itself inward.

(5) The progressive separation of 11/12 from the deck facilitated the deck to leave the bearing support and drop to the ground while 11/12 was caught by the pier and did not fall.

(6) The shear weight of the dropping deck produced some consequential damages. Among them are lower PT peeled off from Member 11 and ripped open the confining shear links and a blow out of concrete when 11/12 was forcibly separated from the deck.

RE: Miami Pedestrian Bridge, Part XI

Earth314159 (Structural) was right. The lower PT rod did not shear but able to rip out the ties in Member 11 as depicted below.



The plastic sleeve/duct was cut during the ties rip out. NTSB video shows the a number of ties rip open by the lower PT rod.



I believe the sheared lower PT rod in OSHA report Fig 63 may have been artificially cut to show the construction joint. Apart from Fig 63 I could not find a reference the lower PT sheared elsewhere in the report.

RE: Miami Pedestrian Bridge, Part XI

I suspect the code in Florida is similar to other concrete codes. For most of the codes I know, the tie arrangements would not have meet code. Is the concrete code used in Florida different in this regard? How did they get away with this tie arrangement for the compression diagonals?

RE: Miami Pedestrian Bridge, Part XI

MikeW7 Thanks again for putting your time into the video. Here are my notes:

On image 74, potential worker on deck (green), potential debris blowing out north of 12 (pink)


On image 75, potential puff of debris (yellow), potential deflection of 10 and 11 (red arrows)


On image 80, bottom of 12 dislodged off of diaphragm (orange)


On image 82, potential furthest extent that PT rod is pushed through the canopy (cyan)


On image 83, PT rod is sucked back in as the deck falls off the pylon (cyan)

RE: Miami Pedestrian Bridge, Part XI

I hope the NTSB takes the Miami-Dade Transit Operation Center to task for not recording the collapse on their Traffic Camera. They had two different protocols to save the feed in a 30 minute window. 1) they could have bookmarked the feed to prevent the system from dumping it & 2) every work-station had a one button record screen peripheral device at each keyboard. You know they got a nice Fed Grant for the brand new Upgrade of the center.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119)

The information you asked took place on Sep 15 2016

Thank you saikee119. That is an astonishing piece of information. This redline drawing of the crack location was known in 2016, but the ultimate bridge designers ignored it. No doubt that was because they were unable to also draw free hand where the failure point would be, but had to rely on a huge and faulty computer program to tell them. As we now know, the computer program was filled with garbage.

RE: Miami Pedestrian Bridge, Part XI

Quote (Saikee)

I believe the sheared lower PT rod in OSHA report Fig 63 may have been artificially cut to show the construction joint. Apart from Fig 63 I could not find a reference the lower PT sheared elsewhere in the report.

Ok.

I mistakenly assumed the caption "Lower PT bar of diagonal 11 sheared” meant sheared during the collapse, as opposed to cut off later.

RE: Miami Pedestrian Bridge, Part XI

Quote (Sym P. le (Mechanical) 23 Jun 19 01:10)

On image 74, potential worker on deck (green)...

I went back to the Collapse - N view video and that NW corner is where Hanson was standing just before the collapse. He was wandering back and forth from NW to NE corner of the deck, and at 2:30 he got on his knees to look at the east side of the 11-12 junction, then at 2:40 he was again on his knees at the west side. Here's the video, set to start at 2:20

RE: Miami Pedestrian Bridge, Part XI

If the lower rod of member 11 was passing thru the crack at a shallow angle and was anchored into deck wouldn't retensioning 11 pull the crack open?

RE: Miami Pedestrian Bridge, Part XI

@Mike - You're right. I had looked for him in the video but wasn't sure of his location or the time frame of the video but there are a bunch of indicators (delivery truck in background leaving yard) that he was standing in the corner at the collapse.

RE: Miami Pedestrian Bridge, Part XI

Sym P. le (Mechanical)23 Jun 19 01:59 - The new video is only 15 frames long, or 3 seconds real-time at 5fps, so Hanson didn't have time to move from his last position in the Collapse - N view video. I rechecked that video and he kneeled twice to look at the junction, the last time right before the collapse. Something bad was happening, and he knew it....

RE: Miami Pedestrian Bridge, Part XI

I wonder if NTSB has a few more frames of that time lapse video right at the moment of collapse.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

If the lower rod of member 11 was passing thru the crack at a shallow angle and was anchored into deck wouldn't retensioning 11 pull the crack open?

Yes, that is correct. It adds some clumping force to increase the friction capacity but it also increases the shear across the concrete surface which makes the whole condition worst.

RE: Miami Pedestrian Bridge, Part XI

MikeW7 Sym P. le Thank you both for your combined efforts. I think this is the clearest analysis of the dashcam footage yet. I know it is painstaking work as I have tried it myself.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

epoxybot I had hoped that the NTSB had the traffic cam footage. I know it was captured by an iphone at about 4 iphone frames per traffic cam frame. I've looked at it frame by frame. The deck appears to crack diagonally (farther south on the east side.)

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie (Computer) 23 Jun 19 03:04)

The deck appears to crack diagonally (farther south on the east side.)

That would explain why the canopy appears to tip toward the camera at frames 82-83, just as the deck hits the highway. I didn't want to say anything before, but to me less of the canopy top would be visible at the end (frames 84-85-86) if the canopy had remained level E-W as the deck settles on the ground. Ground images should provide some clarification.

RE: Miami Pedestrian Bridge, Part XI

Is it possible to damage Member 11 by the adopted construction sequence?

I open up this line of investigation after Tomfh raised the point and Earth314159 concurred.

Quote (Earth314159 (Structural))

Quote (Tomfh)
If the lower rod of member 11 was passing thru the crack at a shallow angle and was anchored into deck wouldn't retensioning 11 pull the crack open?

The evidence

(a) After collapse the bottom end of the Lower PT rod was attached to the deck. Its upper end was in the canopy

(b) Upper PT rod had both ends remained with the separated 11/12. Rod’s bottom end was on top of pier and the upper end in canopy.

(c) There was a construction joint between the deck and the separated 11/12.

(d) This photo shows the the lower PT rod intercepts the last transverse PT rod, 4’-2.5” from the end, in the design. No transverse PT cold be installed to intercept the upper PT rod due to the obstruction by the longitudinal anchors. Evidently if the Lower PT rod were to be separated from the deck like the upper one it had to rip out the affected transverse PT rod and break up part of the deck but this did not happen.



(e) An evidence showing the one PT rods effective on the deck while the other on 11/12 is OSHA report's Fig 35. Along the sloping crack in Member 11 the concrete on one side was still rigid with the deck but the other side was with 11/12 according to the condition of the construction joint.



My interest now is how the two PT rods were de-tensioned. OSHA report does not mention the procedure. It would not be surprising the rod tension was removed in a single operation instead of by stages of 50kips per rod alternately.

If the stress were fully released in one rod while the other was not then there would be a substantial stress differential between the deck and 11/12 due the fact one rod was effectively tied to the deck and the other to 11/12. The stress differential was also exacerbated by the construction joint.

The construction photos and the OSHA report have already confirmed the serious crack damage occurred immediately after the stress in 11 was removed on March 10.

Theoretically the stress release in 11, even in stages of 50kips per rod alternately, could still inflict damage to 11/12 joint with the deck because structurally one rod pulls the deck while the other 11/12 over a plane of weakness created by the construction joint. In stages the damage would be less.

When 11 was re-stressed again on March 15 every application of the staged 50kips to either rod could grind the construction joint if each rod was able to pull it own part of the span. The construction joint (CJ) between 11/12 with the deck looks increasingly likely to play a key role in the demise of the bridge due to the different effectiveness of the two PT rod anchors on either side of the CJ.

It is also interesting to note on June 19 an ENR article reported "FIGG hinted that construction-related activities, rather than design, could be chiefly to blame. According to the statement, FIGG claimed the (OSHA) report “does not include an evaluation of many important factors pertinent to the construction process leading up to the accident. " Does this open the door for FIGG to blame MCM for the bad CJ?

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119 (Structural)23 Jun 19 10:14)

Does this open the door for FIGG to blame MCM for the bad CJ?
FIGG specified the casting of the deck before the "diagonal and vertical members"

FIGG ordered the detensioning.
The cracks were growing before the retensioning.
FIGG ordered the retensioning.
FIGG said safety was not an issue.
How is this MCMs fault, not FIGGs?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

SFCharlie (Computer),

I couldn't agree with you more. I just list the slight possibility FIGG may use to defend its shortcoming by blaming bad workmanship at the CJ which is always a possible weakness no matter how perfect the construction.

I am very clear on FIGG being the sole party controlling everything regarding the timing, magnitude, sequence and method on the tensioning, de-tensioning and re-tensioning of Member 11 that eventually failed.

RE: Miami Pedestrian Bridge, Part XI

saikee119 I took your point to be that the cause layer deeper (literally)between the PT rods and transverse PT.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote:

SFCharlie (] FIGG specified the casting of the deck before the "diagonal and vertical members"]
So FIGG is good so far. (I can't resist) Unless ABC rules say you must cast it top down.

Quote:

How is this MCMs fault, not FIGGs?
Because they touched it.
You don't even have to actually touch it.
When the lift slab construction in NJ or somewhere around there dropped several slabs and killed workmen maybe in the 80s or 90s the guy who operated the roach coach was hit for about $80K. Why was any part of that collapse his fault?
This is a many faceted case and the lawyers are salivating and ordering GulfStreams.
ADD https://www.nytimes.com/1987/10/23/nyregion/record...
28 workers killed, over $5 million in fines, U S Dept of Labor involved. Short article here, worth a minute.

RE: Miami Pedestrian Bridge, Part XI

Vance Wiley Yes, I know, but I don't want FIGG lawyers to believe for an instant, that they can deflect any responsibility...

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Based on the evidence I have seen so far, the bulk of the blame has to go to Figg but that doesn't mean the other parties are faultless. If I was a contractor and saw those cracks, I don't think I would just trust what the EOR was saying without shoring up and getting a second opinion. I think people were putting too much trust into Figg.

RE: Miami Pedestrian Bridge, Part XI

Could a sharp (skilled) contractor have cast a "spine" with "filets" with tops perpendicular to the axises of the diagonals and the central portion of the deck? Wouldn't the cold joints in the deck be constrained by the transverse PT?
Would the highway clearances allowed a reverse taper at the ends of the deck extending to the bottom of the diaphragm? With proper reinforcement, would this have helped?
I guess i'm just beating a dead bridge...

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

My simple speculative root cause of the failed bridge so far

The shear capacity of Member 11/12 with the deck was under-designed.

To assist the transit from roadside to its final position Member 11 was pre-tensioned with two PT rods.

The clamping forces from the two PT rods inadvertently strengthened the joint. The span was able to be simply supported with the ordinary small cracks when the shorings were removed to allow for the SPMT transfer.

After the span was installed on the designated piers the PT rods in Member 11 were de-tensioned. Without the aid of the PT tension the shear capacity of the joint was exceeded resulting extensive cracking, the joint damaged and Member 11/12 deflected outward.

The deflection of Member 11/12 encouraged movements in the construction joint. One PT rod became more effective on one side of the construction joint while the other more effective on another side. This was due to the lower PT anchor able to intercept one set of transverse tendons but the upper PT anchor has no such provision.

The four vertical sleeves, two installed at each face of Member 12, plus a 8” horizontal pipe sleeve below the deck assisted as discontinuities or weak points for 11/12 to detach from the deck.

As a fix to close up the cracks on March 15 tension was re-introduced in Member 11. The sequential tightening of the two PT rods ground the construction joint, increased the cracking, exacerbated the damage, overcome the shearing capacity and initiated the collapse.

RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie (Computer)23 Jun 19 18:24 Could a sharp (skilled) contractor have cast a "spine" with "filets" with tops perpendicular to the axises of the diagonals and the central portion of the deck? Wouldn't the cold joints in the deck be constrained by the transverse PT? Would the highway clearances allowed a reverse taper at the ends of the deck extending to the bottom of the diaphragm? With proper reinforcement, would this have helped? I guess i'm just beating a dead bridge... SF Charlie)

The "skilled" contractor should hire an experienced engineer - but yes, that is one way. Cast the web section on its side and avoid any joints in it - sort of a "tilt-up web" thing.
A different way to skin a cat, if you will.
Clearances could have accommodated thickened deck zones inside (near and approaching) the diaphragms and helped enormously provided they were properly reinforced.
" I guess i'm just beating a dead bridge..." Love it! I think we all are. Maybe along the way there will be an Ace someone can keep. (Kenny Rogers again).

RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie)

FIGG's lawyers will earn their GulfStream.

RE: Miami Pedestrian Bridge, Part XI


SFCharlie (Computer) & Vance Wiley (Structural),


Would the bridge still stand today if the de-tension operation were omitted?

Seem to me this is the simplest solution according to my speculative root cause.

RE: Miami Pedestrian Bridge, Part XI

saikee119 The cracking initiated as soon as the shoring was removed. while these cracks didn't open up immediately, I don't think we know if they opened up before or after the detensioning. The forces in the 11 12 joint would have continued to be conflicted. Would it have been a timebomb? I think we don't know.
Added The bridge was to be designed to withstand extreme conditions. If it fell under traffic conditions, it could have been worse. It was very bad as it turned out.

RE: Miami Pedestrian Bridge, Part XI


SFCharlie (Computer)
,

From the CBS timeline six crack photos were reported on Feb 24, 2018 when shoring was removed. There three cracks reported on Member 1 and three on Member 11. Here are the photo 4 and 4 when the span was fully stressed up and supported at the two ends ready for the transfer.



After de-tensioning on Mar 10, 2018 these two cracks grew to the condition below



The latest crack width of Photo 4 was measured 1" in OSHA report Fig 35&40 and the depth of crack of Photo 5 was 6" as Fig 39. I use Photo 4 and 5 because Photo 1,2,3 & 6 do not have later photo to compare.

Therefore we do know the de-tensioning has damaged the bridge and the re-tensioning caused it to collapse.

RE: Miami Pedestrian Bridge, Part XI

There were already light shear friction cracks before the move. I believe the PT rods did more harm to the joint resistance than good (even before the major cracks).

RE: Miami Pedestrian Bridge, Part XI

Quote (Saikee)

Would the bridge still stand today if the de-tension operation were omitted?

Maybe. But given it was cracking under its own weight it may well have gone on to be another Hyatt walkway. Thousands of people walking across it for the grand opening....

RE: Miami Pedestrian Bridge, Part XI


Tomfh (Structural)
,

I can only talk about facts. When the span was stressed up ready for transit it could support itself by two ends so only the live load was missing. The design live load is 90 PSF and represents about 25% of the bridge self weight. If the bridge was designed to take 1.4 DL it should have no problem to take the live load too as it is generally not possible to make self weight heavier by 40%. Thus it may not in compliance with the code to have the full safety factor in every department the structure should have no risk of a structural failure like it did in Marc 15 , 2018.



Hyatt walkway was a case of a theoretical designer not very practical followed by an experienced fabricator lacked theoretical knowledge.

In the original design two decks were supported by one long threaded rod. Each washer/nut was sized to take one platform. The design was adequate.

When the design went for fabrication the fabricator dismissed a single rod to be threaded the whole length for two storeys stupid and used two short rods threaded only the ends. The fabricator adopted the same washer and nut.

In the end one rod still hung one platform and upper platform used as anchor for the lower platform.

The upper rod had no problem to hold two platforms but its washer/nut was unable to cope with a 100% load increase brought about by the lower platform attached to the upper one. So the system failed by bearing stress when the washer/nut punched through the upper platform.


RE: Miami Pedestrian Bridge, Part XI

Big call to say the bridge would have happily carried 1.4G (and by extension G+Q) if they didn't do the final destressing/stressing operations. It was already struggling and cracking under G, before those operations.

RE: Miami Pedestrian Bridge, Part XI

The Knack (Explains this thread's quest for understanding)
https://www.youtube.com/watch?v=g8vHhgh6oM0

Two Rods Don't Make a Right
Hyatt Regency Walkway Collapse
(NASA's take on the Hyatt incident, different from what I was taught)
https://sma.nasa.gov/docs/default-source/safety-me...

Question, was omitting sleeves and drain pipe from solid model at node and diaphragm trivial for FEA? In FIGG's presentation I see a large stress concentration in the vicinity of the drain pipe. The drain pipe passage was omitted from the model.

For those interested about chains used in move. I found documents that instructed movers to tighten chains before horizontal motion and slacken them before lifting and lowering structure. Also, they were not allowed to dip east or west more then 1/4 inch. They did not achieve that goal.

What was the purpose of the pipes attached directly to diaphragm II while it was in the casting yard? Were they to prevent rotation from PT forces?

RE: Miami Pedestrian Bridge, Part XI



Quote:

saikee119 Would the bridge still stand today if the de-tension operation were omitted?
 

It is basically impossible to predict when a compromised structure will collapse, but it is possible it would still stand. I think there is a very low likelihood, however.
Stressing was to proceed by ( 1) partial stressing the deck, D1 tendons only,
( 2 ) partially stressing canopy , and ( 3 ) Stressing 2 and 11 (in anticipation of the end bays cantilevering during transport).
While supported by the transporters at the first interior bottom panel points 3/4 and 9/10, members 2 and 11 were in tension so they were prestressed to about 560 kips, in the case of 11. The cantilever created about 250 kips tension in 11 so it remained under about 310 kips compression.
The self weight force in 11 when spanning the full 174 feet has been presented as 1300 kips compression, and I agree with that approximate load. Live load will create about 0.25 X 1300 = 325 kips. So the combination of PT force (530 kips) and self weight (1300 kips) exceeds the anticipated Total Load (unfactored - the “real” load) by about 200 kips. Without consideration for (or knowledge of) damage to node 11/12 it was appropriate to remove the PT forces in 11 when the weight of the structure was transferred onto the pylon and south support.
As I understand things, cracking was observed at node 11/12 just after the PT forces were removed. Because the 560 kip PT force was internal to member 11 it loaded that member unnecessarily. And the angle of 11 was 31.8 degrees (less than 45 degrees) so the PT caused more force thru the cold joint than it provided perpendicular clamping force. But ( my current thought is) apparently the compression under the anchor plates at the 11/12 end also provided confinement to the concrete in that zone, which was providing unexpected resistance - assisting the “R” in LRFD.
I pose for consideration that the unexpected resistance was anchoring the lower zones of what became the break out failure mode.
The deck level portion of the final failure plane had slipped earlier, and without the backup of the lower zones presumably mobilized by the compression under the PT anchor plates the failure progressed in an increasingly dramatic manner.
The “shear friction” resistance of the cold joint was not uniform across the contact plane. This allowed incremental failure of the different zones, with the demand increasing on those remaining zones. Releasing the PT in 11 likely allowed this failure mode.
Maintaining the PT forces in 11 was not intended and would likely have masked what was a very questionable connection. Maintaining the PT force might have allowed time for remedial work, but without additional support that work would have been performed at great risk.
In short, I am amazed that a firm with the record of FIGG experienced this problem we see in 11 and the 11/12 node. I am astounded that they could not see a failure in progress and imminent.

RE: Miami Pedestrian Bridge, Part XI


Vance Wiley (Structural)


We all know the tension in 11 was temporary and had no use once the span had been installed at its final position.

I used the proposal to highlight the facts :

(1) The de-tensioning of 11 damaged connection beyond repair. (strengthening modification would be required if the connection were to be rebuilt)

(2) The re-tensioning of 11 pushed the bridge to collapse. (proved to be a bad choice to cure the cracks)

I like to know why doing something harmless (to reduce the compression in 11 by removing the PT rod tension) could damage the structure so seriously. And then restoring the bridge to its previous condition could kill it.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87 (Mechanical))

Question, was omitting sleeves and drain pipe from solid model at node and diaphragm trivial for FEA? In FIGG's presentation I see a large stress concentration in the vicinity of the drain pipe. The drain pipe passage was omitted from the model.

For those interested about chains used in move. I found documents that instructed movers to tighten chains before horizontal motion and slacken them before lifting and lowering structure. Also, they were not allowed to dip east or west more then 1/4 inch. They did not achieve that goal.

What was the purpose of the pipes attached directly to diaphragm II while it was in the casting yard? Were they to prevent rotation from PT forces?

(1) Omitting the sleeves and drain pipe for FEA is due the complication they introduced making the analysis extremely costly. There is no money in a footbridge budget to go that far.

(2) Damage inflicted during the SPMT transfer could be difficult to control and quantify. Knowing what we know now the contractor should have photographed every crack in the structure before proceeding to de-tensioning.

(3) I believe there were small flexible black pipe/hose installed for grouting the PT duct interior. One can see it in some OSHA photos.

RE: Miami Pedestrian Bridge, Part XI

Quote (Saikee)

I like to know why doing something harmless (to reduce the compression in 11 by removing the PT rod tension) could damage the structure so seriously. And then restoring the bridge to its previous condition could kill it.

It’s something I’ll be mindful of in future. Tensioning and detensioning are not necessarily reversible. It can be one way, with no going back.

RE: Miami Pedestrian Bridge, Part XI

These are mounts for the formwork props (pipes) I was referring to. These props were mentioned in OSHA report.

s

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119)

I like to know why doing something harmless (to reduce the compression in 11 by removing the PT rod tension) could damage the structure so seriously. And then restoring the bridge to its previous condition could kill it
Because it wasn't harmless. Removing the PT rod tension changes the stress distribution which if it causes cracks then those cracks to glue themselves back together when you attempt to add compression and return the structure to its previous state. The previous state could no longer be achieved as cracks had formed and grown.

Lets not forget that there is significant moment being transferred through those truss members.

RE: Miami Pedestrian Bridge, Part XI

human909 (Structural)24 Jun 19 11:28

Would it be reasonable to suspect the moment at 11/12 was the most highly concentrated moment in structure? This thing was not really a truss because trusses don't transmit moments. Someways it was a (thin web) I-beam with hidden shear lag. < Just ideas, I don't know.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87)

Would it be reasonable to suspect the moment at 11/12 was the most highly concentrated moment in structure? This thing was not really a truss because trusses don't transmit moments. Someways it was a (thin web) I-beam with hidden shear lag. < Just ideas, I don't know.

I see a problem with calling it a truss. If we chose to be that pedantic then most trusses aren't trusses. Most connections transfer some moment. Throw it into your favourite structural modelling software and you can get and VAGUE idea about the forces in the uncracked structure. That truss member has significant moment AND the vertical member 12 has a very significant moment caused by the roof. This moment again tries to kick out the bottom of member 12 all while 11 is also trying to punch it horizontally.

But really this is still kids playing in the sandpit stuff. I think there has better discussion in the pages previously and in the OSHA report. All simple modelling does is help display approximately the nature of the forces its the connections design and potentially the reverse cantilever load during the move that are the critical aspects.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87 (Mechanical))

Would it be reasonable to suspect the moment at 11/12 was the most highly concentrated moment in structure? This thing was not really a truss because trusses don't transmit moments. Someways it was a (thin web) I-beam with hidden shear lag. < Just ideas, I don't know.

In Moment distribution method by hand, if one has done it in University and still remember it, the moment at a joint is distributed according to the ratio of their stiffness or the "I-value" or the second moments of area.

Member 12 has the largest I-value in the joint of 11/12 with the deck as it is the deepest (I-value is governed by the height to the power of 3). The deck is a lot thinner and its effective breadth is likely to be a limited width permitted by the design code.

Moment exists because the joint was made rigid. The truss the students analyse in their early years has pinned connections to simplify the calculation to just axial forces and shear. In a real structure people make the joint rigid because it is cheaper and structurally stronger. There are moments in 11/12 but I would say the axial forces and shears should be dominant.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87 (Mechanical))

These are mounts for the formwork props (pipes) I was referring to. These props were mentioned in OSHA report.

If it was OSHA report Page 32 you were referring to my interpretation is four hard bearing pads are the standard bridge bearings. The drawings signed for construction show 2 bearing per diaphragm but one construction photo does show 4 bearings.



The B46 and B47 official construction drawings show no side attachments so I tend to regard them as temporary work to be removed after completion.

The attachments pointed out by you cannot be bearings because in the final position there is no concrete support underneath them as depicted by this photo.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119 (Structural) 24 Jun 19 14:01)

The drawings signed for construction show 2 bearing per diaphragm
Here are some cropped images taken from the DROPBOX drone footage:

DJI_0013.mp4 at 00:59 (your image was taken from the same video, at a later time)


DJI_0014.mp4 @ 01:10

RE: Miami Pedestrian Bridge, Part XI

Quote (Saikee119)

Omitting the sleeves and drain pipe for FEA is due the complication they introduced making the analysis extremely costly. There is no money in a footbridge budget to go that far.
Modeling the drain pipe should not have been expensive. Modern modeling software allows one to "subtract" a solid from an existing model. specify a cylinder (the pipe) and subtract it. Of course, they may not have had modern software?

RE: Miami Pedestrian Bridge, Part XI

I could only find one drawing below showing bearing details.



MikeW7 photos seem to suggest the bearing arrangement was different between the span's two ends. The above drawing has no information on Bent2. Looks like Bent1 is the south end of the doomed span, Bent2 the midpoint the doomed span meets the back span and Bent3 the north end of back span.

RE: Miami Pedestrian Bridge, Part XI

Dumb question about pads on the north pier: Shouldn't the canal span also sit on pads? In the picture I posted there doesn't appear to be room for pads because of the vertical rebar.

RE: Miami Pedestrian Bridge, Part XI

Quote:

(1) The de-tensioning of 11 damaged connection beyond repair. (strengthening modification would be required if the connection were to be rebuilt)

There are arguments for the "beyond repair" part. That joint shows significant damage in photo 4 well before the PT rods were de-tensioned. Is it actually practical to repair that area of the bridge with it overhead? That would likely require shoring the bridge and de-tensioning the end of the deck and 11/12 members could be completely rebuilt. So, could the issue even be repaired? I'm suspect the experts would say no, it's not possible. Don't forget, the other end likely had the same issue and would also need a similar re-built to also build it right.

You seem to be pushing the opinion that it was properly constructed but either the tensioning in 11 or the move broke it. I doubt there are many others who share that view.


Quote:

I like to know why doing something harmless (to reduce the compression in 11 by removing the PT rod tension) could damage the structure so seriously. And then restoring the bridge to its previous condition could kill it.


The de-tensioning wasn't harmless, but that's because the structure had design issues and changing the forces let those design issues behave differently. Overall, I would bet that a similar failure would have eventually occurred even if the PT rods in 11 were left tensioned. The joint was failing from the start, the speed just increased once the tension was released.

As for your second point - the joint started to fail and the releasing of the PT rods let the failure progress much further. It should be rather obvious that re-tensioning the rods can't undo the damage in the joint.

RE: Miami Pedestrian Bridge, Part XI


LionelHutz (Electrical),

From concrete repairs experience as someone who had to design the remedial schemes, write the specification, specify/approve the materials, select contractor/suppliers, provide procedure for the execution/implementation, supervise the repair and finally certify completion I will not be able to guarantee my work if the repair done to 100% of the original but still produce the same defects which are shear cracks in this case.

Thus my definition of a strengthening scheme is to modify the structure to cure the defects. Such strengthening scheme will have to be based on theoretical calculations after the root cause has been correctly identified.

If the work has been constructed properly but failed it could also be due to a deficient in design. OSHA has already pointed out some installed rebar were ineffective in resisting shear as they did not meet the code requirements. The structural arrangement of the failed 11/12 joint is vital. All plastic sleeves remain in place. Were they stronger than concrete that broke off first? Many rebar remained with little distortion when the surrounding concrete disappeared. Were they useless in resisting load?

Whatever has been arranged between 11/12 and the deck leads to damage if it were de-tensioned and re-tensioning it back kills the bridge. This is based on historical evidence. My language is just to focus readers' attention.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7 (Electrical))

Dumb question about pads on the north pier: Shouldn't the canal span also sit on pads? In the picture I posted there doesn't appear to be room for pads because of the vertical rebar.

Not a dumb question. We just have no information on the north end arrangement.

As an idiot trying not to be one I would say it is unwise to slide the bridge bottom on the bare concrete north pier and to drop a thin layer of something hard and noncorrosive would be ideal. A proper bearing, allowing the standard horizontal bridge movements, has already been specified at the south pier.

OSHA report has the following references:

"The diaphragm sat over the bearings consisting of four hard plastic pads."

"FIGG asked MCM at around 9:45 am to put an additional plastic shim “right away” similar to the existing shims on the pylon directly under the diaphragm."
after large cracks were discovered in 11/12 on Mar 13, 2018.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87(Mechanical)24 Jun 19 01:42)

For those interested about chains used in move. I found documents that instructed movers to tighten chains before horizontal motion and slacken them before lifting and lowering structure.

I started working on a clearer explanation of the chains a couple of days ago, and I will use your comment as a convient starting point.

OVERVIEW: From what I can piece together from several videos:
  • The spiderweb of tower-to-base chains were tightened at the casting yard and never loosened until the bridge was just west of the piers.
  • Before the bridge was moved over the piers the tower-to-base chains were removed, the bridge raised and leveled, and the chains reattached.
  • The chains remained in place until the bridge was squared up in the E-W and N-S directions over the piers, ready to drop into place.
DETAILS:

There appear to be 3 separate chain groups, each with a different purpose:
  1. Both canopy ends are chained to the deck curbs to prevent the canopy from rocking side-to-side. (my assumption)
  2. The canopy above each SPMT group is chained to the arms of the SPMT towers to hold the bridge against cribbing on top of the towers. (my assumption)
  3. The largest group of chains connect the SPMT tower tops to the mover bases and spreader bars. My guess is that these chains act as a stop so the SPMT towers can be raised until all the slack is out of the chains, causing each tower to become a rigid extension of its base.
The group 3 chains remained tight during the move. As the SPMTs traverse the road median at the 00:59 mark of the Move Part 2 - ground view video you can see each wheel raise and lower as is passes over the median, but the tower chains are never relaxed.

When the bridge is moved into position just west of the piers, several incremental adjustments are made, as shown in the Move part 3 - SW view video. Open the video in a separate window so you can follow along:
  • 00:10 - Starting at the north end, jacks on the wheel trucks of the SPMT bases are individually lowered and raised to adjust the east-west and north-south tilt of the bridge. The presence of these jacks is my assumption, but some mechanism is clearly elevating the SPMT bases at ground level.
  • 00:20 - The group 3 chains are removed.
  • 00:25 - The SPMT towers are raised so the bridge will clear the piers, and to level the bridge north-south.
  • 00:41 - The group 3 chains are reattached.
  • 00:56 - The bridge is moved over the piers.
Once the bridge is over the piers, more adjustments are made, as shown in the Move Part 3 - ground view east 2 video:
  • 00:39 - The SPMT wheel trucks are used to turn the bridge, and crab it sideways.
  • 01:31 - The SPMT Towers are lowered slightly, causing slack in the group 3 chains. As far as I can tell, the bridge wasn't completely lowered onto the piers during this phase, because the group 2 chains would collapse the canopy if weight is taken off the SPMT cribbing.
  • 01:35 - The SPMT towers are used to make final tilt adjustments.
Before the bridge is lowered onto the piers, workers remove the Group 1&2 chains, as shown at the 16:47 mark of the Move Part 3 - SE view video.

The chain removal, final positioning, and SPMT withdrawal can be watched starting at the 01:22 mark of the Move Part 3 - SW view video.

RE: Miami Pedestrian Bridge, Part XI

Quote:

saikee119 (Structural)24 Jun 19 10:46 I like to know why doing something harmless (to reduce the compression in 11 by removing the PT rod tension) could damage the structure so seriously. And then restoring the bridge to its previous condition could kill it.
In my mind member 11 and node/joint 11/12 to deck were seriously under capacity , particularly in the stand-alone condition of March 15. Movements in the joint were apparently slight prior to transport and de-tensioning.
Releasing the PT was not harmless, IMO, because it relaxed internal stresses in the zones just ahead of the PT anchor plates, allowing some strain and deformation from the now first time 1300 kips member 11 was loaded with.
So there was some mis-alignment due to cracking, resulting in loss of interlock of previously homogeneous concrete, and after the cracking had progressed to a visibly recognized failure in progress, the PT was restored. The misalignment of pieces was not restored because the movement had been too great, and internal grinding and crushing was the result. This did not restore the joint to its previous condition.
At this point, the joint has completely failed, and collapse is underway.
Had there been time to restore the support of the transporter (or otherwise provided support), epoxy repairs could have been considered.

RE: Miami Pedestrian Bridge, Part XI

Assuming the posting by FIU of the documentation on their website of the Contracts, Plans, etc., was because of a FOIA request; I'm sure I'm not the only one wondering why the RFI's were not published. The RFC plans do mention Shims for the pylon but there isn't any mention of sizing or locations. The base of the north pylon was to grouted shortly after it was set, subsequent to grouting of the vertical PT bars for 12.

Absent from the Stage 3 sequence was detensioning of 11 & when it should occur in relation to the grouting procedures.

I just don't understand how Denny Pate or any engineer or VSL could consider introducing PT forces into member 11 when it was damaged. Not without first conducting some NDT.



Information on the Bearing Pads. They probably had about a 50% horizontal displacement/accommodation.






RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7 (Electrical)24 Jun 19 15:27 Shouldn't the canal span also sit on pads? In the picture I posted there doesn't appear to be room for pads because of the vertical rebar.)


The pylon at the north end of the main span is intended to “anchor” this complete bridge to the world. Low friction bearing pads are intended at the south bent and the north bent, allowing for minor movements due to expansion and contraction and other causes. Sheet B-104 provides for replacing these low friction bearing pads at the ends of the 270 foot structure.
At the pylon, the main span is to be grouted solid to the concrete. See Stage 3 notes, sheet B-109.
The north span is cast in place and can therefore be cast onto the pylon, creating only a construction joint there, with no bearing pads required. See notes for Stage 4, sheet B-109.

RE: Miami Pedestrian Bridge, Part XI

There have been a lot of information on the SPMT movements and activities. I just wonder has anyone discovered any disconcerting anomaly?

The transfer was executed by a specialist contractor in accordance with a method statement submitted and approved.
The submitted document does allow certain movements, deflections and tolerance, like a maximum 0.5" downward at the centre line and a difference of rotation by 0.5 degree.

The contractor also stated in his submission

"Once the span is in the final position and the torque/twist is removed, any cracks that may have occurred during the movement would likely close and would be very small in width, if measurable at all. There may be select non-structural cracks in select areas, if any, that may need to be sealed (per FDOT Standard Specifications for Roadway and Bridge Construction, 2015 – Section 400 Concrete Structures) once the span is in the final position. As part of the Bridge Movement Monitoring Plan, the "Recommended Strain Gauge Locations - Movement Plan" sheet shows the recommended strain gauge locations that are in the localized areas where temporary cracks may be observed during the bridge movement"

My point is OSHA has in its report stated "Truss was transported by SPMT and placed on the pier and the pylon, and
were generally free of cracks other than those mentioned above".

Anyone has found something useful to add to or disagree with the above?

RE: Miami Pedestrian Bridge, Part XI

I can't see that the transportation did any damage that would significantly contributed to the failure mechanism. Except for overall twisting of the bridge and bending at the joints, the structural form is essentially determinant. The only reported large cracks were from the bottom of #11. I can't see how these would be affected much by twisting during the transport. Other than the initial smaller cracks prior to the move, there was no reports or cracks until after the move.

RE: Miami Pedestrian Bridge, Part XI

Quote (epoxybot (Structural)24 Jun 19 19:35)

RE: The 2 photos posted.
I had not seen the left photo - am I correct in thinking that is the west side of member 11 and the tape measure is touching the top of the deck?
Both the member 11 in the right photo and the horizontal filler formed between 11 and 12 shown in the left photo are BURSTING. Not a good thing in a compression element, particularly when the stability of a 950 ton structure depends on that single member supporting that compression.
Judging by the exotic crack monitoring technique of an "X" across the crack, it appears the surface is ruptured outward also. Note the offsets visible in the lines. (What was the budget for construction inspection? We appreciate someone marking the "X" - it shows better in a photo than a glued on grid the size of a credit card.)
At the time the left photo was taken the failure was in progress, but the force in 11 was somehow being supported by the portion of the deck which blew out later.
I see the top of the fillet between 11 and 12 is 14", not the 15" called for on the drawings. MCM gets one demerit. It will be interesting to see how much a lawyer can charge to that variation.


RE: Miami Pedestrian Bridge, Part XI

Quote (Earth314159)

I can't see that the transportation did any damage that would significantly contributed to the failure mechanism.
That's what I though until I read the OHSA report which focused on it heavily and ran basic calcs.

Even with the PT stands member 11 was under tension and moment. A prime scenario for initiating cracks and weakening an already weak joint.

RE: Miami Pedestrian Bridge, Part XI

Quote (human909 (Structural)24 Jun 19 22:00 Quote (Earth314159) Even with the PT stands member 11 was under tension and moment. A prime scenario for initiating cracks and weakening an already weak joint.)

My rough calculations showed 11 to be in compression during support conditions during transport.
From Vance Wiley 24 Jun 19 04:35June ( member 11 was) " prestressed to about 560 kips, in the case of 11. The cantilever created about 250 kips tension in 11 so it remained under about 310 kips compression."
What did I miss?
EDIT I see I used the upper node dimension instead of the distance from 9/10 to 11/12 which I see is about 40 feet. 11 k/ft x 40/2 = 220 Kips. Diagonal = 220k/tan31.8 degrees =354 Kips tension. So compression remains = 560K - 354 k = about 200 kips compression.
EDIT 2 220 kips/sin 31.8 degrees = 420 kips. So 11 is under 560 - 420 = 140 kips compression.
Time for me to hang it up.



RE: Miami Pedestrian Bridge, Part XI

Quote (Vance Wiley)

west side of member 11 and the tape measure is touching the top of the deck?
Yes, it is the west side of #11, but the tape measure is penetrating the deck by about an inch. ( see the inset at the top right in that left photo.)

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Vance Wiley (Structural) The cracking in #11 is part & partial of the cracking after detensioning.

RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie (Computer)24 Jun 19 22:18 Yes, it is the west side of #11, but the tape measure is penetrating the deck by about an inch. ()

Thank you. I have struck that part of my post. MCM recovers the demerit.
And I must adjust my screen settings. You had specifically indicated that penetration.

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance Wiley)

My rough calculations showed 11 to be in compression during support conditions during transport.
You are correct. I got my units mixed up in my quick calculation. I don't normally deal with kips. winky smile

RE: Miami Pedestrian Bridge, Part XI

The photo is from Fig 42 of OSHA report which also shows crack depth of 6" to 7" by Fig 39. The deck itself has crack depth of 3" and 4" according to Fig 31 and 30 too. The entire construction joint between 11/12 and the deck had slid.

RE: Miami Pedestrian Bridge, Part XI

Presumably the cracks were full depth? The diagonal cracks on the top surface are the same failure surface as the cracks on the back side of diaphragm II?

Because the cracks are jagged and change direction the straight tape measure only go so far in.

RE: Miami Pedestrian Bridge, Part XI

Tomfh,

Fig 29 of OSHA, enlarged below, is probably the most significant on the fate of 11/12 joint.




The centre line of north south 4" ID sleeve is only 5.25" from the north south face of the 2' wide Diaphragm II but has a 1" deep crack according to OSHA Fig 42. After passing the south north sleeve the deck had totally sheared off by about 5 to 10mm (say 5mm on either side of south north sleeve). Concrete has little area and hence little resistance/bond between the two sleeves.

Thus on Mar 13 the evidence in the field indicated 11/12 was departing from the deck with 5 to 10mm outward deflection and crack depths of 1", 3" and 4". The cracks traveled possible 45 degree downward and eventually took out a large prism of the south North face of the Diaphragm II as indicated by OSHA Fig 62. The shape of the prism was already visible by Fig 25 and 27 before the collapse.

The failure mode was as predicted by FDOT engineer Andres who marked up a drawing on Sep 15 2016 when FIGG was still developing the design. Despite FDOT's alert given 18 months in advance FIGG only managed to arrange the diagonal crack shorter by diverting it to the 11/12 face with the use of two pipe sleeves.

RE: Miami Pedestrian Bridge, Part XI

Quote (Human 909)

That's what I though until I read the OHSA report which focused on it heavily and ran basic calcs.

Even with the PT stands member 11 was under tension and moment. A prime scenario for initiating cracks and weakening an already weak joint.

The temporary PT force on #11 was higher than the diagonal tension force. The shear on the connection was reversed, much smaller in magnitude and directed towards the deck.

If the contractor had shored under the #9/#10 joint once the cracks opened up, it would have helped close the cracks.

For certain, it is something to calculate and check but I can't see it as a major contributing factor. I will go over the OSHA calcs on this issue to make sure I am not missing something (I can always be wrong and might find something new). The fundamental issue was the over stress of the shear friction in stage 3. Without this, you don't have a failure of the joint.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee)

The failure mode was as predicted by FDOT engineer Andres who marked up a drawing on Sep 15 2016 when FIGG was still developing the design.

Yeah, a fairly clear prediction in those markups.

Where did those FDOT markup plans come from? Who released them?

RE: Miami Pedestrian Bridge, Part XI

Quote (earth)

The fundamental issue was the over stress of the shear friction in stage 3. Without this, you don't have a failure of the joint.

You also wouldn't have had a failure if the back of diaphragm II didn't punch out.

The shear friction plane failed. The back of diaphragm II failed. All these things broke.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119 (Structural)25 Jun 19 00:43)

The centre line of north 4" ID sleeve is only 5.25" from the north face of the 2' wide Diaphragm II
I comment here only for the sake of clarity, not to be critical. Your posts contain much pertinent information and are helpful.
I think your reference to north face and north pipe sleeve is influenced by the OSHA report which was misleading. I suspect their orientation was influenced by the position of the deck segment in their yard and not as it existed on the construction site.
OSHA has their north and south reversed as it applies to deck segment at 11/12.

RE: Miami Pedestrian Bridge, Part XI

Tomfh,

One thing stands out in this project is FDOT during previews of the submissions marked up FIGG drawings on arears of concern. I agreed with nearly all of them as the cracks were predicted at locations where stress changes abruptly. CBSNBS has done a timeline called "The path to FIU bridge disaster" and release the above information to the public.


Vance Wiley (Structural),

OSHA did not make the mistake. I made the mistake. Thanks for pointing out. I now had the north and south directions corrected in the post.


SFCharlie (Computer),


I agree OSHA Fig 30 shows the worst condition for the cracks. To me Fig 29 is more important because on its own I know the west side has gone and amount of movement can be estimated by the gaps around the north sleeve.

The bridge can have a small non-structural part broken off and still would not compromise the bridge's integrity. So Fig 30 on its own is not decisive until people notice the same pattern was being repeated on the east side with the same ferocity. Once 11/12 had serious 45 degree cracks on both sides the next concern is how much this most critical joint had moved.

OSHA Fig 30 is same as NTSB Fig 1. NTSB Fig 2 is also same as OSHA Fig 26 but taken at a different angle. The NTSB information, except Fig 3 which was used by OSHA as Fig 39, did not show how much 11/12 had moved. OSHA Fig 29 is the corroborating evidence on the 11/12 deflection movement. NTSB Fig 3 or OSHA Fig 35 is the confirmation 11/12 was separating from the deck at front or the last remaining side.

The seriousness of the cracks is that the already deflected structure is nearly impossible to repair.


RE: Miami Pedestrian Bridge, Part XI

I am laying bare the professional conducts of some of the parties in the bridge collapse to make the lawyers more difficult to cheat the public.

We all know it was the MCM subcontractor VSL's workmen who was the first party alarmed by the extent and severity of the cracks to the point they made a photographic record and showed them to the management. The inspection party BPA also participated in actively recording the cracks.

On Mar 15, 2018 three workmen from VSL, two from BPA and one from MCM were present in the re-tensioning work of Member 11. MikeW7's 23 Jun 19 01:38 post commented VSL employee Hanson kneeled twice to look at the 11/12 junction, the last time right before the collapse.

The Collapse - N view video does show this group of workmen stopped momentarily to examine the 11/12 joint several times, they needed to come down from the canopy, during the re-tensioning work. They were the only people really care about the bridge. Not being professional engineers able to diagnose the root cause of these cracks they nevertheless knew the cracks were extraordinary. I believe their interest was ensure no construction error from them to worsen the cracks.

We now know the MCM construction trailer nearby on that day was full of professional engineers who were better qualified to do the crack evaluation/assessment. Most of them were steamrollered to believe the cracks were not safety-related. The steamroller party even unilaterally gave an early instruction the to MCM that Member 11 were to be re-tensioned, without any repair now and obviously over the live traffic. The Mar 15,2018 meeting broke off at 11am according to FIU and CBSNBS investigation timeline reported that no one took steps to close the road while work proceeded on the cracking structure.

The real surprise to me is the re-tensioning work commenced approximately noon according to the OSHA report and so if any of the Mar 15, 2018 meeting attendees wishing to see the cracks and the result after the Member 11 was re-tensioned he/she could do so after leaving the meeting at 11am. None of the attendees did. The party who claimed the cracks were not a safety concern had no interest in seeing see the progress of the fatal work it instructed.

I would moderate the above by pointing out Contractually there were really two main parties in the FIU bridge; the owner and the consortium who designs and builds the required structure. The Owner is represented by BPA as according to the organization chart in OSHA report, BPA should be directly answerable to FIU and possibly viewed as Owner's Engineer or representative. BPA had two engineers present in the collapsed bridge and does have the Owner's power to stop the re-tensioning work if BPA deems it would be unsafe to allow traffic to go underneath. The least BPA could discharge its duty is formally to write to FIU the risk associated with the known cracks. BPA has to justify any action to stop work and face the massive claim by MCM. I am sure FIU would have within its establishment some professors able to provide a third party opinion to the cracking problem. This action should have taken place on Mar 13, 2018 after the cracks were fully photographed.

On the consortium side MCM was well represented by the subcontractor's workmen and one of its own staff. Nearly all contractors I know would position their position at the right contractual position when it come to any technical matter/decsion. In the next bridge MCM may be wise enough to stop work but in the FIU project MCM only concentrated on making sure every tensioning, de-tensioning and re-tensioning work was carried out as per the designer's instruction. MCM could be criticized for the lack of safety appreciation on the cracking bridge but on record MCM was disturbed unhappy with them in the correspondences. I also of the view that MCM can pay and obtain any third party professional opinion quickly to assist its private evaluation if MCM deems the matter were disturbing enough.

The designer was conspicuously missing in the remedial work instigated by him. It makes people wonder on what basis could he claim the distressed bridge wasn't a safety issue and could be so relax to manage everything remotely. On record the designer was the first one to conceive the FIU bridge but the last one to think it could collapse.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119 (Structural)25 Jun 19 13:03)

MikeW7's 23 Jun 19 01:38 post commented VSL employee Hanson kneeled twice to look at the 11/12 junction, the last time right before the collapse.

I looked at the video again and noted more details - Video link set to start at 1:57
  • 1:59 - Person talking with Hanson leaves. I wonder who this is?
  • 2:01 - Hanson begins continuously monitoring 11-12. Is this when tensioning begins?
I went thru the video frame-by-frame and compiled a lengthy list of events, then lost the edit. Grrrrrr.......

RE: Miami Pedestrian Bridge, Part XI

saikee119 (Structural)25 Jun 19 10:01 I agree that by 13 Mar the bridge was visibly structurally damaged. I also believe that the 14 Mar photo shows that it is continuing to fail, retensioning or no.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

The order to de-tension and then re-tension the rods in #11 shows how completely confused the design team was. These rods could add and/or subtract ZERO force to the region of failure, which was outside of the #11 envelope. The rods would have had no effect on the failure mode whatsoever. What caused the failure was a total absence of any steel tying #11 to the deck.

RE: Miami Pedestrian Bridge, Part XI

There have been criticisms leveled at FIU for doing nothing. I think FIU/BPA should come clean on this one.

I said this because OSHA sourced 21 out of the 23 photos on the cracks photographed on Mar 13 from BPA. Thus BPA had to be party with the most information on the cracks. Did BPA shared these photos and concerns fully with the others?

BPA was selected by FIU and so it had to communicate with FIU first on the seriousness of the cracks before jointly seeking a solution from the Design and Build Consortium. Thus BPA had to have a view and more importantly a criterion on how the cracks could be fixed. After all BPA would at the end has on behalf of FIU to inspect the repaired bridge and recommend acceptance or rejection.

May be I am naive because if FIU/BPA were unsure of the crack severity there is nothing to stop them from picking a telephone to summon an army of free experts from the University payroll to offer quickly a third party opinion. My expectation from the experienced academics is the majority would not accept the risk of carrying out re-tensioning on a badly cracked critical Member 11 while the live traffic passing underneath. Thus I believe FIU/BPA could have done a lot more to control the situation or even avert the disaster. If FIU were so readily to believe what the bridge designer said it should have saved money not to hire BPA. BPA was hired to protect FIU's interest on the technical acceptance of the work so has BPA failed its duty?

RE: Miami Pedestrian Bridge, Part XI

Quote (FortyYearsExperience (Structural))

What caused the failure was a total absence of any steel tying #11 to the deck.

OSHA Fig 61, 62, 64 to 70 show bars left in the deck when 11/12 was pulled out by the falling action of the deck. Thus you may like to replace "any steel" with "sufficient effective steel" in your post. OSHA does condemn some steel bar ineffective as they failed to comply with the code on development length.

Also the sketch below shows at least 4 shear links intercept the construction joint.



OSHA Fig 63 has hints the above links had sheared across the construction joint

On whether the PT rods effect on the failure mode OSHA Fig 19 and Fig 30 show the same crack when the Member 11 was first tensioned and de-pensioned respectively. The crack of Fig 39 was 7" deep and serves as strong evidence that the one rod was effective in pulling the deck while the order was 11/12 and split Member 11 at the bottom end. It was the action of the PT rods that first damaged the joint and later triggered the collapse. To suggest the PT rods to have no effect on the failure mode will require challenging proof. The historical development of the cracks has already proved beyond doubt the construction sheared and cracked as soon as PT tension was first introduced.

RE: Miami Pedestrian Bridge, Part XI

Quote (FortyYearsExperience)

and saikee119
I would like to have seen member 11 and the deck connected through a "strut and tie" method with the reinforcing of 11 lapping and transferring the thrust to an imaginary "strut" within the deck with the capacity to handle all the horizontal component from the load in 11 and extending maybe 40 feet toward the center of this span. That would have required maybe two tons of reinforcing and cost maybe ten grand.
I think this would have satisfied the concerns that have been expressed by FortyYearsExperience earlier in this forum and with which I agree.
I also question the wisdom of a sloping (forcewise) construction joint in a member of the importance of 11 and with loads of this magnitude. Creating a square face on the joint at 11 and deck would have avoided the shear friction issue. It would have required a formed "socket" in the deck, perhaps, with more concrete below the deck and with proper reinforcing around that "socket" but it may have provided a more predictable transfer of the forces from 11 to the deck.
Also there should have been dedicated confinement reinforcing in members like 11, as well as at all nodes of the truss.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119 (Structural)25 Jun 19 15:58)

For the first time I have discovered what has bothering me about the detailing of node 11/12 and deck, as shown in your referenced post. At least two things, there are more remaining.
First, the 4 - #7 hoops designated "7S01". The bottom cross leg of those hoops is shown to pass through the formed channel for the drain pipe below the deck. Am I not correct in thinking these are closed ties? This would have been sorted out during the placing of reinforcing, I am sure. Regarding same 7S01 hoops, the one in the fillet between 11 and the deck explains why that small fillet stuck to the deck in the early cracking of this joint. The fillet was anchored by the southmost hoop.

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance Wiley (Structural)25 Jun 19 17:39)

The bottom cross leg of those hoops is shown to pass through the formed channel for the drain pipe below the deck. Am I not correct in thinking these are closed ties?
I admit it is hard to tell from the cropped drawing , but the drainpipe is outside the bottom of the deck in a channel.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

saikee119 (Structural) - The Timeline was created by NBC not CBS.

For the record, Structural Technologies, a subsidiary of Structural Group was the subcontractor responsible for all the formwork, reinforcement & place & finish. VSL, a subsidiary of Structural Group was the PT subcontractor. MCM seems to have been mostly construction management.

I've been exploring the TiO2 issue with respect to the pristine rebar. Even the corrugated PT sleeves came away without even tiny scabs of concrete attached. There doesn't appear to be research regarding the bond strength of TiO2 modified concrete to rebar. There is a lot of positive research about nano-TiO2 and a resulting increase in strength. This seems to be the case with many "nano" constituents. Wonder of wonders, filling in the empty spaces between the sand, cement & aggregate makes concrete more solid.

The "Environmental" hype about TiO2 and NO2 conversion to an NO dust & subsequent "self-cleaning" has me wondering about how TiO2 may be modifying the surface energy of concrete. Epoxies have a surface energy of 35 to 45 dynes/cm. Based on the information I can find, TiO2 has about the same surface energy as epoxies. The thing about surface modification is that sometimes it doesn't take much of an ingredient to completely change the performance of a mixture. Epoxy coated rebar require greater development length. Maybe there needs to be some research done to determine if the addition of TiO2 isn't affecting rebar strength development.

RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie)

Thank you - that detail shows 7S01 is an open bottom with legs turned out and clears the drain channel. All is good.

RE: Miami Pedestrian Bridge, Part XI

Quote (epoxybot (Structural)25 Jun 19 20:04 filling in the empty spaces between the sand, cement & aggregate makes concrete more solid. )

A diamond is pretty solid - and shatters easily. I wonder if the introduction of TiO2 is reducing the ability of the concrete to accommodate strains?

Quote (Maybe there needs to be some research done to determine if the addition of TiO2 isn't affecting rebar strength development.)

A new industry on the future - using TiO2 to clean used rebar. All the visible rebar appears to be a lot cleaner than I expected. No adhesion?
Did the TiO2 affect or totally defeat "cohesion " in the cold joint? Seems possible as a side effect or unintended consequence, considering it reacts with things that contact it and paints itself white (grows a white coating). Should cohesion be a no-no if TiO2 is used? Development lengths increased as you suggest?
I have mentioned before how brittle the concrete seems to appear. What is the elasticity?
This may bring a rich chemical company into the nest of defendants.
"Surface Energy" - I have encountered that term. Perhaps you could provide an explanation of how it works and might apply in this case?


RE: Miami Pedestrian Bridge, Part XI

Quote (epoxybot (Structural) 25 Jun 19 20:04)

I've been exploring the TiO2 issue ...

Is it clear if "plain" Ti02 was used, or NT (nanotitanium)? Here's some info on both on pages 34-35 of this research paper (end page dated 2017):

Quote:

To be effective, the nanotitanium needs to be at the ‘interface’ of the concrete, i.e. at the point where it can react with sunlight and organic debris (which then breaks down to CO2 and water). For this reason, and also to reduce cost, it is more usefully added as a surface coating or thin slurry rather than being mixed into construction concrete.
It also says

Quote:

Titanium dioxide is also photocatalytic in its non-nano form which is considerably cheaper than nanotitanium, and potentially less toxic. This may explain why the use of nanotitanium in concrete is currently extremely limited.

In either case, it appears there is no practical reason to incorporate Ti02 or NT into the concrete-rebar interfaces, so is it known if the TiO2/NT was used merely as a topcoat? I remember several comments about a "white slurry".

ADD: If it's expensive and used primarily as a topcoat, that would explain why there's no literature about its effect on rebar, etc.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7)

Titanium additive -
A surface treatment makes sense. Did they paint the diagonals white or is that a Titanium coating that has (had) already reacted? Is it to be sprayed on or surface applied to the bottom of the canopy and deck?

RE: Miami Pedestrian Bridge, Part XI

The diagonals look like they received a parging of additional TiO2 surface coating but integral TiO2 was called for in the design.





Surface energy is probably best understood by the contact angle of water on a given surface. Water being a liquid with a very high surface tension @ 72 dyne/cm.

RE: Miami Pedestrian Bridge, Part XI

epoxybot (Structural) 25 Jun 19 21:12 - So adding Titanium to the concrete mix increases the cost by a factor of 2.5 to 3.2, yet it has no known benefit. Hmmm.... And there also doesn't appear to be any literature explaining how concrete properties are affected by its addition. (...cricket noises...)

EDIT: I'm referring to my post of 25 Jun 19 20:48 which explains that TiO2 only has useful properties when it's used as a surface coating.

RE: Miami Pedestrian Bridge, Part XI

Quote (epoxybot (Structural))

The Timeline was created by NBC not CBS.

Thanks for pointing out. I have corrected the previous posts except one. The uncorrected post may have the edit privilege expired.

RE: Miami Pedestrian Bridge, Part XI

For those who were surprised to find the rebar, left behind after the collapse, were so clean with little or no concrete fragments attached it would be useful to remind ourselves that when the 11/12 joint failed the deck dropped to the ground but the 11/12 was caught by the pier. Therefore there would have been some heavy tensile forces by the dropping deck to pull itself out of 11/12.

Thus the 11/12 failure mode might have be a horizontal shear failure to start with but the dropping action of the deck would have led to a blow out of the joint when a large number of rebar did not have the adequate development length or the concrete surrounding the rebar was simply inadequate for the embedment purpose. It goes without saying the rebar next to the four vertical sleeves had the least chance of being gripped firmly by the concrete.

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance)

I would like to have seen member 11 and the deck connected through a "strut and tie" method with the reinforcing of 11 lapping and transferring the thrust to an imaginary "strut" within the deck with the capacity to handle all the horizontal component from the load in 11 and extending maybe 40 feet toward the center of this span.

Yes.

A thickening of diaphragm II would have been good too (to reduce punching shear stress), or it that was deemed unsightly a massive steel plate on the end of diaphragm II with welded bars reaching back a long way back into the deck.


Quote (Vance)

I also question the wisdom of a sloping (forcewise) construction joint in a member of the importance of 11 and with loads of this magnitude. Creating a square face on the joint at 11 and deck would have avoided the shear friction issue

Yes with the benefit of hindsight it was a bad idea to align the construction joint with the shearing force. It should have been rotated to the compressive stress direction, with a socket or a stub.

I still don’t know if the shear friction would be ok though. The bridge could still shear at that junction.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7 (Electrical)25 Jun 19 21:32 epoxybot (Structural) 25 Jun 19 21:12 - So adding Titanium to the concrete mix increases the cost by a factor of 2.5 to 3.2, yet it has no known benefit. Hmmm.... And there also doesn't appear to be any literature explaining how concrete properties are affected by its addition. (...cricket noises...))

UH-OH.
From an earlier post - "Don't be the first guy to try something and don't be the last guy to use something".
FIGG may have hit a double. First AND Last.

RE: Miami Pedestrian Bridge, Part XI

Quote (FortyYearsExperience)

The order to de-tension and then re-tension the rods in #11 shows how completely confused the design team was. These rods could add and/or subtract ZERO force to the region of failure, which was outside of the #11 envelope. The rods would have had no effect on the failure mode whatsoever. What caused the failure was a total absence of any steel tying #11 to the deck.

I can't disagree with the conclusion, but I have to say that there is a certain internally consistent logic to the re-tensioning. When you take an action and then something bad happens, most often the best thing to do is undo the action and see if that fixes it. For example, you deploy the wing flaps and the airplane starts rolling uncontrollably left. What to do? Put the flaps back where they were, of course! That might buy you some time to go do some troubleshooting and save the day.

Of course, the logic at work assumes that a) correlation means causation and b) the action is in fact reversible. It appears that the cracks started widening precipitously when the PT in 11 was de-tensioned, so they ordered the tension restored, hoping for both. However, it appears now that the widening cracks were not necessarily caused by the de-tensioning, but rather symptoms of deeper problems and merely exacerbated by the de-tensioning. And also that the action was in fact not reversible.

But in the heat of the moment, with engineers and contractors suffering from the narrowing tunnel vision that too often comes from a stressful situation, it probably seemed like a good idea at the time.

--Bob K.

RE: Miami Pedestrian Bridge, Part XI

CODE --> FortyYearsExperience

The order to de-tension and then re-tension the rods in #11 shows how completely confused the design team was. These rods could add and/or subtract ZERO force to the region of failure, which was outside of the #11 envelope. The rods would have had no effect on the failure mode whatsoever. What caused the failure was a total absence of any steel tying #11 to the deck. 

I agree the design team was confused. They detensioned the rods and cracks opened up. They made an association with a correlation and confused it with a cause. However, the retensioning of the lower rod added a substantial shear stress across the concrete joint. The member force was unchanged (equal and opposite reaction) but the shear stress on the concrete was substantially higher on an already over-stressed surface.

RE: Miami Pedestrian Bridge, Part XI

The detensioning did cause the cracks to open up further (it wasn't mere correlation), which put the structure into a state that retensioning caused the structure to collapse. Like a ratchet, or a wedging mechanism.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

The detensioning did cause the cracks to open up further (it wasn't mere correlation), which put the structure into a state that retensioning caused the structure to collapse. Like a ratchet, or a wedging mechanism.

I don't think a definitive statement like that can be made. It may or may not have help initiate the movement but there was already signs of movement before the transportation. The release of the PT did reduce the shear friction clamping force but it also reduce the shear stresses on the concrete. Based on calculation, the release of the tension decreased the shear on the pour joint more than it decreased the shear friction capacity. These is not to say the theory is 100% correct/accurate but it is all we have to go off of. The only definitive way to confirm this is through a test. Regardless, we can say the joint was over stressed in shear friction. I think we can also say that it was a significant error to re-tighten the PT.

RE: Miami Pedestrian Bridge, Part XI

I'll throw in this mark up for thought. The 11 lower PT cable had the transverse deck PT cable to pull against (cyan). I think the node is more apt to be described as the 11/deck/diaphragm node. 11 had various longitudinal cracks and a number of longitudinal rebar had seemingly little deformation.

RE: Miami Pedestrian Bridge, Part XI

Quote (Earth)

Based on calculation, the release of the tension decreased the shear on the pour joint more than it decreased the shear friction capacity.

The cracks widened significantly when it was detensioned. If calculations say detensioning improves the situation (by reducing shear stress more than it reduced shear friction capacity) then the calculations are wrong.

RE: Miami Pedestrian Bridge, Part XI

I think the 2 vertical plastic sleeves on each side of 12 have played a significant role in the demise of the 11/12 joint.

In Sym P. le (Mechanical) 26 Jun 19 01:22 sketch it can be seen the two vertical sleeves occupy about half the area the deck bonding with the Member 12. We now know the front and the rear of these sleeves have cracked through on both east and west of Member 12. Thus the deck was likely to have debonded from the two vertical sides with Member 12 after the tension in Member 11 had been released. The upper PT rod was therefore unable to distribute the stress into the deck and became more effective in compressing Member 12.

Lower PT rod on the other hand was able to anchor more effective into the deck as it is wholly inside the deck and close to the transverse PT tendons. Since the deck was likely to have debonded from Member 12 and so the lower PT tension would not be effectively felt by Member 12.


The above sketch shows if the deck were not monolithic with 11/12 then the upper PT rod will not be effective on the deck as it is nearly outside the deck area.

The photos after Mar 13 have confirmed serious movement in the construction joint and so the the two haves of either of the CJ would have been debonded too.

Thus the re-tension scheme was operating on the condition totally different from that in the casting yard. It is also instructive to note 11/12 had cracked CJ, at least partially, even before leaving the casting yard according to OSHA Fig 18.

The condition of the CJ prior to re-tensioning can be seen in OSHA Fig 32 to 38 inclusively.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

The cracks widened significantly when it was detensioned. If calculations say detensioning improves the situation (by reducing shear stress more than it reduced shear friction capacity) then the calculations are wrong.

I don't think that is true. It is easy to confuse correlation with causation. This is human nature. The cracks happened to get worse after then tension was released. You can not say that was the cause of the cracks getting worse. It does not conclusively say the calculations are wrong or that the release of the tension initiated the movement. We know the movement started before the transportation but we can not conclude one way or the other that the cracks got worse because of the release of the tension.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

The cracks widened significantly when it was detensioned. If calculations say detensioning improves the situation (by reducing shear stress more than it reduced shear friction capacity) then the calculations are wrong.
Exactly. And there is more going on than simple shear friction. For example a tense PT member will take some shear. A slack one won't. Likewise in a perfect theory world shear friction should be pretty linear. In reality you can get non linear behaviour.


Quote (Earth314159)

I don't think that is true. It is easy to confuse correlation with causation.
I think everybody here is aware of the difference. However the immediacy of the occurrence is an extremely big hint. If I accidentally hit my thumb with a hammer and it starts hurting I don't think it makes much sense to argue correlation vs causation.

RE: Miami Pedestrian Bridge, Part XI

Quote (human909)

I think everybody here is aware of the difference. However the immediacy of the occurrence is an extremely big hint. If I accidentally hit my thumb with a hammer and it starts hurting I don't think it makes much sense to argue correlation vs causation.

There is also reason to believe it had nothing to do with the initiation of the shear friction failure. We know that the pour joint was over stressed and there was some small amount of sliding before the transportation. Calculation indicated the detensioning helped the condition. If the failure was going to occur anyways and the plan was to detension, then it is just a correlation. For all we know, the detensioing may have improved the condition. I think you need more evidence to back up the statement. I also think it is irrelevant in the sense that there was a problem with this joint regardless of whether there was detensioning or not. There were technical, judgement and ethical errors made.

Your example is misleading since you pick an example where there is causation. Look at another example. My child was vaccinated. My child was diagnosed with autism. Vaccinations cause autism. It is the same thinking and it is human nature.

RE: Miami Pedestrian Bridge, Part XI

Quote (earth)

We know that the pour joint was over stressed and there was some small amount of sliding

Yes it was already cracking. The joint appears to have been struggling from the outset. The point is that the situation became much worse once it was detensioned. The members shifted and the cracks opened up to nearly an inch wide.

Quote (earth)

For all we know, the detensioing may have improved the condition.


How can it have improved the condition if it "cracked like hell" once they loosened the bars?

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

How can it have improved the condition if it "cracked like hell" once they loosened the bars?

How do you know it wouldn't have "cracked like hell" if it wasn't loosened?

RE: Miami Pedestrian Bridge, Part XI

Quote (earth)

How do you know it wouldn't have "cracked like hell" if it wasn't loosened?

I don't know for sure. But it makes sense. There are two possibilities:

A- releasing the PT made things worse, causing it to "crack like hell"

B- releasing the PT actually made things better, and by coincidence the bridge "cracked like hell" at that moment for a different (unknown) reason.

Barring good evidence for option B, option A makes a lot more sense to me.





RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

I don't know for sure. But it makes sense. There are two possibilities:

A- releasing the PT made things worse, causing it to "crack like hell"

B- releasing the PT actually made things better, and by coincidence the bridge "cracked like hell" at that moment for a different (unknown) reason.

Barring good evidence for option B, option A makes a lot more sense to me.

The movement took days. It is not reasonable to say "at that moment". If it was instantaneous, the course of action would have been much different.

RE: Miami Pedestrian Bridge, Part XI

Quote (earth)

The movement took days. It is not reasonable to say "at that moment".

The OSHA report describes the appearance of the severe cracking following the detensioning as follows:

Quote (OSHA REPORT)

As they began to de-stress the PT bars of diagonal 11, cracks began to appear at multiple locations, most prominently at the construction joint of diagonal 11 and the deck and at the top of the diaphragm II. There were three VSL employees performing the de-stressing – Kevin Hanson (supervisor), Navarro Brown and Chester Ashley. Kevin is regarded as one of the most knowledgeable PT field personnel in South Florida. After observing the cracks, Kevin became visibly disturbed and informed other VSL employees of the situation. Kevin took pictures of the cracks, and sent them to his supervisor, Sam Nunez, stating that “it cracked like hell”

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119 (Structural)26 Jun 19 02:25)

Quote (I think the 2 vertical plastic sleeves on each side of 12 have played a significant role in the demise of the 11/12 joint.)

I agree. A tear on the dotted line.

Quote (The above sketch shows if the deck were not monolithic with 11/12 then the upper PT rod will not be effective on the deck as it is nearly outside the deck area.)

Consider that the force from each rod is applied to the top end of member 11 and it is 27 feet away. The forces have aligned themselves wherever they want to in 11 so both rods can apply their force to the cold joint from the top. The location of each is critical to events at the deck zone, and the upper PT may have influenced the splitting in 11.
So if we consider member 11 as extending into the deck and including the bottom anchor plates, adding PT force affects only 11 and nothing else if we ignore the integral nature of the deck. Likewise releasing the PT in 11 also affects only 11. But 11 is not isolated from the deck - it is intended to be a part of the deck. How well it is integrated into the deck is the thing that is/was undefined and improperly evaluated.
By failing and blowing out the end it has shown us where it needed reinforcing and where attention was lacking.
Consider please my previous post posing the idea that loosening of PT and subsequent increased cracking caused loose pieces in the joint and the re-tensioning crushing those now mis-aligned pieces, leaving crumbles and sand in the joint with even less capacity to transmit loads to the deck.
In reviewing FIGG calcs in the March 15 PowerPoint Party, they did not address the failure plane/surfaces that actually developed. It appears that the contact surface of the deck was basically smooth and the factor Mu should be 0.6 instead of 1.4 or 1.0. Applying that to both the shear friction steel across the joint and the perpendicular force of 950 kips reduces those contribution significantly. Consider also that the early cracking and movements had broken any bond and the cohesion was lost and movements were underway. At what point can this zone at the deck surface no longer support any horizontal load?
With the remaining blow out being almost diagonal tension with little reinforcing, is it possible to now calculate the contribution of the breakout zone at failure? Considering reductions for sleeves, etc? This was step 2 of the two part failure, as I see it, and step 1 had already given its all.

RE: Miami Pedestrian Bridge, Part XI

To tension or not to tension - - -
An added thought to tensioning of member 11 -
After releasing the PT loads and evoking the resulting "it cracked like hell" remark, there could have been new cracks or just larger existing cracks, but the failure plane was likely defined. If so, the upper PT rod in 11 was above the actual failure plane and was effectively isolated from the deck, therefore the effect of retensioning of the upper PT rod was primarily internal to 11.
The lower PT rod remained anchored in the deck, even after the collapse, and any force in the lower rod created more shear force along the sloping portion of the failure plane, which was basically the slope of member 11 and this lower PT rod. If this sequence has merit, the tensioning of the lower PT hastened the collapse.
EDIT ADD "I suggest there is more certainty that the condition just described existed by the time node 10/11 had dropped a foot. It makes a good triggering scenario."
I think collapse was inevitable before retensioning began.

RE: Miami Pedestrian Bridge, Part XI

Quote (epoxybot (Structural)25 Jun 19 21:12)

Thank you for the primer/refresher on surface energy.

RE: Miami Pedestrian Bridge, Part XI

Vance, I think you’re generally correct.

My impression is the CJ was already starting to shear, but the node was still hanging on. detensioning reduced confinement (or otherwise weakened) the node allowing the prism punch out to occur. It isn’t clear to me whether that failure prism was already a hairline crack, a latent crack, or completely uncracked, but in any case after detensioning the failure surface was a complete crack.

Apparantly it was continuing to slip bit by bit over the next day or two, but was still hanging on by its fingernails. Retensioning then gave it a final push (by adding shear force at the crack), causing it to let go.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

The OSHA report describes the appearance of the severe cracking following the detensioning as follows:

I am not too sure why it even maters. In this particular case, what recommendation can come of determining when or why the an over stressed shear friction crack started to opened up. Calculations imply the rod tension was worse than no tension. The coincidence of the timeline implies the detensioing made it worse. The best we can say is the smaller cracks started before the move, larger cracks were first notice shortly after detensioing, and the cracks grew larger. At least we have some kind of timeline but to say what exactly was the perturbation that started the cracks is somewhat irrelevant because without a physical rationale, you can't really based any recommendations on this. Why not just use it as a timeline and say the detensioing may have been the likely disturbance that initiated the cracks on an already over-stressed joint. We know retensioning was a big mistake since that is backed up with substantial evidence and numbers.

RE: Miami Pedestrian Bridge, Part XI

Quote (Earth)

Why not just use it as a timeline and say the detensioing may have been the likely disturbance

That’s what is being done. The sudden dramatic worsening of the cracking occured when they began to detension, so it is likely that detensioning was a causal factor, as opposed to it just being a coincidence.

Quote (Earth)

Calculations imply the rod tension was worse than no tension

And? If this saga has one thing to teach us it’s that we shouldn’t be beholden to our numbers when the structure is telling us something different.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

And? If this saga has one thing to teach us it’s that we shouldn’t be beholden to our numbers when the structure is telling us something different.

What was the structure telling us that was different from the numbers? The detensioing is irrelevant. The calculation said that reshoring was the proper course of action. The cracks said reshoring was the proper course of action. What did we learn about the detensioing possibly initiating the crack? Both the numbers and behaviour said what the proper course of action was. The possible disturbance that started the sliding is not relevant. We can't use that to predict anything.

The detensioing wasn't the casual factor. The over-stressed joint was the causal factor. We learn a lesson about the over stressed joint. We learn to check our calculations when we see an over stressed joint. We learn to keep looking if we can't find the cause. We learn to get assistance if we can't determine the cause. We learn not to risk the public safety if we can't determine a cause. We learn to identify a joint that is over stressed in shear friction.

RE: Miami Pedestrian Bridge, Part XI

Quote (earth)

The detensioing wasn't the casual factor. The over-stressed joint was the causal factor.

They were both causal factors. The node was too weak, and was already showing signs of cracking before detensioning. The detensioning made things a lot worse.

RE: Miami Pedestrian Bridge, Part XI

Quote (Earth314159)

The detensioing wasn't the casual factor. The over-stressed joint was the causal factor. We learn a lesson about the over stressed joint. We learn to check our calculations when we see an over stressed joint. We learn to keep looking if we can't find the cause. We learn to get assistance if we can't determine the cause. We learn not to risk the public safety if we can't determine a cause. We learn to identify a joint that is over stressed in shear friction.
I agree, and all very good points.
But this is a discussion forum and many points raised will not directly be causal in nature but will receive intense attention and require lengthy discussions in the legal system procedures just ahead.
And the examination of how this event developed and progressed can provide warning signs in ones future practice. This is not a common event, the firms involved represent the basic "standard of care" in their respective industries, and yet it did happen.
Hopefully it will never happen in your career. I have sat beside the Judge and addressed the jury, as an "expert witness". I have seen instances where "details matter". Even small details.
So I encourage deeper investigation. It cannot change the outcome, particularly for those who lost their lives and those injured by the event. It may further some level of understanding. That can't be a bad thing.

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance Wiley)

I agree, and all very good points.
But this is a discussion forum and many points raised will not directly be causal in nature but will receive intense attention and require lengthy discussions in the legal system procedures just ahead.
And the examination of how this event developed and progressed can provide warning signs in ones future practice. This is not a common event, the firms involved represent the basic "standard of care" in their respective industries, and yet it did happen.
Hopefully it will never happen in your career. I have sat beside the Judge and addressed the jury, as an "expert witness". I have seen instances where "details matter". Even small details.
So I encourage deeper investigation. It cannot change the outcome, particularly for those who lost their lives and those injured by the event. It may further some level of understanding. That can't be a bad thing.

I agree. These events also do make me think and to be careful.

RE: Miami Pedestrian Bridge, Part XI

2
It pays to remember the "swiss cheese" analogy of catastrophe. Normally the bubbles in swiss cheese create small depressions, but they don't all line up to create holes entirely through the wheel. But sometimes a whole bunch of independent bubbles can line up and cause a whole all the way through.

A similar thing applies in most disasters. There's almost never one root cause, but instead a set of causative and exacerbatory factors. The closest to a single root cause here is that the design was insufficient to carry even the self-load of the bridge. The 11/12/deck node was particularly bad. The detensioning altered the stress patterns in and around the node, probably helping it crack. The retensioning caused further movement and the collapse of the entire structure. The decision not to close the road and shore up the bridge was also a cause. The decisions made in modeling and calculating the forces involved were causes. The regulatory oversight enforcement structure that allowed for fewer design checks than legally required is a cause.

If you dig far enough you hit the second law of thermodynamics, statistical mechanics, and the rest of the laws of physics as ultimate causes. It becomes useless in preventing future disasters. But finding the various causative factors and changing procedures to fix them can help. If any of the causes I listed (or several others) hadn't happened the structure wouldn't have collapsed when it did. It still might have collapsed, eg if the only change was to not detension the #11 rods it probably would have held up until a large live load of people was placed upon it (Murphy's law).

RE: Miami Pedestrian Bridge, Part XI

I don't consider the detensioning the root cause, nor the retensioning, but the person who ordered the de- and re-tensioning.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Reading over the OHSA report and FIGG's actions, it seems FIGG were pretty worried despite their repeated reassurances their was no risk.

They spoke of the need to capture the node, and were urgently preparing the strapping details.

What was their rationale in insisting propping/shoring wasn't necessary? Were they concerned in may inadvertently worsen the situation? Or was it more about saving face and avoiding the embarrassment of having to shut down the road and install emergency shoring?

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural)27 Jun 19 02:11 What was their rationale in insisting propping/shoring wasn't necessary? Were they concerned in may inadvertently worsen the situation? Or was it more about saving face and avoiding the embarrassment of having to shut down the road and install emergency shoring?)

I'm betting it is the latter. This "Signature Bridge" came with a Prime Directive of glorifying ABC methods.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

Reading over the OHSA report and FIGG's actions, it seems FIGG were pretty worried despite their repeated reassurances their was no risk.

They spoke of the need to capture the node, and were urgently preparing the strapping details.

What was their rationale in insisting propping/shoring wasn't necessary? Were they concerned in may inadvertently worsen the situation? Or was it more about saving face and avoiding the embarrassment of having to shut down the road and install emergency shoring?

I suspect it would be a huge embarrassment to have to close a number of the lanes after bragging about the ABC system. There is also a money issue.

Figg's comment about capturing the node is very interesting. It suggests they did know what the issue was but ignored the shear plane calculation at the pour joint in their presentation the morning of the collapse. How did they know they had to capture the node but also made the comments about not knowing where the cracks came from? It seems contradictory.

RE: Miami Pedestrian Bridge, Part XI

Quote (earth)

Figg's comment about capturing the node is very interesting. It suggests they did know what the issue was but ignored the shear plane calculation at the pour joint in their presentation the morning of the collapse. How did they know they had to capture the node but also made the comments about not knowing where the cracks came from? It seems contradictory.

I suspect they knew it was bad. Their recognition of their need to capture the node, and their haste in developing emergency strapping suggests they didn't have full confidence in their powerpoint calculations.

RE: Miami Pedestrian Bridge, Part XI

Quote (Earth314159)

How did they know they had to capture the node but also made the comments about not knowing where the cracks came from? It seems contradictory.
Good point. It is contradictory. Not the first instance.
They were gambling on having time to make corrections and stated that when the backspan was complete all would be good. Unfortunately the chips on the table were the lives of 6 people. I challenge the statement about the backspan providing all the stability needed. And I do not know the schedule but it would be at least 4 months before the backspan would be constructed, cured, and have falsework removed. That is 4 months of the public being exposed to a serious risk.
Kenny Rogers sings softly in the background . . . .
You gotta know when to hold'em
Know when to fold'em
Know when to walk away
Know when to RUN

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance)

Unfortunately the chips on the table were the lives of 6 people. I challenge the statement about the backspan providing all the stability needed. And I do not know the schedule but it would be at least 4 months before the backspan would be constructed, cured, and have falsework removed.

They intended for the steel beams to act in the interim, prior to backspan being installed. They don't appear to have figured out how to make the backspan provide the necessary stability, but once the steel straps were in they would have some breathing room to figure it out.

That meant they only needed to worry about the period until the steel straps were installed, which is presumably where the decision to retension came in. They emphasised the need to retension ASAP.

RE: Miami Pedestrian Bridge, Part XI

2
I find some of the discussion of detensioning and retensioning of the PT rods, as if the concrete responds elastically (if detensioning widened the cracks, retensioning should have closed them), to be a bit strange. Detensioning of the rods reduces the confinement, allowing the concrete to crack more extensively and allowed the already cracked concrete to move. Retensioning the rods does not put the genie back in the bottle - you can't carry stress across the crack anymore; all it does at that point is produce new stress concentrations and local crushing (concrete shearing) in new places, speeding the demise of the connection.

RE: Miami Pedestrian Bridge, Part XI

Quote (HotRod10)

I find some of the discussion of detensioning and retensioning of the PT rods, as if the concrete responds elastically (if detensioning widened the cracks, retensioning should have closed them), to be a bit strange.

I think most of us appreciate that it didn't work that way, and that releasing the PT allowed the cracking to dramatically worsen, and that retensioning didn't close them up but instead triggered collapse.

However it seems to have been FIGG's working assumption that retensioning would help the joint, which is why it's being discussed. Without the benefit of hindsight I don't think it was an entirely unreasonable assumption on FIGG's part. Still quite risky to not immediately shore it up, but I don't know how many of us could have readily predicted that the retensioning would cause it to fail completely.

RE: Miami Pedestrian Bridge, Part XI

Quote (HotRod10)

I find some of the discussion of detensioning and retensioning of the PT rods ... to be a bit strange.

I cringe everytime I hear the term used in association with this incident, especially when used by the media which lends it an air of authority. This is not a meccano set.

I prefer to think of them as having been decommisioned, having served their purpose. The fact that they ended up keeping the structure together before being used to destroy it are different matters.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

... I don't know how many of us could have readily predicted that the retensioning would cause it to fail completely.

There, but for the grace of God...

I disagree.

RE: Miami Pedestrian Bridge, Part XI

Quote (Sym)

I disagree.

You think you would have predicted it?

RE: Miami Pedestrian Bridge, Part XI

Is it time to regroup and refocus thread?

RE: Miami Pedestrian Bridge, Part XI

Quote (HotRod10 (Structural)27 Jun 19 05:26 Detensioning of the rods reduces the confinement, allowing the concrete to crack more extensively and allowed the already cracked concrete to move. Retensioning the rods does not put the genie back in the bottle - you can't carry stress across the crack anymore; all it does at that point is produce new stress concentrations and local crushing (concrete shearing) in new places, speeding the demise of the connection.)


I think I agreed with that. But much less succinctly.
See Vance Wiley (Structural)24 Jun 19 18:01

RE: Miami Pedestrian Bridge, Part XI

Quote (Sym P. le (Mechanical)27 Jun 19 06:20 This is not a meccano set.)

I had to search that reference -
In 1950 mine was called an Erector Set. Have not thought of it in years.
Thanks for the memory.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural)27 Jun 19 06:14 Without the benefit of hindsight I don't think it was an entirely unreasonable assumption on FIGG's part. Still quite risky to not immediately shore it up, but I don't know how many of us could have readily predicted that the retensioning would cause it to fail completely.)

I must agree. In the investigation of damage to tilt-up panels due to shortening in a tee beam/PT slab roof, I used the phrase "There but for fortune, go I" - Joan Baez, maybe 1962.
The potential for failure was probably recognized but not accepted, and the urgency was paramount. With the PT rods in place and capable of rapid restoration of the force, it "seemed like a good idea at the time".
Mark Twain said "When a hammer is the only tool you have, everything looks like a nail".
With the benefit of hindsight it is easy to see the only appropriate and immediate action was to close the street and call for shoring. To be installed by remote control.

RE: Miami Pedestrian Bridge, Part XI

Old story, another bridge.
"The bridge is not in any danger of collapse," state Department of Transportation Secretary . .
https://www.eng-tips.com/viewthread.cfm?qid=352416
Perhaps it's the fear of crying wolf that leads to such statements?

The host of this video on bridge had better idea at 20 second mark.
https://www.youtube.com/watch?v=xPULiFwZ06M

The host of this video confused sag with span at 1 min 56 sec.
https://www.youtube.com/watch?v=9YKOOr_OVgc

Amateur bridge inspector: https://www.youtube.com/watch?v=GF4S0ceYkZQ

How they fixed it: https://www.youtube.com/watch?v=BLViBG-BA5Q

Explaination: http://www.fox11online.com/news/local/green-bay/do...

RE: Miami Pedestrian Bridge, Part XI

I agree with Tomfh that not many of us could have readily predicted that the retensioning would cause it to fail completely.

However FIGG and most of us know prior to collapse the 11/12 had cracked 45 degree (the geometry is the warning sign that the crack is structural in nature) on the west, cracked and separated from the front, cracked 45 degree on the east. The only remaining that did not crack was space at the back to which the 11/12 was moving slowly away from the deck.

The fact FIGG wanted to "capture" this runaway node and divert some of the load from 11/12 to the next bay is a testimony he knew the structure was in serious trouble. It is also an admission that 11/12 was under-designed and unable to withstand that temporary condition.

Based on the severity of the cracks I would say most responsible professional would not proceed with any remedial attempt unless it is beyond doubt that the remedial action could rescue this distressed structure or help to stabliize it.

Had FIGG sought formal permission, with detailed justification on how the cracks could be controlled or reduced, prior to re-tension the doomed bridge there is a good probability that the FDOT, with their own prior theoretical predictions of the cracks and saw some of them had occurred in the field with such ferocity, would not risk to have the traffic there during the remedial work. In which case the bridge could still collapse but no public would lose their lives needlessly.

RE: Miami Pedestrian Bridge, Part XI

Thinking back to Pate's phone call to Tom Andres' voicemail: https://www.huffingtonpost.com.au/entry/florida-en...

Would Pate have been mindful that these cracks were the same cracks Andres had predicted with lots of red exclamation marks? Was that the reason for the call? To hopefully intercept and defuse Andres before Andres found out?

RE: Miami Pedestrian Bridge, Part XI

Where the sound goes depends on the curvature and the location of the sound source. Unlike flat parallel surfaces that will certainly echo, curved surfaces may or may not. It looks like the center of curvature is at or near the surface of the centerline of the deck, so only sounds at the deck surface center will be reflected back to the source. All other sound will be dispersed.

There is a monument in Washington, D.C. for Einstein that has audio-retroreflective contours; the effect is only noticeable in a very small area at the center of the arcs that make it up. From the wikipedia article "To a visitor standing at the center of the dais, Einstein appears to be making direct eye contact, and any spoken words are notably amplified." https://en.wikipedia.org/wiki/Albert_Einstein_Memo...

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural)27 Jun 19 12:31)

Was that the reason for the call? To hopefully intercept and defuse Andres before Andres found out?

Yes. Edit: Retracted until I find source > Also, if you browse the email records you will find the purpose of the on-site presentation was to "calm everyone down". We can only imagine what MCM was telling FIGG before that.

I think if EOR ever is on the stand for this, he will say bridge was not deflecting and that is why safety was not a concern.

There has to be more to the story... too many professionals failed in the same manner...

At this time, I feel nothing is to be gained by looking at this accident any further until NTSB report comes out or suits go to trial. I feel more comfortable looking at similar solved events in the past for insight instead. That's just me.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs)

Also, if you browse the email records you will find the purpose of the on-site presentation was to "calm everyone down".

Do you have a link to that?

RE: Miami Pedestrian Bridge, Part XI

I think someone with a solid understanding of the structural mechanics involved should have known better than to think that retensioning the rods, with cracks in the deck of that size, would improve the situation. I'm no expert when it comes to the structural mechanics of concrete, but several of the people directly involved with this supposedly are. With my limited experience and understanding, even without the benefit of hindsight, I think I can honestly say, I would not have suggested retensioning the rods given the size and location of the cracks. Trying to pull yourself up from your bootstraps doesn't work and often breaks the bootstraps.

RE: Miami Pedestrian Bridge, Part XI

Reading this thread, I sense that I'm not the only one who feels some emotion over the tragic loss of life, and, yes, the thread has gotten emotional, but understanding the human aspects of the cause is as important as the mechanism of failure. Yes, I know the NTSB will address human factors, and has more access to communications and interviews, than we do, but I think it's important for all of us in the engineering professions to have a chance to explore our own feeling with regard to this event. Let's not get bogged down in quibbling about wording, but let's keep talking about what allowed the event to occur.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural) 27 Jun 19 13:25)

Do you have a link to that? (referring to email records)

I think it's in the BT-904 Document List, Item 4.

I collected a bunch of investigation related links and put them in the description box for the Links Playlist on my Whirled Gnus YouTube page.

RE: Miami Pedestrian Bridge, Part XI

I just don't agree that they couldn't have foreseen that re-tensioning #11 was a non-starter. Denny Pate did a site inspection before the meeting. When he saw the crack at the base of the 11/12 node migration up the 11 diagonal, any prospect of re-tensioning #11 was off the table. How do you move forward without knowing the soundness of the #11 diagonal without doing an impulse echo or impact echo test. There is no way to know the amount of section loss by simple visual examination. It is like taking an already cracked concrete cylinder and putting it back in the compressive test rig and loading it again without any idea of an ultimate failure load.

RE: Miami Pedestrian Bridge, Part XI

Has this been addressed?

v. There is insufficient details of the walkway deck web interface and the canopy web interface where there is significant interfacing shear between the elements.

Clarification: Here, as with any structural system which has connecting members, there is a transfer of force between the various members at each nodal point. FIGG would not classify these behaviors solely as “shear” per se. However LRFD’s strut and tie design philosophy (section 5.6.3) will assure the required transfer of loads at the nodal transition points. Relative to this comment, no changes will be made that would alter the 90% foundation design plans.



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From: "Feliciano, Manuel" <mfeliciano@figgbridge.com>
To: "Andres, Tom" <Thomas.Andres@dot.state.fl.us>
Cc: "Dempsey, Dwight" <ddempsey@figgbridge.com>,
"Pate, Denney" <dpate@figgbridge.com>,
"Robertson, Robert" <Robert.Robertson2@dot.state.fl.us>,
"Theryo, Teddy" <Teddy.Theryo@dot.state.fl.us>,
"Reyna, Alfredo" <Alfredo.Reyna@dot.state.fl.us>,
"Alberto Delgado"
filename="Responses to Central Office Foundation Related Comments.pdf"


RE: Miami Pedestrian Bridge, Part XI

Interesting email exchange about TiO2:

RE: FIU Concrete Mix Designs

From: Arcalas, Ronald <Ronald.Arcalas@dot.state.fl.us>
Sent: 10/25/2016 7:40 AM
To: Reyna, Alfredo <Alfredo.Reyna@dot.state.fl.us>
Cc: Patel, Mayur <Mayur.Patel@dot.state.fl.us>; Bergin, Michael <Michael.Bergin@dot.state.fl.us>; Espino, Barbara <Barbara.Espino@dot.state.fl.us>; Vieira, Alexis <Alexis.Vieira@dot.state.fl.us>; Sajadi, Jamshid <Jamshid.Sajadi@dot.state.fl.us>; Andres, Tom <Thomas.Andres@dot.state.fl.us>; Alberto Delgado
Archived: 6/16/2019 2:47 AM
Folder: Outlook Exchange

Alfredo,
To clarify this again as discussed in our previous meeting, the job is "On
System" classified as "Class A" with Type I Construction Administration
that requires our FDOT Specifications. Please comply with our process in
the approval of concrete mix design as indicated by our District
Structural Materials Engineer.

We will issue the Final Project Materials Certification Letter for this
project.


Thanks,
Ronald D. Arcalas, P.E.
Dist. Materials Operations Engineer
Dist. Materials & Research Office 4/6
Tel: (954) 677-7035
Fax: (954) 677-7090
Cell: (954) 290-4607
Email: ronald.arcalas@dot.state.fl.us

-----Original Message-----
From: Sajadi, Jamshid
Sent: Tuesday, October 25, 2016 10:12 AM
To: Arcalas, Ronald
Cc: Patel, Mayur; Bergin, Michael; Reyna, Alfredo; Espino, Barbara;
Vieira, Alexis
Subject: RE: FIU Concrete Mix Designs

Greetings all,

Ron,

As we understand, this is an on-system, critical project; consequently,
all required concrete mix designs and relevant testing should be in
accordance with the FDOT specifications requirements and approved by FDOT.
In other words, this project shall be treated as a regular FDOT project,
and is to be certified by FDOT at the end.

As I indicated in my previous e-mail, dated October 17, 2016, those
submitted concrete mix designs are not FDOT approved mixes. Consequently,
a FDOT-approved concrete plant shall be retained to produce those mixes
(with FDOT approved constituents). In addition, the proposed admixture
(TIO2) and the color pigment shall be approved by the State Materials
Office prior to the concrete trial mixes and production. Please let us
know if we can be of further assistance.

Best Regards,



-----Original Message-----
From: Arcalas, Ronald
Sent: Monday, October 24, 2016 1:12 PM
To: Sajadi, Jamshid; Vieira, Alexis; Dano, Robbin
Cc: Patel, Mayur
Subject: FW: FIU Concrete Mix Designs

FYI


Thanks,
Ronald D. Arcalas, P.E. Good morning Ron,


Dist. Materials Operations Engineer
Dist. Materials & Research Office 4/6
Tel: (954) 677-7035
Fax: (954) 677-7090
Cell: (954) 290-4607
Email: ronald.arcalas@dot.state.fl.us

-----Original Message-----
From: Reyna, Alfredo
Sent: Monday, October 24, 2016 1:07 PM
To: Alberto Delgado
Cc: Yassin, Aiah; Espino, Barbara; Nunez, Xiomara;
'JMorales@BPAMiami.com'; Arcalas, Ronald; Andres, Tom; Bergin, Michael;
Zhang, Hailing
Subject: FW: FIU Concrete Mix Designs

Alberto see the comments below regarding the Concrete Mix

Alfredo Reyna, P.E.
LAP Coordinator - Keith and Schnars, P.A.
Florida Department of Transportation
(305) 470-5288 Fax (305) 470-6737
Alfredo.Reyna@dot.state.fl.us

-----Original Message-----
From: Andres, Tom
Sent: Monday, October 24, 2016 11:50 AM
To: Bergin, Michael
Cc: Reyna, Alfredo; Zhang, Hailing
Subject: RE: FIU Concrete Mix Designs

Thanks Mike - this helps a lot.

Alfredo,

Could you forward Mike's comments back to the DBF and FIU? Also it would
be great if Mike could be kept in the loop regarding the Mass Concrete
results.

Thanks,

Thomas A. Andres P.E.
Assistant State Structures Design Engineer
605 Suwannee St., MS 33
Tallahassee, FL 32399-0450
(850) 414-4269

-----Original Message-----
From: Bergin, Michael
Sent: Friday, October 21, 2016 1:58 PM
To: Andres, Tom <Thomas.Andres@dot.state.fl.us>
Cc: Reyna, Alfredo <Alfredo.Reyna@dot.state.fl.us>; Zhang, Hailing
<Hailing.Zhang@dot.state.fl.us>
Subject: RE: FIU Concrete Mix Designs

Tom

I reviewed the mixes and they appear to be within our specification limits
(Section 346) for the specific applications. There was no material sources
indicated so the mix would not be approved by the district office if
that's a concern, this being a LAP project (and a research project??)
maybe the purchaser doesn't care.

As far as I know there is no approved sources of TiO2, but again this may
not be an issue since the mix will not be approved by the Department.

Is the intent to instrument the mass concrete placement(s)? If so I would
like to see the data after the placement. We've recently seen some
temperature increases in the cement/slag combinations for mass concrete
and I'd be interested to see if the temperatures come down as a result of
the addition of the fly ash.

I noted that color is mentioned in 3.4.6.5, but it doesn't specifically
address the color by standard number as we have done in our TSP
specifications that call out colored concrete. If there is an intent for
color, I suggest that the standard color number is referenced and that the
only supplementary material added to the mix is slag. Silica fume, or fly
ash will change the final color of the finished concrete and may not
provide a consistent color from one batch to the next.

I hope this helps, but let me know if there are any questions, ---- Mike

Michael Bergin, P.E.
State Concrete Materials Engineer
Florida Department of Transportation
State Materials Office
5007 NE 39th Avenue
Gainesville, Florida 32609
Michael.bergin@dot.state.fl.us
Office 352-955-6666

-----Original Message-----
From: Andres, Tom
Sent: Thursday, October 20, 2016 6:26 AM
To: Bergin, Michael <Michael.Bergin@dot.state.fl.us>
Cc: Reyna, Alfredo <Alfredo.Reyna@dot.state.fl.us>; Zhang, Hailing
<Hailing.Zhang@dot.state.fl.us>
Subject: FW: FIU Concrete Mix Designs

Mike,

Could you review and provide comments? This is for a LAP Project in D6.

Alfredo, I will provide comments to you via email.

Thanks,

Thomas A. Andres P.E.
Assistant State Structures Design Engineer
605 Suwannee St., MS 33
Tallahassee, FL 32399-0450
(850) 414-4269

-----Original Message-----
From: Reyna, Alfredo
Sent: Monday, October 17, 2016 8:39 AM
To: Espino, Barbara <Barbara.Espino@dot.state.fl.us>; Andres, Tom
<Thomas.Andres@dot.state.fl.us>; Arcalas, Ronald
<Ronald.Arcalas@dot.state.fl.us>
Subject: FW: FIU Concrete Mix Designs

All,
Question who need to approved this?
Your help is appreciated... Do I need to upload it to the ERC?

Alfredo Reyna, P.E.
LAP Coordinator - Keith and Schnars, P.A.
Florida Department of Transportation
(305) 470-5288 Fax (305) 470-6737
Alfredo.Reyna@dot.state.fl.us

-----Original Message-----
From: Rodrigo Isaza [mailto:risaza@mcm-us.com]
Sent: Friday, October 14, 2016 8:25 PM
To: Feliciano, Manuel
Cc: Reyna, Alfredo; Eugene Collings-Bonfill - P.E., PMP; Dempsey, Dwight;
Hango, Erika N.; Alberto Delgado; Alan Ruiz, P.E.
Subject: Re: FIU Concrete Mix Designs

Thank you Manuel

Alberto/Alfredo, please ensure it gets into the ERC right away and help us
expedite the review and approval process.

Thank you

RODRIGO ISAZA | Sr. Project Manager | PH: 305-541-0000 - Ext 371 | M:
305-970-6989 MCM | 6201 SW 70 St., 2nd Floor, Miami, FL 33143 |
www.mcm-us.com<https://urldefense.proofpoint.co...
-2Dus.com&d=AwIFAw&c=1QsCMERiq7JOmEnKpsSyjg&r=mEvReUV_8mhKOet9i_HfnQ&m=9H8
A15mnTMgwa5ViBtviWTnSQR4T4K1ygywDOLEyUAY&s=Kchn79SVUars4cQCrRx4OgWP50fq1Ld
p4G3maz8JW0o&e= >.
Please consider the environment before printing. A reminder from MCM....
Building Excellence.


On Oct 14, 2016, at 7:30 PM, Feliciano, Manuel
<mfeliciano@figgbridge.com<mailto:mfeliciano@figgbridge.com>> wrote:


Alfredo,

Please find the concrete mix design and the technical special provisions
for the photocatalytic concrete. It is my understanding that you will
upload this submittal to the ERC system for review.

Let us know if you need anything else.

Thank you.


Manuel Feliciano, P.E.
FIGG
Regional Bridge Engineer
mfeliciano@figgbridge.com<mailto:mfeliciano@figgbridge.com>
O: 850-224-7400 ext. 1235


<FIU8500_1_MetaK.pdf>
<FIU5500_1.pdf>
<FIUMass.pdf>
<Technical Special Provision Photocatalytic concrete.pdf>

RE: Miami Pedestrian Bridge, Part XI

Quote (Epoxybot)

I just don't agree that they couldn't have foreseen that re-tensioning #11 was a non-starter

It’s not that they couldn’t have foreseen it, it’s just that it’s a lot easier to foresee with the benefit of hindsight.

They were all very worried about the cracks, but none of them seemed too concerned with retensioning prior to installing the emergency steel braces. That’s why I’m wondering if we’re applying hindsight bias when we say it was obvious at the time they shouldn’t have retensioned.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87 (Mechanical))

I think if EOR ever is on the stand for this, he will say bridge was not deflecting and that is why safety was not a concern.

It is physics that every structure deflects in response to load. The bridge must deflect when it has to support its own weight of nearly 930 tons. In fact if you take a look at the 11/12 drawing the Member 12 is not vertical but kicks in by 2.125" at the bottom so that at the equilibrium condition with the permanent load Member 12 should be nearly vertical to match the side span which were to be constructed in situ.

The photos taken by BPA before the Mar 15 collapse have conclusively showed that there were crack separations between concrete and between concrete and plastic sleeves. Anyone cares to examine the OSHA photos, which repeated what NTSB had previously published, would know 11/12 had cracked separations on the west, front and east sides. The remaining side that did not crack was against air so 11/12 was able to move away from the deck by the amount indicated by the crack widths.

The geometry of the cracks confirms they were structural in nature and as predicted by FDOT 18 months previously. Concrete is a brittle material. The geometry, severity and extent of these cracks are enough as proof in court to justify the 11/12/deck has already failed structurally before its collapse. I can justify this statement because in the structural analysis this node was assumed rigid, as every reinforced concrete structure joint. If any member were permitted to move then such specified amount of degree of freedom must be included to design calculation.

Many of us are now convinced by the Minutes of Mar 15, 2018 meeting that the bridge designer knew the problem. He was more interested in protecting his reputation than the workmen instructed to re-tension the bridge and those motorists using Highway 41.

Many flaws will come out of the Mar 15,2018 presentation by the bridge designer. Among them is the bridge designer trying to justify the presence of cracks by using the Diaphragm II as an example. He proved the 2' wide by 4' deep Diaphragm II was structurally adequate when it was simply supported on 4 shims with half of the bridge weight acting downward centrally by Member 11/12. The diaphragm in such case would have compression at the top and tension at the bottom. No photo was ever taken on the bottom of Diaphragm II which was inaccessible. The resulting cracks at the bottom if existed were irrelevant to the 11/12 cracked separations with the top and rear faces of the Diaphragm II.

I have seen many such moves in meetings when an attendee wasted others' time by arguing something theoretically sound but irrelevant to the issue sought by everybody. Or the designer in this case was simply barking on the wrong tree.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87 (Mechanical)27 Jun 19 20:24 Has this been addressed?
v. There is insufficient details of the walkway deck web interface and the canopy web interface where there is significant interfacing shear between the elements.
Clarification: Here, as with any structural system which has connecting members, there is a transfer of force between the various members at each nodal point. FIGG would not classify these behaviors solely as “shear” per se. However LRFD’s strut and tie design philosophy (section 5.6.3) will assure the required transfer of loads at the nodal transition points. Relative to this comment, no changes will be made that would alter the 90% foundation design plans.)

If that clarification is the FIGG response I read it as boiling down to "F U".
And apparently section 5.6.3 did not assure anything except failure.

RE: Miami Pedestrian Bridge, Part XI

Is there anyway this thread could get broken apart maybe a sub thread that just focuses on the technical faults. There could be another thread that just focuses on the chain of command or legal side or whatever.
There's been so much good work done here, but can't help feeling a bit overwhelmed by the information.
It seems like there was an actual calculation error or bad assumption by the design team, but it's kinda hard to track it all through the enormous number of posts.

Anyway, just a thought. Very impressive stuff here though by all contributors.

EIT
www.HowToEngineer.com

RE: Miami Pedestrian Bridge, Part XI

After a quick scan of the FIU information outlined by MikeW7 I found some flaws in the Mar 15, 2018 Presentation

(1) Confining force by averaging the lateral tendons not justified.



The 11/12 area did not have any transverse tendons at all and the next nearest one was 4.04’ away from the Diaphragm II or 4.915’ from the Member 12. In the presentation 11/12 was assumed 4.75’ long to have transverse confinement based on a total force from 65 tendons divided by the 175’ length.

(2) Altered failure mode by the shifted construction joint (CJ) not appreciated and reductions in strength caused by the presence of the five cast-in sleeves and pipe were omitted in the presentation.



In the presentation it was assumed the rebar crossing in area the Member 12 bonding with the deck (including Diaphragm II) would participate in resisting shear. This area shown in the presentation is a triangular area (from the deck) plus a full cross section of 2’ wide by 4’ deep from Diaphragm II.



In the field the construction joint had shifted. Therefore 11/12 shear capacity based on the above assumption is no longer justified and the reason is explained in a sketch I prepared below (unit in inches).



It is obvious once the CJ had shifted the bonding area A on the two sides of 11/12 would cease to resist shear against 11/12 but stayed as integral parts of the deck. The resistance then would be transferred to the vertical face between the deck (Area A) and Member 12 (Area B).

The presentation had assumed the full cross section of Diaphragm II, on both sides of Member 12, would be available to resist shear. However by placing two vertical 4” ID sleeves on each side and one horizontal 8” ID pipe this bonding surface B was compromised leaving a mere 17.22% area effective.

The designer in the presentation did ignore the concrete contribution to resist shear and relied heavily on the rebar crossing the bonding areas he had assumed. In the field the 11/12 slid along the CJ, pulled out the vertical face between Area A and B and managed to shear out of position along the 8” horizontal pipe. This is because along the 8” pipe the concrete area is at its minmum while at the same time the rebar are embedded with the least amount of concrete due to the sheer volume taken up by the 8” pipe. The lack of concrete surrounding the rebar, at the failure plane, has eventually led to many steel bars seemingly clean and undamaged without any concrete fragment attached after the collapse.

In summary the failure mode brought about by the shifted CJ was not appreciated. The significant capacity reductions in the 11/12/deck joint, caused by embedding four vertical 4” sleeves and one horizontal 8” pipe at the critical positions, were not considered. These shortcomings led to the bridge failed materially different to the Mar 15,2018 presentation. Therefore the assurance of the bridge was safe on Mar 15, 2018 was based on some serious flaws.

It would be fair to say the designer did not have the time to go over the cracks photos as we did and so his presentation could not have the refinements I have introduced. However the presentation was crucial to persuade every attendee to the Mar 15, 2018 meeting that the bridge was safe. Therefore any flaw that concealed the true condition and led to an inaccurate assessment of the bridge safety should have been responsibly avoided.

RE: Miami Pedestrian Bridge, Part XI

The shear plane in the presentation was not the critical shear plane. That was the main issue.

RE: Miami Pedestrian Bridge, Part XI

Quote (earth)

The shear plane in the presentation was not the critical shear plane. That was the main issue.

Yes. The calculation presentation focussed on non-critical aspects of the design e.g. deck flexural strength and non critical shear planes. The calculations ignored the criticial failure surface. FIGG was presumably aware of the crictical failure surface as they were devising a method to "capture the node". So it would appear the calculation presentation was somewhat of a ruse designed to placate the others, to give FIGG a bit of breathing room to devise the fix.

RE: Miami Pedestrian Bridge, Part XI

It would be interesting to see FIGG giving testimony about capturing 10/9 yet claiming they did not know where the crack came from.

RE: Miami Pedestrian Bridge, Part XI

@saikee119 - well done, now we are making progress! So is block B presenting itself as a wedge to split 11?

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural))

So it would appear the calculation presentation was somewhat of a ruse designed to placate the others
Lets not forget the Power Point Presentation had to have been prepared before March 15, and that was the only thing the EOR had available at the meeting. So I have to think he was presenting something he did not totally support. That does not excuse him for all the "no safety concerns" after he had seen the cracking just before the meeting. Those statements are not unlike those a coach at a Junior College might say to his team before a football game with Clemson or Alabama.
All in all, he is left in a world of hurt. It has been a particularly tough time for him - a bridge in the street and his phone went thru the washer. What's next?

RE: Miami Pedestrian Bridge, Part XI

Or better, the deck starts to sag, taking the lower PT bar with it while the upper PT bar is still embedded in 12. This splits 11 while also creating the twisting in the slab around 12.

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance)

Lets not forget the Power Point Presentation had to have been prepared before March 15

Sure, but the presentation included photos of the big cracks. So they can't claim they had no idea it was cracking like that when they put together that presentation.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119)

The four photos you have presented of the member 11 and 12 to deck area - show serious horizontal splitting of the east face of that zone, particularly the fillet between 11 and 12 - am I right?
It looks like each of the photos laps the adjacent pic by about 50% but the takeaway is the same. Serious cracking about 8" above the deck surface. That is about where the top of the 7S01 shear friction bars was. Such a lack of confinement reinforcing.
In your sketch of the blow out area - it seems the departing block was defined by the PT confinement of the anchorages of D-1 at the upper region and expanded diagonally below that zone. Much of what I see in the end of that deck section/diaphragm below that PT anchorage (from OSHA pics) looks like diagonal tension.
The pipe sleeves probably defined two zones - one south of and one north of the sleeves, with a weakened zone at the sleeves. That likely allowed each side - A and B - to fail separately, IMO. If that is a valid idea then there were 3 zones of failure, each with different modes and stiffnesses. +




RE: Miami Pedestrian Bridge, Part XI

saikee119 (Structural)28 Jun 19 02:41

Good post and drawing.

On page 111 of OSHA report, this quote appears:

Quote (OSHA Report)

There was a 4” and 4½” diameter plastic pipe placed right against the columns in diaphragm II.

Elsewhere in report 4" only is shown.

RE: Miami Pedestrian Bridge, Part XI

Saikee119 has shown the failure zone definitively. Consider that block A remains intact after the collapse and block B is scooped out.

I propose that the deck and diaphragm form a yoke around 11/12 (under 11 and around 12) and are attached at the base of the diaphragm, otherwise detached and free to pivot about the attachment (rebar can only offer limited resistance and acts as a damper to movement). The detachment of Saikee's Block B from the diaphragm allows the structure to sag. It is the sagging structure that twists the diaphragm/slab off of 12 and also opens the crack beneath the heel of 11. Note the point of tight contact between 11 and the deck highlighted below with the green oval.

The sagging structure is actually pulling the lower 11 PT bar down and tears 11 longitudinally. This tearing is what was initially visible as the fine longitudinal cracks from the time when the form work was removed. The upper 11 PT bar is not free to move down. The tension cracks at the upper fillet between 11 and 12 is further proof of 11 being pulled apart by the two PT bars anchored in the conflicted members (yellow oval).

This complex cause and effect would not likely have been apparent to the workers, nor was it apparent to the Engineers though all the signs were visible from the outset.





RE: Miami Pedestrian Bridge, Part XI

I'm not making anything of it, but it's interesting how the span is providing the rigidity when SPMTs retract their wheels. Observe the subtle cross-beam reactions mid-span and with the movers.

https://www.dropbox.com/sh/u8qicuu22xmfhg1/AABr9FV...

RE: Miami Pedestrian Bridge, Part XI


Sym P. le (Mechanical),

Block B could come out as a wedge had the CJ cracked through. I am not suggesting this had happened and the cracks photos do not support it either. I am preparing another sketch to show the possible failure plane. An accurate knowledge of the actual failure plane is essential to understand why putting the tension back in Member 11 killed the bridge.

Vance Wiley (Structural),

The four photos of the CJ cracks are intended to demonstrate movement covers a distance up to but not including the vertical sleeves. After the sleeves the separation changed direction and I am about to show it with a sketch. I only lined up the four photos but couldn't overlapped them as their magnifications may be different to each other.

The triangular fillet can be seen without the bottom crack so it was still part of the deck. How much 11/12 moved away from the deck can be estimated by the horizontal width of the fillet's diagonal crack. I would say the movement could be easily 5 to 10mm. This magnitude corroborates with the separation between concrete and sleeve at the west side. The fillet crack shows conclusively 11/12 has separated from the deck at the front or the south side.

I believe there should be only two zones of failure. Every part on the deck should be substantially rigid but minor cracks and separation of inconsequential nature could be present. The failure zones with 11/12 can be seen to have lot more serious within-zone cracks and separations.

RE: Miami Pedestrian Bridge, Part XI

In an earlier post (12 Jun 19 01:31) I mentioned that FIGG's main business was designing immense structures, and alluded to the fact that American automakers took years to figure out how to properly design smaller automobiles. One of Detroit's first downsizing "tricks" was to start with a familiar design and remove short sections of length and height from a large clay model and blend the edges together (this was an era when CAD meant Clay Assisted Design). Is it possible FIGG used a similar approach (to save money) and "downsized" what they were familar with by simply grouping CAD building blocks (drains, etc.) closer together, and this close packing caused a superposition of stresses and weaknesses that were much higher (exponential instead of linear) than what they expected? Is it possible their FEA software failed for the same reason - it wasn't designed to model closely-packed voids?

Back in college I wrote a finite-element simulation for magnetic fields in solids, and I remember that managing the boundary conditions at a solid-air interface was a nightmare. I assume the same holds true for solid-air interfaces in structures.

I'm not trying to absolve FIGG from blame, I'm just trying to understand why they were so adamant about their simulation results.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119)

... to understand why putting the tension back in Member 11 killed the bridge.

Re-engaging the PT bars in 11 was a reckless move based on elementary principles. The structure needed to be shored up while decisions were made to rehabilitate the structure or remove it. Understanding this failure sequence/mechanism adds to the library of information for future reference and would have been useful for rehab purposes.

RE: Miami Pedestrian Bridge, Part XI

A prediction of the failure plane based on OSHA report photos

Using the photos from OSHA report I venture to suggest a possible failure below.


Section of 11/12/deck joint with PT rods, transverse tendons and cast-in items added (edited)

The firs part of the above failure plane is the shifted CJ. OSHA Fig 18 shows cracks appeared as soon as the bridge was first required to support its own weight by removing the shoring while the bridge was still at the roadside casting yard on Feb 28, 2018. Subsequent OSHA Fig 32 to 38 show the CJ had shifted about 5 to 10mm after its tension was removed and the bridge was resting at its final position on Mar 10, 2018. OSHA Fig 63 to 70 indicate the lower PT rod firmly anchorded to the deck.

I have assumed 45 degree stress distribution from the anchors and the lines of influence are depicted by brown dotted lines.

After passing through the lower PT rod anchor the last part of the failure plane is less certain because the initial failure plane may have been complicated by the consequential damages caused by the final separation of 11/12 from the deck.

An importance feature of the above sketch is that the lower rod was clearly holding up the deck while the upper rod was more effective on 11/12. The alternate stressing and de-stressing of the two rods would encourage movements and exacerbate the distressed joint. This is because the lower rod was compressing concrete stiffer than the upper rod. By releasing or applying same tension, typically 50 psi per rod at a time, the strain would be different in the two rods. Different strains cause concrete to deflect by different amount and the difference widens and elongates the cracks. Personally I consider the two rods in Member 11 badly arranged at a position seriously compromised by the embedded items. The upper rod especially became more flexible because as it had more self-arranged weaknesses than the lower rod. Subsequently a simple tensioning or de-tensioning of the two rods could generate uneven strains leading to detrimental damages to the bridge. The uneven strains from the same tension force of the two rods could be the very culprit responsible for the bridge collapse.


Rear view of Member 12 with Diaphragm

It is certain that bonding surface of the diaphragm to Member 12 would be compromised by the 4” vertical sleeves leaving just 41% concrete effective in the vertical direction.

Additionally due to the presence of the 8” pipe cast horizontally the concrete at the centre line of this pipe was necked to have just 30% concrete on either side.

Bounded by the above weaknesses of a shifted CJ at the top, vertical sleeves on both sides and a horizontal pipe at the bottom the enclosed concrete suffered a blow out during the collapse.

Before the collapse 11/12 was retained in position by the rebar many of which could act as dowels. The ferocity of the cracks would suggest some yielding in the steel and internal local concrete crushing, especially surrounding the rebar.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119 (Structural)28 Jun 19 10:33)

I believe there should be only two zones of failure.
Your "yoke" reference is quite good to describe the more direct shear zones which you have designated "A"
I suggested 3 zones can be defined, as follows:
1) cold joint at top of deck - extends to north of the lower PT, Mu = 0.6
2) zone "A" - integrally cast zone of primarily "shear friction" defined on each side by the PT anchorages of D-1
3) blow out zone "B" of primarily diagonal tension.
A different method of calculating resistance/capacity is appropriate for each zone. Whether the resistances are additive or not is another issue. If they fail simultaneously then it may be appropriate to combine them. But it appears that the "cold joint" failed first and somewhat independently of zones A and B. But we can't see what was happening at zones A and B until the cracking at the deck surface beside 11/12 became visible and the cracking in the north face of diaphragm 2 also became visible.
As to A vs B, I totally agree that the pipe sleeves created two zones, with a soft zone between from the sleeves.
Whether A and B did or did not fail simultaneously may be of little import, because it all ended up in rubble over a time span of about two blinks, maximum.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7 (Electrical)28 Jun 19 15:09)

I'm just trying to understand why they were so adamant about their simulation results.
We think airplanes can fly themselves so we crash. We think they can land themselves so they crash. We think trains can run themselves so we text our girlfriend and we crash. We forget phone numbers because we use speed dial - "you are #3" - and if I wash my phone I don't know how to call you.
We think a computer can tell us everything so we do not need to pay attention. We are overwhelmed by the volume of printout produced so none of the numbers are significant. We do not touch the problem with any depth of understanding or delve into it because we trust the computer to do it for us.
Why the hell are we even necessary? We are abdicating our position of authority at the top of the intelligence pyramid and apparently willing to fade into oblivion like the neanderthals did.
I understand the feeling - if the computer has already solved it, my hand check is like watching a football game after it has been played - the score is in, so its over.
We can get engineering answers without even understanding elementary engineering principles.
I spent a long time translating Ed Wilson's structural program from Fortran to interpretative Basic because we were a small firm and cold not afford an IBM 1130 (quarter million in 1980 dollars). This was 1980 period - just before the PC was introduced so I was using a CP/M computer with 64K memory and an 8" 600K floppy.
So I thought about selling copies and advertised in ENR. Then I began to think I could be an accessory to a collapse. This program would allow untrained people to find and "present" a complicated analysis of an indeterminate structure when they had no idea what was going on. No copies were sold.

RE: Miami Pedestrian Bridge, Part XI

Vance Wiley - My point was that FIGG were experts at building large structures, and were very confident in their simulation expertise, but were obviously surprised at the discrepancy between what they predicted and what actually happened, as in "this has never happened before".

My question is "Was this because their hand calculations and simulation methods/models were inappropriate for this small-scale task with tricky geometry, but they didn't know it?" Were there non-linear complications present that they literally had no way of predicting, either by simulation or hand-checking. Were there hidden, "day one" problems in the software that hadn't been exposed before, but appeared because of the unique nature of 11-12 junction. These are important questions that need answers to prevent the next "never happened before" disaster. Hopefully the NTSB report will address this.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7 (Electrical)28 Jun 19 17:54)

These are important questions that need answers
So correct. Questions that need to be answered throughout the industry and anywhere technology is used and lives are in the balance.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87 (Mechanical) 28 Jun 19 18:53)

For the record, FIGG successfully produced at least one small bridge.
Small, but simple. It's not the smallness of the bridge that's the problem, it's the complex close-packing of elements in the small volume of the 11-12 junction/diaphragm area. Did FIGG "shrink" the element spacing of a structurally sound design and not expect any problems? Was the FEA software smart enough to compensate for non-linear (or exponentially greater) effects that resulted when elements (especially the drain and conduits) were packed closer than normal?

EDIT ADD: Apologies for posting three nearly identical rants. Working in the garage on a hot muggy day....

RE: Miami Pedestrian Bridge, Part XI

MikeW7 (Electrical)28 Jun 19 19:32

Agreed. This project was such an unusual aberration, it's important for the engineering community to uncover every relevant detail. I can tell you this about this bridge> I felt offended the first time I saw it. It was so patently absurd to me right off the bat. But it was indeed built, so that means I'm the absurd one? Do you get the point I'm trying to make? Perhaps our interest in this thread is an effort to help others not fall into the same trap FIGG fell in? But then I saw this video with the school buses and I became became indignant. Thirty feet wide cracking bridge, 30 foot school bus under it. Do the math.

This cannot go unanswered.

RE: Miami Pedestrian Bridge, Part XI

Quote (Mike W7)

Was the FEA software smart enough to compensate for non-linear (or exponentially greater) effects that resulted when elements (especially the drain and conduits) were packed closer than normal?

You do calculations like shear friction by hand. These types of calculations are not black and white and are difficult to program into FEA. You would have to program all the rebar through the critical shear sections. The program would have to figure out where the critical sections and pour joints are. I haven't seen any software that can do that yet. You would need a pretty good GUI to efficiently input all the rebar.

Part of the issue is that engineers get answers from software but it doesn't give you all the critical answers you need. The computer turns off the mind.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs 87)

Agreed. This project was such an unusual aberration, it's important for the engineering community to uncover every relevant detail. I can tell you this about this bridge> I felt offended the first time I saw it. It was so patently absurd to me right off the bat. But it was indeed built, so that means I'm the absurd one? Do you get the point I'm trying to make? Perhaps our interest in this thread is an effort to help others not fall into the same trap FIGG fell in? But then I saw this video with the school buses and I became became indignant. Thirty feet wide cracking bridge, 30 foot school bus under it. Do the math.

I have designed a lot of difficult structures. I don't see this bridge as particularly difficult. I think the hardest part to design would be the live load all on one side of the deck. The torsion is difficult to get out of this bridge. The mistake that was done was fundamental and could have been made on a more typical structure. A lot of people criticized the shape, the fact that it was concrete, the determination of the pylon height etc. I think it is good to push the limit of design and aesthetics but when you do that, you can't cut corners and you need thorough reviews. If you can't build bridges and buildings based on aesthetics, our urban landscapes would be depressing to live in. Good aesthetics are important, complying to structural codes is mandatory.

RE: Miami Pedestrian Bridge, Part XI

You can’t design a junction like that with FEA, hence the failure surface not showing up in their rainbow plots.

RE: Miami Pedestrian Bridge, Part XI

Quote (Earth314159)

I haven't seen any software that can do that yet.
Modern integrated circuit design software has progressed over the past 40 years to the point where desktop CPUs now contain over a billion transistors, so I'm absolutely amazed that structural design software is that primative. Any idea why?

RE: Miami Pedestrian Bridge, Part XI

The structural design software isn't primitive, but concrete isn't a homogeneous or linearly elastic material. It's non-linear and reacts differently to tension and compression, making its 'shear' behavior very complex and subject to wide variation. There are numerous different ways that are used to model reinforced concrete, and none of them are really very accurate. You really can't break concrete down into 'finite elements'; it can really only be analyzed on a macro scale.

There are just some things that even the most sophisticated computers and software still can't handle very well, like weather prediction and predicting the response of concrete to loading.

RE: Miami Pedestrian Bridge, Part XI

Quote (Mike W7)

Modern integrated circuit design software has progressed over the past 40 years to the point where desktop CPUs now contain over a billion transistors, so I'm absolutely amazed that structural design software is that primative. Any idea why?

A lot of people are surprised by that. Structural engineering is not as black and white as people think. The behaviour of concrete (and other materials) is complex. There have been attempts to write non-linear analysis with an-isometric materials but even with these programs the input is too complex and it takes far too long to run to be of any practical use. To properly capture the true behaviour of concrete, you need a lot of elements and they are all non-linear. I can't even imagine how that information can be inputed into a computer.

A typical design approach is to start with a concept and do some rough hand calculations. You do some 2D FEA with what you believe are the critical combinations. You then do a 3D analysis with a whole bunch of load cases and combinations. If you get the results you expect, you are likely on the right track. The analysis gives you a reasonable load path but it is not necessarily 100% correct. Minimum rebar requirements assures the load path can adjust from the theoretical load path.

It takes intelligence to figure out what is critical and how to bound you solutions. Once you do a linear analysis, you have to know what will be critical and where your assumptions will likely be conservative or un-conservative. That is why review is important in structural engineering. A structural engineer also needs methods to double check themselves. You have one kick at the can. There are no prototypes with what we do. It takes a lot more in the analysis than just working with a computer.

RE: Miami Pedestrian Bridge, Part XI

FIGG used FEA to analyze local stress/strain change vs PT bar tension. They used a difference filter to look for rainbow at the deck and did not find one. BTW, FIGG in presentation stated calculation reviews were done by independent party. Who?

The FEA model in no way has enough data to predict chaos in a cracked structure.

Not to put down engineers, but medical doctors know better how to diagnose because they deal in almost 100 percent failure conditions. We can learn from them. You have to know when to discard a test result. FEA test fail>close road/not embarrassed about it, FEA pass>close road/feel sheepish/keep checking NOT FEA pass>road open/feel confident/checking can wait 2 days

RE: Miami Pedestrian Bridge, Part XI

Thanks for the feedback. So the root problem is really unknown variables in the manufacturing process - things like aggregate distribution and rebar adherance that can't be modeled even as statistical anomolies. Very enlightening.

My rants earlier today were based on an assumption that structural design was modular, like semiconductor design: each structure "module" had well-defined internal properties, and connecting modules together was a process of insuring the boundary conditions matched. Apologies to everyone for my profound ignorance....

RE: Miami Pedestrian Bridge, Part XI

I have long been concerned about the stability of this structure under unbalanced linve loads and as a structure with flange stability requirements.
So lets try this - URGENT !! PEER REVIEW REQUESTED. Please.
I did some calcs - unfortunately I have no skills at graphics and minimal if not negative skills at typing. So the calcs are in felt tip pen. 3 sheets, with cap letters in a circle to aid in referring to something.
And I fear I have made an error so this calc is NOT FOR CONSTRUCTION. But perhaps it will stir some curiosity in someone.
I hope the calcs are kinda self explanatory.
Now I must grit my teeth and await the criticism.







OK - hit me with what you got. I'll get over it - maybe.


RE: Miami Pedestrian Bridge, Part XI

I am thinking I used wrong value for "E". What is proper "E" in units of feet?
KSI/inch = KSF/foot or Kx12x12/1 footx12 = 72000 ksf? So I am off by factor of 12 ? That feels better.
It has been about 15 years since doing this stuff - -
EDIT
Did a bit of unit conversion - found a reference that addressed E in different units.
Found E = 864000 Ksf which means I only overestimated the deflection of the canopy as a diaphragm by a factor of 144.
Dividing the 9.91 feet by 144 gets a deflection of 0.069 feet or 0.82 inches.
That is stiff enough to do the job.
Clearly the 9.9 feet would be a rotation in the 30 degree range and would be self un-loading.
The canopy as a beam is 174/16=span depth of 11. That looks stiff enough.
Sorry for the big number.
https://civilengineeringbible.com/article.php?i=14

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance Wiley)

I have long been concerned about the stability of this structure under unbalanced linve loads and as a structure with flange stability requirements.

I'll try to go over it but I have to work tomorrow. Check the out-of-plan bending in the diagonals as well which is required to distribute the warping torsion. It looks like in C that you assumed the canopy has lateral resistance at both ends.

This is a similar problem to cable stay bridges with only a central line of cable stays. In these cases, you only have St. Venant torsion in the deck (and may be some local warping torsion near the ends of the deck). I can't see second order effects on the cable stays helping too much. This is why I think getting the torsion out is one of the hardest parts of the design. You also have to think that there are significant aero-elastic concerns in the torsional mode.

RE: Miami Pedestrian Bridge, Part XI

Quote (Earth314159 It looks like in C that you assumed the canopy has lateral resistance at both ends.)

Thank you - and yes, the only thing I see that provides a lateral support to the canopy is member 1 and 12. 12 is / will be/ would have been OK when the pylon is cast around it.
EDIT In thinking about it, member 2 and 11 will help reisst the out of plane reactions from the canopy, because they are also "fixed" at the deck and backed up with the diaphragms. End Edit.
And as I added in a subsequent post, I think my deflections are off by a factor of 12 - plus whatever I missed in choosing E=6,000,000 (inch units).
Lateral bending in webs is maybe a combo of bending and torsion because of the sloping diagonals ? Ignoring torsion and Using two 24x21 sections only and about 33 feet of trib to a node I see:
90#LLx15'x8'x33'=356 '-kips. M/S = 356'k/(2x24x21x21/6)= 1210 psi. This is to be combined with prestress remaining and/or tension in one diag and compression in the other diagonal.


RE: Miami Pedestrian Bridge, Part XI

Earth314159 (Structural)29 Jun 19 03:14

Quote (Earth314159)

...whole bunch of load cases and combinations...
Earth Irrational π, your entire post is spot-on. In fact, all the posts this week have been well worth the time to read. Thanks.


Introduction to Nonlinear Finite Element Analysis: http://www2.mae.ufl.edu/nkim/INFEM/

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7)

Modern integrated circuit design software has progressed over the past 40 years to the point where desktop CPUs now contain over a billion transistors, so I'm absolutely amazed that structural design software is that primative. Any idea why?
That is an excellent question. And I have no ready answers apart from the results aren't readily applicable to legacy codes.

Quote (HotRod10)

The structural design software isn't primitive
Most of the stuff I see is. For SpaceGass which is big in a few countries is based of windows 3.1 and is still used as a primary structural tool,

Quote (HotRod10)

concrete isn't a homogeneous or linearly elastic material. It's non-linear and reacts differently to tension and compression, making its 'shear' behavior very complex and subject to wide variation.
All of which can be accounted for.

Quote (HotRod10)

There are numerous different ways that are used to model reinforced concrete, and none of them are really very accurate. You really can't break concrete down into 'finite elements'; it can really only be analyzed on a macro scale.
Finite elements make up the macro scale. Sure there is plenty of non homogeneity but that can just as easily be considered on the micro scale as it can on the macro scale. The macro is just the summation of finite element interactions.

Quote (HotRod10)

like weather prediction and predicting the response of concrete to loading.
Not even close. Weather is fundamentally a chaotic macro system and one that doesn't readily partition. At the micro level you have your element size is tiny, the current properties are unknown and the scale is insanely massive.\\\

FEA with sensible element sizes is possible for most structural analysis particularly because you can generally partition off sections. Material properties are very well know within though there is also know variance.

RE: Miami Pedestrian Bridge, Part XI

"Weather is fundamentally a chaotic macro system and one that doesn't readily partition."

So is concrete. At least fluid compression and expansion is linear; not so with concrete. As I said non-linear and non-homogeneous makes for large variability, especially on the micro scale (and I'm talking inches, not microns).

RE: Miami Pedestrian Bridge, Part XI

Concrete is very hard to model. We can’t even model a concrete slab from first principles. We still rely on empirical methods to assess basic things like shear capacity, bar anchorage, etc.

Add reo, PT rods, complex geometry to the mix and it’s near impossible.

RE: Miami Pedestrian Bridge, Part XI

Got a feeling some of us are getting confused when it comes to modelling RC structures.

To start with in global analysis of the full structure, using 3D finite element or 2D frame analysis, it is sufficiently accurate using the linear model even a concrete section cracks under load and so its second moment of area changes. This is because the equilibrium computation is based on the relative stiffness of the members joining at a node. The old engineers would know that we could calculate all the moments and shears of a continuous beam by using the ratios of the I-vales. The actual I-values are needed only when we have to estimate actual quantities like deflections.

All international RC design codes allow us to use gross sections for each member in the structure analysis because the relative stiffness ratios of the cracked concrete sections, with concrete resisting only in compression but no tension on the other part of the neutral axis, will not be materially different from those based on the full sections when the structure has no load. It is an approximation but has been accepted as good engineering practice and able to produce acceptable and safe design.

An other example is we all know a structure must deflect under load so the equilibrium condition can only exist in its deflected geometry. However everybody uses un-deflected geometry when the structure has no load for the analysis. This is again an approximation and accepted as good engineering practice for the majority of the application. People would bother non-linear analysis with flexible structure known to produce large deflection like a a suspension bridge. Last time when I looked at one the difference between geometrical nonlinear and linear solutions was only 4%.

In element or node analysis we put in rebar to carry their structural duties. This is done according to the code so if we need the rebar to give its maximum steel stress the bar has to be embedded to adequate length to give the full stress development.

As long as the rebar is sufficient and arranged in manner according to good engineering practice the design assumption that at each joint the moment, shear, torsion and axial force can be distributed according to their stiffness will be realized.

What we have here is a joint failure because it was rigid enough. Therefore the deficiency should be either the rebar was inadequate or some bar ineffective or some bars placed in a manner at variant with the good engineering practice or concealed construction defect or a combination of several flaws.

Finding fault with the structure modelling or the analysis techniques is barking on the wrong tree.

RE: Miami Pedestrian Bridge, Part XI

TITLE: 11/12 vs. Deck/Diaphragm

My revised drawing and images from OSHA for comparison of left/right side diaphragm and 12 cracking as well as indicating damage to underside of diaphragm.

saikee119's B shear plane is the only significant shear zone as the deck surface is not in contact with 11 and there is no indication that the cold joint shears through 12.





RE: Miami Pedestrian Bridge, Part XI

It has bothered me, as others have stated, on how the decision to restress the PT bar in Member 11 was made. I would like to think it went beyond, seems like it was working better with the force in the rods, lets put it back on.

A thought I heard many years ago on the safety of a structure that has undergone distress is that whatever happened, the structure that remains not only can resolve the static forces, it was able to stop the movement after the event occurred, and should be relatively safe until another event makes it mad again. Further it is generally better to do nothing than do something wrong and creates a situation you do not fully understand.

A scenario, you get a call late one night. My roof trusses are failing due to very heavy snow fall, don't worry we've already installed shores, please come out tomorrow and take a look. You arrive the next morning to find they installed shores right in the middle of panel points of the bottom chords. Now the stiffest axial support path creates bending in members that were primarily designed for tension. Now you not only have to design a fix, you have to design something to fix new situation before the snow begins to fall again and only hope that they didn't send someone in while the snow was still falling to install the shores.

Most of the time you have a good idea what happened, heavy snow the structure has never seen before, someone drives a fork truck over metal roof deck, someone removes a load bearing wall that they thought was just a partition. Side note, it always amazes me on the home fix-it shows that they suddenly find a load bearing wall. Because all homes built in the 1920s had floor joists that span 32 feet.

If the event occurs quickly, then most prudent people would "red tape" the area to restrict access until the situation is fully understood. Events occurring over time are much more complex. It can be as simple as, yes the cracks in your sidewalk are going to get bigger over time, to determining what is happening in a very complex structure. It would still seem prudent to me if you do not understand the situation, "red tape" the area and do not do something that could make the structure madder than it was before.

Also remember that construction workers want to go home at the end of the day too. Never assign a task that you wouldn't stand beside them while they do it.

RE: Miami Pedestrian Bridge, Part XI

Quote (HotRod10)

"Weather is fundamentally a chaotic macro system and one that doesn't readily partition."

So is concrete. At least fluid compression and expansion is linear; not so with concrete. As I said non-linear and non-homogeneous makes for large variability, especially on the micro scale (and I'm talking inches, not microns).

I don't want to sidetrack this too much with a tangent debate but lets have some real context here. Weather is a dynamic system, a concrete structure isn't (well at least it shouldn't be. Like saikee119 said, "finding fault with the structure modelling or the analysis techniques is barking on the wrong tree". FEA is used to design and test much more complex structures than this bridge including structures involving multiple materials and non homogeneous behaviour. Sure you can get it completely wrong if you don't model it correctly but that is the case with all analysis.

RE: Miami Pedestrian Bridge, Part XI

The construction joint, Egad, I still can't believe anyone would have a construction joint at such an angle to the primary axial force in Member 11. Still thinking peer review, ASI, RFI, shop drawing markup, field observation, or something would have changed this from flat across the deck. Although as others as posted, it would appear that FDOT did raise concerns and was politely told to sit down and shut up.

Why would you even consider creating such a situation. ACI 318 Commentary, "Construction or other joints should be located where they will cause the least weakness in the structure." Below the deck, now you have to actually resolve the forces in a thin section of a strut and tie model, above the deck, "ugly construction joint" in a structure that is supposed to meet the "Bridges As Art" criteria.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee)

Finding fault with the structure modelling or the analysis techniques is barking on the wrong tree.

They used 3D finite element analysis to reassure themselves and/or others that the node was safe. Such software cannot properly analyse a joint like this from first principles, so they should have been doubly cautious when the software said it’s ok.

RE: Miami Pedestrian Bridge, Part XI

I have prepared a break down of the failure surfaces and explain them with the following sketches.

The center line through the truss/bridge should look like the sketch below.

Fig-1 Deck center line not meeting center line of 11/12

An interesting feature is the deck does not join the centers of 11/12. It eventually broke off from Member 11 leaving 11/12 substantially as one entity despite serious spalling at the joint.

I then looked at how the deck was bonded to 11/12. This is shown in the sketch below.

Fig-2 slid CJ altered the internal resistance pattern of the deck/11/12 joint

Area C is deck only. One can say the deck could be bonded to 11/12 by one vertical interface between Area A and Area C plus two side interfaces of Area A+B with 11/12.

FIGG is his Mar 15 presentation had assumed the Area A to be triangular but I have assumed it to be rectangular here. The shape doesn't matter because we all know now the construction joint (CJ) slid and so any concrete bonded to the two sides of 11/12 would have been irrelevant and so is the shape of Area A. The resistance would then be transferred to the surface D, the interface between the north edge of the deck (in AREA A) with the south face of Member 12 (Area B). Area A therefore had no shear failure and remained part of the deck even after the collapse.

The concrete bond from the deck with the two sides of Member 12 is therefore left to Area B which is the same full cross section as Diaphragm II.

However as I have pointed out FIGG's designer had chosen to put 2 No. of 4" ID vertical sleeves and one 8" ID horizontal pipe at this critical location to weaken the joint dramatically. The consequence is shown in the next sketch.


Fig-3 side shear resistance of 11/12 significantly compromised by the embedded sleeves and pipe

The Area A was divided into three vertical strips of 3", 2.25" and 9.75" wide. The first two were too flimsy and would break quickly when they were pressed against the flexible sleeves.

The 8" horizontal pipe occupies 40% the concrete sectional area. Thus the section with remaining 60% area became the weakest plane against the horizontal force. I denote this failure surface as Area E. It has two strips each with 30% of the original area on either side of the pipe.

These two strips now form Area B1 which has 17.3% of the original section area of 24" wide by 40.5".

I summarize each available failure areas or surface, including the easily broke area B2 and B3, by the last sketch plus a spread sheet showing their dimensions and the likely resistance by concrete, rebar and clamping force.

I have labeled the failing surface starting from CJ as a, b, c, d and e. The two sides area now have B1, B2 and B2 for completeness.


Fig-4 Details of failure surfaces and areas

The above sketch is relevant if we want to assess why the two PT rods could have different strains when the same tension force was use in tension and de-tension.


Fig.5 A tabulation of the failure surfaces/areas

The use of this spreadsheet is for us to examine what rebar crossing each failed area or surface and sum the total shearing resistances to prove if the joint was strong enough against the collapse.

RE: Miami Pedestrian Bridge, Part XI

Quote (hpl575 (Structural))

it would appear that FDOT did raise concerns and was politely told to sit down and shut up.

I comment on the above as I don't want people misunderstand the good will gave by FDOT.

The bridge was never FDOT responsibility. FIU owns this bridge and so it paid a consultant like BPA to do check the construction. The peer review was done by Louis Berger who remains the sole authority to determine the structural integrity of this bridge. That peer reviewer wasn't properly engaged to do a full review to include the temporary conditions during construction is another story.

My interpretation of FDOT's responsibility is that it oversees the prject to ensure the right code, loads and materials were used. It liaises with the design consortium on all matter interfering with usage of the public highway. FDOT has to ensure the finished product would not have maintenance problem jeopardizing the function of Highway 41. FDOT did intervene to revise the lanes arrangement to suit the future expansion of the traffic.

FDOT has considerable experience in bridge design and so everyone was making use of its expertise. FDOT did put down a lot of very useful comments. They were valid, fair and not pessimistic or nitpicking. FDOT even marked down comments to remind FIGG's omissions.

What FDOT gave in design matter has no regulatory or contractual power and FIGG could dismiss them all. FIGG can design its bridge differently to address the concerns. Not many realize FDOT advice was free and represented no more than a good will to the FIU project.

To build this bridge FIU should have its own engineering representative in verifying the design and the same or different representative to oversee the construction. It appears FIU had been over reliant on FDOT to save the cost/need to hire a professional representative to check every stage of the bridge design.

In conclusion without contractual obligation or regulatory power FDOT would not fight back if its advice were disregarded. By giving out the advice FDOT can consider its duty discharged.

The lawyers may have a go at FDOT citing bad advice. However I am confident the expert witnesses will be on FDOT side. FDOT did issue a fact sheet to clarify its position but FIU thought it was a fake news! Such was the depth of misunderstanding.

Had FDOT owned the project the game would have been played differently.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119)

I have prepared a break down of the failure surfaces and explain them with the following sketches.

Saikee119. Appreciate the effort you have gone to in preparing these sketches.

However, there is a major flaw in your analysis. Your sketches show only a vertical view of the failure zone: there is no identification of the failure zone as seen from above. No proper analysis can be performed without this aspect being taken into account. This is because failure was caused by excessive stress. Without the shear zone from above being known, we cannot calculate stress which is a two dimensional unit (force per area).

I believe that if you take into account the failure zone as seen from above, your analysis of the failure zone you identify as seen from vertical aspect will substantially change.

RE: Miami Pedestrian Bridge, Part XI

Quote (Saikee)

The above sketch is relevant if we want to assess why the two PT rods could have different strains

Saikee, how come your failure surfaces are horizontal and vertical? Only limited portion of the actual surface was horizontal and vertical. Most were classic cone shaped cracks...

Maybe I’m missing something. Not sure. The previous green crack sketch seems a more accurate reflection of actual failure planes.

RE: Miami Pedestrian Bridge, Part XI


FortyYearsExperience (Structural),

My spreadsheet describes the area of each failure surface. This is the detachment of the deck from 11/12.

The information is based on the cracked on condition prior to the collapse and the failure mode afterward.


Tomfh (Structural)

I notice some cone failure at the rear of diaphragm but that is likely tensile failure due to the principle stress at play.

The failure B1 did not occur as a perfect vertical shear but spreading 45 degree out in the field. The purpose of the sketch is to show what amount of concrete left to resist shear. How the principle stresses changes the failure pattern is just the result of the shear failure. It is the same when crushing a concrete cylinder it can often fail by shear at 45 degree and not by compressing the concrete to disintegration.

The failure surfaces are important for us to count the rebar participating the resistance. FIGG in his presentation had disregarded the contribution from the concrete and concentrated on the contribution from the rebar. FIGG's assumed total failed surfaces is much larger than mine.

In the RC design against shear we used the concrete surface parallel to the shear and count the cross sectional areas of the concrete and rebar across the shearing face. The actual shearing pattern can be different and may not resemble the design.

RE: Miami Pedestrian Bridge, Part XI

Saikee, a curious phenomenon with expert witnesses is that they tend to be on the side of the person paying them. If someone wants money from FDOT, they'll find an expert who'll say there was a problem in what FDOT did.

I assume USA has a duty of care for engineers, and that FDOT has authority over road opening/closure. If you can show FDOT had structural concerns but allowed the road to be opened to traffic anyway, the water is muddied. Then the question of settlement cost vs litigation cost trumps 'right' and 'wrong'.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee)

I notice some cone failure at the rear of diaphragm but that is likely tensile failure due to the principle stress at play.

Oh, I see. You are treating it as a shear cross section akin to shear in a beam, and not really looking at the actual failure surface.


To my mind the cone failure is the critical feature. The cone snuck around all the critical PT reo, so I dont' believe shear theory can be applied. It's more akin to an unreinforced punching/cone failure.

RE: Miami Pedestrian Bridge, Part XI

We need saikee119's 2D drawings converted to 3D interactive drawing.

RE: Miami Pedestrian Bridge, Part XI

TITLE: Door Closers vs. Door Stops

Consider there is no effective resistance to shear at saikee119's plane B. The fracture merely jumps over to the longitudinal deck PT cable interface against the back of the 4" sleeves (OSHA pg 97, Figs, 64/65). The result is that there are no shear planes of any capacity in play, the model becomes one of a giant torque load between the base of 12 and the diaphragm/deck duo, and 11 becomes a giant point load on 12 just above the torque application.

This model anticiptes movement from the outset and builds load on 11 through to failure. Any rebar in the system merely acts as a slow release dampening device like a poorly adjusted door closer with big kick in the ass on the clasp setting. There is no door stop. The only good news is that in this instance you have at least five days to figure it out.

RE: Miami Pedestrian Bridge, Part XI


steveh49 (Structural),

I agree if FDOT had a safety concern or knew about the risk but did not act then it will be culpable.

FIU bridge isn't a FDOT project so its involvement is on the as-needed basis.

The cracks reported by FIGG to FDOT by the well known voicemail on Mar 13 however assure the recipient party no safety issue.

There is no evident that the bridge was in imminent collapse until FIGG instructed MCM to re-tension the bridge. On record the bridge collapsed just before the re-tensioning operation could complete the very last step.

I am not sure how much FDOT know about the problem until it sent Afredo Reyna to attend FIGG's presentation on Mar 15. The OSHA photos were essentially BPA's work. BPA was employed by FIU and may not even send out its photo collection to every party. Mar 15 would be the key date for others to learn about the crack problems.

FIGG's presentation was designed to assure everything was under control and no need to worry. Under such atmosphere I doubt if FIGG would allow BPA to distribute the crack photos to undermine its assertion. SO it would be useful if we know exactly what information the attendees got on Mar 15. OSHA report mentioned only three photos of Fig 24, 25 & 26 were used by FIGG's presentation.

The curious feature of the Mar 15 meeting both FIGG and FDOT were not interested in seeing the re-tensioning work, apparently ordered by FIGG on Mar 13 and due to commence after the site meeting had finished. OSHA described the FDOT engineer wasn't a structural guy who did seek advice from Tallahassee FDOT office on the re-tensioning operation as well as on FIGG's presentation but didn't get the right person, who did FIU bridge reviews, for help.

From the Mar 15 attendees BPA's JOse Morales and Carlo Chapman and MCM's Pedro Cortes were the only ones recorded by OSHA went to witness the re-tensioning operation.

The story could be entirely different had Tom Andres, who was the one predicting the shearing cracks, attended the Mar 15, 2019 meeting.





RE: Miami Pedestrian Bridge, Part XI

Does this concept make sense? > When concrete is under increased compression (PT bar), can we think of it has being harder? So with taut PT bars member 11 between dead and live plates was similar in hardness to deck and forces penetrated to dead end. When bars went slack, the forces from the softer 11 concentrated above the harder deck interface. With this in mind, what would re-tension do?

Also, unrelated, please explain tributary area (https://www.eng-tips.com/search.cfm?q=tributary&am...)

Tributary Area (http://www.engineersdaily.com/2014/05/how-loads-fl...)
The tributary area is related to the load path, and is used to determine the loads that beams, girders, columns, and walls carry. The reader is expected to be familiar with the concept of tributary area from other design courses, as it also applies to design of timber and steel structures; however, a brief overview is presented in this section. The tributary area for a beam or a girder supporting a portion of the floor is the area enclosing the member and bounded by the lines located approximately halfway between the lines of support (columns or walls), as shown in Figure 4. For example, a tributary area for the reinforced concrete beam AB that is a part of the one-way floor system is shown hatched in Figure 4a. A typical column has a tributary area bounded by the lines located halfway from the line of support in both directions (shown hatched in Figure 4b). In the case of uniformly loaded floors, tributary areas are approximately bounded by the lines of zero shear, that is, the lines corresponding to zero shear forces in the slabs, beams, or girders supported by the element for which the tributary area is determined. Zero-shear locations are generally determined by the analysis. For buildings with a fairly regular column spacing, the zero-shear locations may be approximated to be halfway between the lines of support.
Figure :4 Tributary area for reinforced concrete members: a) beams; b) columns.
Figure :4 Tributary area for reinforced concrete members: a) beams; b) columns.

RE: Miami Pedestrian Bridge, Part XI


Sym P. le (Mechanical),

I know you have been preoccupied with the joint rotation at the support.

Even the concrete has sheared off the joint could be kept going by the rebar crossing the shearing face. The rebar could be acting as dowels then.

The collapse was likely by the failure of the rebar. Some could have sheared off completely. Other may lost the concrete surrounding the steel bar if there was insufficient concrete around or the embedded length was too short.

RE: Miami Pedestrian Bridge, Part XI

jrs_87 (Mechanical),

The way I explain one PT rod may experience different hardness in the tensioning is the lower rod was secured to the deck already highly compressed. The upper rod on the other hand was physically outside the deck and diaphragm if you take a look at the first sketch of my 28 Jun 19 16:58 post.

Both rods compressed axially Member 11 but the lower rod was additional deck to compress. Therefore by applying the same tension to both rods the axial shortening in the lower rod has to be less than the upper rod. The difference in strains means the two rod anchors distance were progressively changed.

RE: Miami Pedestrian Bridge, Part XI

The neighboring compression in the concrete may be different near each rod, but the extension of each rod should be the same for the same applied tension. Since the amount of concrete the rods pass through is majority in the diagonal even the differences in the deck area are small contributors; possibly a few percent.

RE: Miami Pedestrian Bridge, Part XI

One thing I've been wondering: Why did both ends of the bridge not crack similarly? And can anything be learned from the fact that they didn't?

Up to a point, it seems they did. On Feb. 6, 2018, Bolton Perez & Associates performed a visual inspection of cracks on diagonals 3 and 10 after PT bars tendons No 2 and No 11 were stressed. They stated, "We believe, this first stressing operation has temporarily created tension on members No 3 & No 10; thus, creating cross sectional cracks transferring the tension loads to the steel on these members." (p. 1, FIU Pedestrian Bridge / BT-904 Crack Inspection Report). The OSHA report then indicates that on Feb. 26, after hearing a loud popping sound when the shoring under the span was removed and it was self-supporting, "The employee from the TSG and one of the employees from the MCM walked over the bridge and noticed cracks at the bases of diagonal 2 and 11." (p. 24).

Thus, up to a point, the span ends seemed to be responding similarly to stresses. However, after the move and then the de-tensioning of 2 and 11, that clearly wasn't the case, with only the north end developing progressively larger and, frankly, scary cracks. Is it clear why? Was it because diagonal 2 was much beefier than diagonal 11, with different geometry and maybe more rebar? Or because the deck ends were different, with the north end less reinforced? Or because the north end was moved much farther, over a bumpy median and was also cantilevered farther from the supports? Or because the south end of the span was designed as the actual end of a bridge, and the north "end" was intended to be a temporary intermediate construction phase? Or some combination? Or something else?

I apologize if the comparison of the two end responses has been addressed earlier, or if the question is just plain dumb. I'm not an engineer but I worked with engineers and obtained environmental clearance for many federally funded bridges, so they got used to me asking dumb questions!

RE: Miami Pedestrian Bridge, Part XI

Wetlander (Specifier/Regulator)30 Jun 19 02:55
Your post seems well reasoned to me.

RE: Miami Pedestrian Bridge, Part XI

@saikee119 - Thanks for your response. Modelling unconventional use of rebar might be unconventional or just darn near impossible. I've been thinking out loud and trying to incorporate what I see or don't see into a better understanding of the collapse. My model may not be perfect but it is a work in progress and I welcome anyone to critique what I put out. I'm learning a lot from these threads.

I accept your analysis of the shear planes wholeheartedly because it moves past a lot of what obviously wasn't happening, shear planes that weren't working as such. This allows for more freedom to consider why the bridge would creep for almost a week (if not from the very time the scaffolding was removed) without collapsing. As I looked closer, I noticed that even your concept of plane B, which is perfectly sensible, may have been an illusion. That is not to dismiss your work, but to further embrace it. If it turns out that the design of the north end of the bridge is just an illusion, so be it if that is where the evidence leads. Key design elements are absent and those that are present are pressed into unconventional service ... for a time. I'm hard pressed to accept simple answers, especially in complex circumstances. OSHA's explanation, though it maybe perfectly valid, does not seem fully developed and that opens the door for speculation or preferably a fuller analysis of the matter.

My earlier analysis with a torque element and the door closer was an idea for an explanation of the slow motion train wreck. If all of the conventional door stops are removed, it seems to me that what we are left with is one or two feet of the diaphragm and 12 fully and coherently bonded, with the rest of the structural elements free (cough) to move and straining under loads they were never meant to carry. I did not suggest an ultimate failure mode because once a run away condition is described, the final collapse is a moot point. I could be wrong.

One thing I forgot is that when the north end of the slab slipped off of the pylon, it seems to have sheared the two hold down rods. That would knock a bunch of concrete off of the bottom of the diaphragm.

Anyway, thanks again and I look forward to all of the posts.

RE: Miami Pedestrian Bridge, Part XI

Wetlander (Specifier/Regulator),

The point why the north end crack and not the south end has been raised before in this forum.

The answer was south end has a stronger joint. Although its vertical column is only 5% larger but the 3' diagonal is 50% larger casting with the same deck. There are more rebar and some of them could be placed in more strategic locations than the north end. The diagonal load on the south end is higher though.

RE: Miami Pedestrian Bridge, Part XI


jrs_87 (Mechanical),


Thanks for the link. The link has this diagram.


which is what I have predicted and shown in the CAD figures. The above 3D diagram is on the south and not the north end.

I think people slowly realize the deck was pulling away with the force equal to the horizontal component of the diagonal member axial compression and the connection surface wasn't generous. especially at the north end.

RE: Miami Pedestrian Bridge, Part XI

saikee119 (Structural)30 Jun 19 10:25

Saikee, I'm glad for that image too. Link below is for an old animation that is good in another way. Now, what we need is a true-to-life translucent solid model that can be rotated, sectioned, and animated. Ability to highlight separation interface would be a plus.

Hopefully, precisely how the PT bar tension reversals affected the area will soon be satisfactorily explained.



Source: https://www.youtube.com/watch?v=DmN3tJXQHc8 1:44 mark (please read screamdoctor comment below video)

I don't think the entire node slid together as shown. North face of 12 and diaphragm cracked and flexed at first while holding 11 back. The discrepancy of displacement accounted for by internal crushing and rotation.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87)

High quality blog post about OSHA report from a U.K. bridge designer:

https://happypontist.blogspot.com/2019/06/fiu-brid...
Thanks for the post!

A comment from that blog post has a great short summary of another construction collapse in the Netherlands. The engineering failings are woeful and damn obvious, but the summary of the problems inherant in the industry are equally applicable. (And also quite applicable to the recent Australian construction issues.)

https://www.youtube.com/watch?v=jSmRELbmdV4

RE: Miami Pedestrian Bridge, Part XI

Thanks, jrs_87 and saikee119 for the responses. It is terribly unfortunate that the FIGG analysis resulted in designing a larger diagonal 2 compared to others, but not a larger diagonal 11.

RE: Miami Pedestrian Bridge, Part XI

Quote (human909)


That video was a scorching rebuke. flame I had to watch it a few times to absorb it all. Sad the view count is only 2k.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87)

That video was a scorching rebuke. I had to watch it a few times to absorb it all. Sad the view count is only 2k.
Yes. It doesn't hold back. And what is true there is certainly true in the Australian construction industry and likely also the US and across the globe. The problems go well beyond engineering mistakes.

Further reading on the collapse is also enlightening. What the video also didn't fully elaborate was that the construction joint played a huge role in that incident too. The thin prefab slabs were never roughened. There are some great picture of core sames show pretty much zero cohesion in the slab layer.

A brief english version (paywalled)
https://www.scribd.com/document/369044718/20170929...
A comprehensive dutch version (pictures still tell some of the story)
https://www.onderzoeksraad.nl/en/media/attachment/...

Anyway much further on that needs its own topic! But it certainly is an interesting failure and extraordinarily lucky nobody was killed.

RE: Miami Pedestrian Bridge, Part XI

Quote (human909 (Structural)30 Jun 19 12:33)

... a great short summary of another construction collapse in the Netherlands ... https://www.youtube.com/watch?v=jSmRELbmdV4

Transcript obtained from the video closed-caption subtitles, downloaded with https://www.yousubtitles.com/ followed by format editing to insert spaces, break paragraphs, etc.

Quote:

In recent years several accidents have occurred in which buildings have partially collapsed. Four of these incidents have been investigated by the Dutch Safety Board. The Board's most recent investigation examined the partial collapse of a multi-storey car park, located next to the Eindhoven Airport terminal.

The building had a modern design with open floor plans. It’s floors were kept light and slim by using bubble deck slab floors. A bubble deck floor is constructed from individual prefabricated slabs made of a concrete base, reinforcing steel, lattice beams and plastic spheres. Concrete is then poured onto the floor slabs. The plastic spheres mean that less concrete is required, reducing the total weight.

In a conventional application the Bubble deck floor slabs span the distance between the columns in the longitudinal direction. Deflection of the slabs is the largest in the middle of the floor. The reinforcement bars ensure that the floor does not break. In the case of the Eindhoven car park the decision was made to rotate the floor slabs 90 degrees. The floor supplier and the contractor considered this solution the best to meet the design requirements for a thin floor with a long span and gradient for rainwater drainage.

EDIT: In the video demonstration, the slabs were 10mx3m. To create a 12m span, 4 slabs were turned sideways.

Rotation of the floor slabs resulted in a vulnerable floor design, as it placed seams at the locations where the deflection of the floor is the largest. No additional measures were taken to overcome this vulnerability. Owing to the coupling reinforcement bars being too short, the floor was unable to properly bear its own weight. The warm temperatures of 27 May brought about an additional temperature stress, causing the floor to collapse.

Despite deviating from the usual construction method, no attention was given to the possible consequences of this design choice. Signals were not met with a response. None of the parties saw any reason to have reservations about structural safety, although this could and should have been the case.


A shared concern for safety is not self-evident in the construction sector. The Board sees a sector characterised by a limited ability to learn and a low degree of self-reflection. Previous incidents and investigations have not resulted in improvements, while parties are all too ready to blame someone else.

Construction projects often have a complex organisation, in which multiple parties are involved at different times. As individual parties do not oversee the bigger picture, the chance of error increases. This fragmentation of the construction process also leads to a lack of clarity regarding the allocation of responsibilities. As a result, safety risks go unnoticed and no control measures are being taken.

The lack of one central party to monitor the overall construction process, and to keep other parties on their toes, is one of the most important explanations for the inadequate monitoring of safety risks. This conclusion was previously drawn by the Dutch Safety Board and it was indeed the case in Eindhoven as well.

In addition, the Board notes that the competition in the building sector is above all based on price instead of quality. If clients were to take safety more seriously, the level of safety would be increased considerably. As municipalities conduct less supervision, the final check of building constructions is increasingly skipped in a sector that takes insufficient responsibility for safety. The Board considers it high time that the building sector takes up this responsibility and implements the necessary improvements.

Improved risk management is needed throughout the building sector. Both clients and contractors must bear their share of this responsibility. To this end, the Board makes recommendations in the following areas:
  • Expand the existing Construction Safety Governance Code and make it less voluntary.
  • Ensure clarity in the division of responsibilities and their coordination.
  • Appoint a single party to take overall responsibility for risk management throughout the construction process.
  • Investigate how this can be made mandatory.
  • The Board expects the construction sector to apply the principles of mechanics and the associated diagrams thoroughly.
  • Builders are expected to adopt a professional attitude by addressing any doubts and critically reflecting on their own actions.

RE: Miami Pedestrian Bridge, Part XI

That is quite an interesting collapse. Should that be I’m its own thread?

RE: Miami Pedestrian Bridge, Part XI

The Dutch are very blunt in their discourse, so I'd expect that to show up prominently in their investigations. One needs to know more about how Architecture/Engineering & Construction are treated in both the Public & Private sector to know what might need to change. The way the BubbleDeck was rotated was crazy. Apparently two other contractors turned down bidding on the project because they didn't like the spacing of the floor design. This suggests the engineering to meet the design was part & parcel, the contractor's responsibility.

When it comes to FDOT oversight of the FIU bridge, one has to acknowledge that the LAP (Local Agency Program) isn't unique to Florida and starts with the FHWA. Universities already operate as Quasi-municipalities, so the bureaucracy of qualification probably wasn't a major hurtle. Especially if they had T.Y Linn (FIU Planning Consultant) holding their hand. Universities are nothing, if not Federal Grant & boilerplate savvy.

FDOT's, Mr. Alfredo Reyna, PE. was working on contract from Keith and Schnars, Consulting Engineers. Keith and Schnars, looks to be mostly "Consulting". As a hired gun LAP Coordinator, his job is to merge FDOT requirements with FHWA requirements and make sure that the parties involved are adhering to the LAP & TIGER program protocols. He was very conscientious about wanting to be copied/included in all FDOT communications. As far as can be ascertained, he only saw the few crack photos from the FIGG presentations and may not have fully understood the condition of the bridge. His FDOT role, or his firm may even preclude him from making any engineering judgements.

Bottom of Pg. 84 of the OSHA Report: As a result of the blow-out, three #7 shear reinforcements at the construction joint of diagonal 11 and the deck were sheared but the southernmost shear reinforcement remained intact.
It would have been nice to see a photo showing the full CJ contact surface, after the large debris had been removed. The southern most rebar was supposedly in the crack "separated" chamfer in the 11/12 node.



Something to be considered, the inspectors for Bolton Perez and Associates, Inc., were hired for their knowledge and experience in oversight of the new work. When cracking became a concern, someone experienced in making assessments of cracking in concrete should have been sent out to access the work, even if it meant contacting a third party. If your firm doesn't do remedial work, then they need to recognize when they are out of their depth and get a qualified opinion.

In construction, sooner or later you deal with people who are very adapt at shortcuts, end-runs and playing two ends against the middle. It gives them an advantage over their more scrupled or less experienced competition. If any party was experienced at divining the shortest path through the entanglements of the FDOT, it was FIGG and they look to have exercised their skill in this sphere to maximum advantage; when it came to the requirements of PEER Review.

RE: Miami Pedestrian Bridge, Part XI

> human909 (Structural) 29 Jun 19 12:15

>> Quote (MikeW7)
>> Modern integrated circuit design software has progressed over the past 40 years to the point where desktop CPUs now contain over a billion transistors, so I'm absolutely amazed that structural design software is that primative. Any idea why?

>That is an excellent question. And I have no ready answers apart from the results aren't readily applicable to legacy codes.

(Long) retired integrated circuit development/test engineer here. I'm afraid the answer is that the IC design software has an easier job. Most of it has to do with the modular nature of microprocessors. The basic elements (transistors, resistors) have gotten smaller, but the inherent technology of those devices date back to the 1950s and '60s. Elements in a processor (adders, registers) frequently get reused. Note that some errors in the late Intel processors actually dated back to the Pentium I. It seemed to work, so it got reused.

The other part of it, is the development tools have been working on essentially the same problems as the overall technology improves. How do you get signals around, keep power dissipation, and make it smaller. These questions really don't change; just the controlling parameters.

FWIW, when I retired in 2001, IC layout people usually had an AA degree, though the designers ranged from BS to PhD--usually BS and MS. That layout software was intended for use by that level. It also doesn't hurt that IC designs are essentially 2D, though it's layered, with various connections at different levels.

I've been following this discussion since shortly after the bridge collapsed. Impressive work, gentlemen!


RE: Miami Pedestrian Bridge, Part XI

I would have like the investigator to study carefully the first set of photos to arrive at the probable cause. He/she then returns the next day to concentrate on the selected sections/portions, clear debris to get to the bottom. He should return as many times as needed to capture all the important information before the site is cleared.

I am amazed at no photo available to show the east side of the diagonal 11 after the collapse. It had a massive longitudinal crack.

Sheared off rebar was described by OSHA but without photos showing what happened after the debris and dust were cleared to show the naked steel. Shots should be taken to cover the full extent of the CJ.

The true cause of the collapse would be self evident if there are sufficient photos showing each remaining rebar to allow its bar mark to be identified accurately.

It is quite possible OSHA has access to a lot more photos and published only few good ones with the report. In the digital era I myself often take 10 times more photos than needed for a report.

RE: Miami Pedestrian Bridge, Part XI

2

Quote (saikee)

It is quite possible OSHA has access to a lot more photos and published only few good ones with the report. In the digital era I myself often take 10 times more photos than needed for a report.

I find that no matter how many photos I take I can still fail to take the photo I really need.

RE: Miami Pedestrian Bridge, Part XI

I have seen video of the NTSB taking photos as the remains of the bridge were disassembled (jackhammered). There should be a dossier on the web at the time of the NTSB hearings.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Source: https://www.nbcmiami.com/investigations/Engineers-...

Quote (Tony Pipitone NBC 6 News Article)

The next morning, March 13, FIGG's president, Linda Figg got involved, emailing her engineer of record on the project, Denny Pate: "Wouldn't it have made sense for us to address all of these things while we were on site? … I think it is important for you to stop whatever you are doing and address this so we can put all of these items to rest quickly."

Source: https://www.nbcmiami.com/investigations/Judge-Dema...

Quote (Tony Pipitone NBC 6 News Article)

“Don’t you worry,” replied fellow engineer Eddy Leon, adding Pate was going to travel from Tallahassee to Miami for a meeting “just to calm everybody down.”
...
Pate had already left the worksite and was heading back to FIGG headquarters in Tallahassee a half hour before disaster struck when he texted his boss, firm owner Linda Figg: “Meeting went pretty well. FDOT and FIU were represented. Our attendance and information were appreciated. We will be working on some additional ideas to better the current situations. I’ll get you more details when I get back to the office.”

RE: Miami Pedestrian Bridge, Part XI

From my previous experience the type/magnitude of this disaster would bring down the companies responsible. There should be virtually no prospect of getting future work to continue trading after all the facts have been revealed.

The employees would likely continue doing the same kind of work as before possibly together under new company names. The equity holders of the old companies are expected to suffer financially after the indemnity insurances have taken the first hit.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87 (Mechanical)1 Jul 19 14:12 Minor court proceedings with Pate testifying.)

I have always thought that one's future is never more uncertain than when it depends on either the health care system or the judicial system.

RE: Miami Pedestrian Bridge, Part XI

This larger fragment of 11/12 node (green) is of interest to me. How it separated with this size and shape should be explored.

Turquoise marks 2 whole shims, 1 damaged shim, drain pipe fragment, and concrete fragment with impression of PVC pipe. The steel plate leaning on pier is leftover stock used to fabricate emergency steel shim?

Can rebar (red) be identified? Is it important?

RE: Miami Pedestrian Bridge, Part XI

@jr - that's a good question. It looks like the block even has a rebar mark on the front face. The steel plate is likely the shim that was used under 12.

RE: Miami Pedestrian Bridge, Part XI

Another view of block, and another similar block that was out of frame in previous image.


RE: Miami Pedestrian Bridge, Part XI

My guess is chunks of 11.

RE: Miami Pedestrian Bridge, Part XI

jrs_87 (Mechanical) What is the url of the video(s) your stills are posted from?
Thanks,

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie (Computer)2 Jul 19 18:52 jrs_87 (Mechanical) What is the url of the video(s) your stills are posted from? Thanks,)


They are screenshots from OSHA report, zoomed in with added markups. The page of report and zoom level can be seen in top margin of image. Page 91/115 and 93/115. Zoom 400 or 800 percent.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87)

... Is it important?

Yes. But there comes a point of diminishing returns. Personally, I find the information to date and the touted explanations inadequate. Every piece of debris and every crack in the concrete tells part of the story. Six people died and another is permanently disabled so "it was punch out" is not much different than "it broke" or "it fell down". If the victims aren't enough of a reason, the photos and videos beg for better answers, if not just for a coherent presentation of all the facts.

RE: Miami Pedestrian Bridge, Part XI

Damn some of that court case reports are damning.

Quote:

Louis Berger cited those examples as part of its effort to persuade Judge Bailey to give it access to the draft minutes and communications FIGG and its lawyers created as it developed the “corrected minutes,” which were sent to NTSB six weeks after the collapse.

FIGG claims those records cannot be seen by others because they are privileged work product, created in consultation with its lawyers in anticipation of litigation.

Judge Bailey decided to review the underlying materials herself before deciding if FIGG made “any kind of factual alteration” in its drafts. If so, she said, it is possible she could override the privilege and allow others to see what FIGG and its attorneys were doing in preparing their version of the meeting.

Louis Berger has not yet seen the drafts and communications, but based on what it has seen in discovery, it is saying it appears FIGG did alter the facts.

“A comparison of the corrected minutes to contemporaneous handwritten meeting notes of FIGG employees and other contemporaneous documents confirm that FIGG counsel likely changed or permitted changes to the facts that resulted in corrected minutes that were not fully faithful to what transpired at the March 15 meeting regarding the cracking,” wrote Louis Berger’s attorney.

RE: Miami Pedestrian Bridge, Part XI

It would be fascinating to see the original notes vs the corrected minutes.

RE: Miami Pedestrian Bridge, Part XI

Another fascinating point of this collapse is seeing the party most knowledgeable with the bridge could play dumb on professional and technical matters like:

When something obviously important and wrong could be dismissed lightly with no real sense of urgency.

Knew the bridge in distress but ignored the safety and fantasised the defects could be concealed in future by building quickly its back span.

Unilaterally instigated an unsafe operation that led to the bridge collapse.

Displayed in public various attempts to conceal, alter and destroy evidence.

We all know in engineering we learn from mistakes. If one does not admits mistake then it is possible that he doesn't really know about the mistake and so is unable to correct it. Disturbingly this bridge collapse has such a hallmark.

RE: Miami Pedestrian Bridge, Part XI

Concur with saikee119...

The bridge was screaming for help and to some extent the presentation acknowledges that.

In an instance like this with large and growing cracks at a critical location that you can not replicate with analysis, as the EOR, you have to convince yourself it is not OK until it is.

It appears they convinced themselves it was ok until it wasn't.

IC

RE: Miami Pedestrian Bridge, Part XI

Most of you on this site have vastly more experience with real reinforced concrete than I could ever have. Please excuse my ignorance.
My simplistic understanding of the failure is:
Member 11 cracked at the cold joint and overcame the resistance of the rebar and filet.
It (11) developed enough momentum to take out member 12 and the attached portion of the deck/diaphragm due to insufficient steel connecting the thrust from 12/deck/diaphragm to the PT in the deck.
Is this so over simplified as to be wrong?
It seems to me that at least two insufficiencies were required for the bridge to collapse.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Here, in San Francisco, where we have earthquakes, we have different requirements. Reinforced concrete has to withstand repeated blows from it's foundations. (Yes, I'm still going to talk about the bridge.) First a structure of welded rebar is constructed (It looks like it could hold up the building by itself, I'm sure it can't.) The space for concrete is only a few inches between rebar. then a cage of rebar is put around the whole thing, then concrete poured. some of the retro fit columns even have a cylinder of sheet steel around the concrete to contain it if it crumbles. Now about the bridge (finely.)
Does the lower PT rod pulling out of member 11 indicate insufficient confinement? It prevented the PT rod from having any possibility of working as a dowel. Does member 12 have insufficient confinement? It seems to have "exploded" around it's rebar, leaving the rebar more or less intact.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

SFCharlie (Computer),

I am convinced that 11 and 12 were moving together as one unit at the bottom. Member 12 has some additional resistance against moving outward and this is from bending its connection with the canopy. The resistance from bending is relatively small because a very small deflection at the bottom of 11/12 is enough to fail the joint and collapse the bridge.

Most people outside RC design may not realize the cast-in items, of 4 No. 4" vertical sleeves and one 8" horizontal pipe, could and have destroyed the integrity of this joint because they shortened the load paths.

You can see from my 29 Jun 21:49 posted sketches that the centrelines of 11 was able to join with 12. The deck was only to the side of 11. The one PT rod were also arranged to bit hard into the deck while the other to 12.

Member 12 is not structurally important and it together with the part of the joining canopy are omitted in a standard Warren bridge. As each joint the internal and external forces must be balanced to achieve equilibrium. The horizontal component of the diagonal 11 balances with the tension in the horizontal deck while its vertical component with the vertical reaction (50% of bridge weight) at the pier or pylon. Therefore one can chop off 12 and the bridge will still stand. However the canopy was used for anchoring the longitudinal PT tendons and 12 would have its self weight exerted on the support, other than that 12 had very little resistance against moving outward.

In conclusion 11 only needs one insufficiency from its connection with the deck and carries 12 as a piggyback. Obviously if 11/12 were to break away from the deck it could slid out of the whole CJ or part of the CJ with 12 broken off along the plane with the 8" pipe. It chose the latter.

The crack photos confirm 11 suffered some internal splitting in axial direction first slightly when it was first tensioned in the roadside and then later quite seriously, with 12 wide and 72 deep cracks, when the tension was first removed.

The pull out of the lower PT rod should be looked at a consequential damage. It was from a follow-on consequence when the deck dropped to the ground while 11/12 was caught up with the pylon. The blow out was indeed due to insufficient concrete surrounding some rebar but OSHA has reported a smaller number of rebar were sheared off at the CJ.

RE: Miami Pedestrian Bridge, Part XI

https://www.workflowmax.com/blog/101-engineering-q...

44. “[I am] opposed to the laying down of rules or conditions to be observed in the construction of bridges lest the progress of improvement tomorrow might be embarrassed or shackled by recording or registering as law the prejudices or errors of today.”

― Isambard Kingdom Brunel

86. “Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

― Unknown

33. “Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance.”

― Dr AR Dykes

RE: Miami Pedestrian Bridge, Part XI

jrs_87 (Mechanical),

Quote (Isambard Kingdom Brunel)

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

I am British trained and am pretty sure in Brunel's era engineers used only the elastic theory in the design which is safer with a larger margin of safety factors.

In today's design the envelope has been pushed forward by designing structures at ultimate limit state of collapse with working conditions adequately protected by commonly agreed safety factors stipulated by the codes and national standards. These safety factors change very little between countries.

A Designer is not allowed to build a bridge that barely stands. FDOT and Louis Berger are the authorities stopping it from happening in this project.

So if this bridge design does not have the relevant safety factors it is a non-compliant design and not one with a design error. I believe the former is illegal and so a criminal offence whereas the latter is a negligence.

RE: Miami Pedestrian Bridge, Part XI

saikee119 (Structural)3 Jul 19 19:37

My I kindly point out Isambard Kingdom Brunel did not make that quote.
"Unknown" did.

RE: Miami Pedestrian Bridge, Part XI

jrs_87 (Mechanical),

You are right. My misquote.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87)

A Designer is not allowed to build a bridge that barely stands. FDOT and Louis Berger are the authorities stopping it from happening in this project.
I think you miss the point of the statement, well at least I interpret the statement vastly differently.

Throw enough stone, timber, steel or concrete at a structure and you don't need to be an engineer to make it stand up. You just need a decent budget and a little experience. Do you think the Romans and Greeks or Egyptians were calculating shear and moment capacity of their materials and structures? Sure some builders would have made mistakes in building bridges, aqueducts or amphitheatres but they fell down and they rebuilt with mateThrial so it would stand up.

The point is it takes an engineer to design a structure efficiently so it is 'just' strong enough for the required loads and very little more.

Quote (jrs_87)

In today's design the envelope has been pushed forward by designing structures at ultimate limit state of collapse with working conditions adequately protected by commonly agreed safety factors stipulated by the codes and national standards. These safety factors change very little between countries.
Ultimate limit design highlights this point completely, in fact the clue is in the name! Codes generally require a structure to be strong enough that it "barely stands" under extreme conditions and allowing for variation in material properties. There is no explicit safety factor in ultimate limit state design.

As per: https://en.wikipedia.org/wiki/Limit_state_design

Quote:

In the purest sense, it is now considered inappropriate to discuss safety factors when working with LSD, as there are concerns that this may lead to confusion.

While you may interpret various load factors and capacity reduction factors as safety factors that is not what they are intended to be. While this made seem like pedantry, it underlines the entire difference in philosophy of limit state design vs other methods that explicitly use factors of safety.


To put it another way. A FOS approach is almost like saying I don't know enough so I'll just throw a big factor on top to allow myself to sleep at night. A limit state approach says, I know the properties of my structure and I have very good idea of the loads and environmental loads it could likely experience. I can sleep at night if my design capacity is slightly bigger that my design loads. (For me 'slightly bigger' could mean 10% or 20% if I have tightly calculated design loads that lets me sleep at night just fine.)

RE: Miami Pedestrian Bridge, Part XI

I’ve always found the distinction between limit state strength design and factor of safety design to be rather artificial.

At the end of the day all the fancy limit state factors boil down to a safety factor very similar to traditional approach.

RE: Miami Pedestrian Bridge, Part XI

human909 (Structural),
It is no point going over "barely stands" because each one is entitled to his/her own interpretation.

Regarding the older civilisations building structures without our modern codes my explanation would be they didn't do tension and bending but relied on compression mainly. Ancient massive structures stand today but they are no longer affordable nowadays for what they serve.

In limit state design the safety factor is based on statistics. There is a lot human still do not know but a characteristic strength of steel or concrete is both based on a population of 5% could fail. Every country uses this criterion.

The safety factors on loading are based on good engineering practice. Naturally occurring loads like wind, earthquake, storm, rain, tides etc are based on return periods like one in a 100 years. On tides we use the HAT or Highest Astronomical Tide of the project site plus surge added on top if necessary.

A designer is always given the freedom to select any factor above what the design code mandates as long as he/she can justify it. If one is uncertain one would increase the safety factors to allow for the unknowns. The safety is always in the hand of the designer.

Therefore if a designer loses sleep at night it is mostly likely he/she has identified some uncertainties but hasn't raise the mandatory factors enough to cover the risks.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural))

I’ve always found the distinction between limit state strength design and factor of safety design to be rather artificial.

At the end of the day all the fancy limit state factors boil down to a safety factor very similar to traditional approach.

I must be very old as I went through the elastic design, Load factor design and the limit state design. I can explain the difference as below.

In elastic design, also referred to as the "Modular Ratio method", we design a structure in working condition (load factor=1) by restricting its maximum stresses of steel and concrete to within prescribed permissible limits. In this method we have no idea of and no interest in when the structure would fail.

To save a bit of money people changed the above to "Load factor method". In computation it is essentially the same elastic theory but the prescribed permissible concrete stress is based on a safety factor, with consideration on designed or nominal concrete mix, applied to the specified concrete strength. This is an attempt to estimate more closely the concrete resistance near the failure condition.

The Limit state design is different because for the first time the design is based on concrete at failure with a declared compressive strain of 0.003 (0.0035 in UK and EU). Both the American and UK/EU codes have partial safety factors for load as well as on materials. UK/EU codes have one partial safety factor for steel and another factor for concrete but American uses just one strength reduction factor to represent both concrete and steel. The American strength reduction factor can also change with stress types.

Based on my 30 years work on the analysis of concrete sections subjected to axial load and biaxial bending I can say the elastic theory and Limit State designs give nearly same result. This is because in LSD we design a section at the ultimate limit state of collapse but in its serviceability state or working condition the steel and concrete are mostly still inside elastic range. In UK/EU serviceability limit state the load factors are removed and partial safety factors, in concrete and steel, are adjusted lower according to the codes.

The above makes sense because most of our structures in normal working state (limit state of serviceability) should have the concrete and steel stresses still inside the elastic range. Thus we increased the volume of RC design computation possibly by 3 to 5 times to get back to where we started with the elastic theory.

RE: Miami Pedestrian Bridge, Part XI

2
Designing structures to Codes is risky at best in many cases. Preparing a design that satisfies code requirements will get your project approved by government agencies and may provide some protection in a court of law, but the engineering comes into play when one recognizes the critical members and joints and wisely puts a bit more there "for the wife and kiddies".
Also the needs of a certain project may far exceed the "minimum code requirements".
This 14 million dollar project needed about $10,000 - maybe $20,000 more steel and concrete in 11 and its joint to the deck. It is never cheaper than before the bid.
I see several things that illustrate an oversight as it applies to the north end of this main span. First, the south end is much 'stronger', suggestion it was considered as an end joint and given attention as such. The north end (which failed) was to (supposedly) have more capacity in the final state, leaving the door open to not considering it as an "end" condition in this phase of the construction.
Second, it has been stated that all would be good when the structure was complete.
Then I see very similar treatment of joints at the deck level - with not a lot of reinforcing across the joint, and non specific requirements for preparing that joint, leaving the question of the future performance of those joints.
Perhaps the use of "shear friction" design should not be used to design a non-redundant joint which can let an entire structure fail if it does not perform as needed. Perhaps it was "poor engineering" to design a joint with such a flat angle. But the code does not appear to prohibit such a configuration, and suggests shear friction as a design procedure. The code statement that joints are to be placed where they will least affect the performance of the structure is a cop-out. But it is impossible to anticipate every condition which may arise. That is where the engineering comes into play.
Watch, question, and learn. Confucius (supposedly) said "A smart man learns from his mistakes. A wise man learns from the mistakes of others". May you never be an "other".




RE: Miami Pedestrian Bridge, Part XI

Quote (saikee)

Thus we increased the volume of RC design computation possibly by 3 to 5 times to get back to where we started with the elastic theory.

Yes, that's what I was getting at. At the end of the day things aren't much different. These supposed different philosophies say much the same thing.

RE: Miami Pedestrian Bridge, Part XI

human909 (Structural)3 Jul 19 21:06

Human, thank for your posts. I enjoy reading them. Just a friendly correction, those are not my Quotes, they are from poster quoting me.

While "barely stands" assertion is obviously rhetorical, it does have a small element of truth in some cases. I'm sure the sentiment of the unknown author was complimentary to engineers.

RE: Miami Pedestrian Bridge, Part XI

On the comments above on elastic design vs limit states- in the more recent ASD, there is a different allowable stress for every conceivable configuration of member. The results of this were that simplicity was lost, but much of the advantage of the limit-state approach was already incorporated into the ASD codes.

And, back to the bridge failure, one observation there is that if you routinely design "new" stuff, all of your education and all of your experience is in the field of "how to prevent failure", which means you don't normally deal with failures. In this case, there were two issues, one being deficient design, and the other being the reaction, or lack thereof, to that deficiency. In the reaction to that deficiency, engineers that routinely dealt with evaluation and repairs of existing structures, or engineers that routinely dealt with demolition of structures, might have been better able to deal with the situation.

RE: Miami Pedestrian Bridge, Part XI


Quote (SFCharlie)

Here, in San Francisco, where we have earthquakes, we have different requirements. Reinforced concrete has to withstand repeated blows from its foundations.

In conventional construction, bridges are built in situ. However, given ABC, this span had a number of foundation changes: from removing shoring to being placed on the transporters to moving over irregular ground to being placed on piers. Maybe these were similar to some level of seismic events, and should have been considered in the design?

RE: Miami Pedestrian Bridge, Part XI

People have moved lighthouses, hotels, entire apartment buildings; while it did not go far, much of pre-fire Chicago was raised up and much was moved; too bad all the raised buildings burned in the Great Chicago Fire.

There were many flaws in the design of this bridge; the move only further exposed their lack of care, but did not do specific damage to the bridge beyond uncovering already weakened areas.

https://historydaily.org/moving-a-7600-ton-apartme...
https://en.wikipedia.org/wiki/Raising_of_Chicago

RE: Miami Pedestrian Bridge, Part XI

Quote (Wetlander)

In conventional construction, bridges are built insitu. However, given ABC, this span had a number of foundation changes: from removing shoring to being placed on the transporters to moving over irregular ground to being placed on piers. Maybe these were similar to some level of seismic events, and should have been considered in the design?

The loads from moving are not close to the level seen in earthquakes. Concrete structures are usually quite ductile and give lots of warning before failure. There are of course exceptions. This structure was for the most part determinant (one possible load path) and was supported at two points (during the move and two different points after the move). Up and down movement of the supports has very little effect on the structure. Twisting is different but there were no signs of damage due to twisting or torsion.

ABC is just new name for an old idea.

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance Wiley)

Perhaps the use of "shear friction" design should not be used to design a non-redundant joint which can let an entire structure fail if it does not perform as needed. Perhaps it was "poor engineering" to design a joint with such a flat angle. But the code does not appear to prohibit such a configuration, and suggests shear friction as a design procedure. The code statement that joints are to be placed where they will least affect the performance of the structure is a cop-out. But it is impossible to anticipate every condition which may arise. That is where the engineering comes into play.

Your point are well taken but in the concrete structures that I have designed, it wouldn't be possible to avoid shear friction at critical locations. However, you also don't have to design them to the skin of the teeth. I think the best thing to do is to use a capacity design philosophy for these kinds of joints if at all possible or practical.

RE: Miami Pedestrian Bridge, Part XI

Speaking of earthquakes, I survived my 3rd 6+ magnitude today. Quakes this strong you remember for life and it makes you glad for building codes - no matter what philosophies were used to develop them.

RE: Miami Pedestrian Bridge, Part XI

I believe most of us are quite clear about the failure but a few still talk about the lack of provision on the code, validity of using shear friction, guidance of joint selection/preparation etc as though the designer was confused.

What we had here is a simple connection failure. Had this been steel bridge and the members are structural steel the separation failure would be the inadequate welding or bolts to connect them together.

Since this is a reinforced concrete bridge so a connection failure is just the rebar inadequate or embedded to insufficient depth on either side of the joint or not enough concrete surrounding the rebar to develop the maximum steel stress. The photos have given the clues and confirmation. The OSHA report has declared the margin by which it was deficient.

A seasoned RC designer would be disturbed by the failed joint when seeing :

(1) The steel reinforcement is meant to connect the deck, 11 and 12 together so the design should have bar between deck with 11, 11 with 12 and 12 with deck. It did. However for such a thin deck logically some bar should be bent sideways (or use L bar instead of straight bar) to link the east, west as well as the north/south sides of the deck. There were no bar to link the east and west sides at all. The reason is the thin deck had been stressed by tendons in both directions leaving very little space for such purpose.

(2) The deck has a substantial diaphragm at the end. This 2' wide by 4' deep diaphragm potentially can provide enough rigidity for the connection if enough steel bars were provided between 11/12 with the diaphragm. Using some L bars instead of all straight bar would have a large part of the diaphragm to participate in the joint resistance. Again no such bar from 11/12 were placed to link the east and west directions of the diaphragm.

(3) Not using L bar is NOT a design shortcoming and so cannot be blamed for the bridge collapse. However the straight bar arrangement, straight in direction only as bars could have hooks or small L at the ends that are totally within the member cross section, was compromised by the four vertical sleeves and on horizontal pipe. These embedded items occupied 60% of the vertical and 40% horizontal bonding areas of 11/12 with the diaphragm. These embedded items also took away the possibility of passing rebar through this vital area. Had L bar been used the capacity reduction from these embedded items could have been mitigated because the lengths of the two Ls can be lengthened to allow for interior sections affected by the embedded items. Also L bars are effective when placed on top and below the horizontal pipe as depicted below.



We can all get wiser after the event but my point is the problem isn't high-tech but just down to simple steel reinforcement arrangements to anticipate points of weakness.

RE: Miami Pedestrian Bridge, Part XI

ADD: This video was analyzed quite a bit way back in Part 1, but here is my contribution.
EDIT: Added frame 049 and edited for clarity.

I created a series of 9 cropped and stabilized images from the traffic cam video that Miami Herald reporter Monique O. Madan posted on her Twitter account. The images can be downloaded as a single ZIP file using this link. The image names are numbered according to the frame number of the phone video.

Of interest: two consecutive traffic-cam frames (phone video frames 049 and 057) show that both the canopy and deck were cracked and falling while the canopy above member 12 hasn't dropped significantly (this was also observed in frames 76-77-78 of the truck-cam video, as noted in my post of 22 Jun 19 22:31):




The methodology I used is as follows:
  1. Use VirtualDub to crop a 640x360 section of the original 720x1280 video, resize it by a factor of 400% using nearest neighbor algorithm to 2560x1440, and export a numbered image sequence. As noted in my post of 22 Jun 19 22:31, the nearest neighbor algorithm minimizes enlargement artifacts by duplicating existing pixels instead of interpolating in-between pixels.
  2. Select the 9 "best" consecutive frames of the traffic cam footage. This was difficult because the phone was not held steady, a person to the left was moving into and out of view, and the frame-rate of the monitor and phone were out of sync, causing subtle scan-line artifacts. In a few cases two traffic-cam frames were melded to create double-exposed or half-and-half spliced images, so I decided to not post a cropped video.
  3. Use IrfanView to incrementally crop the edges of the images so the final 1600x900 versions were aligned. None of the images were rotated for better alignment.
ADD: Here is a cropped version of the video - I'm pretty sure it used the same source.

RE: Miami Pedestrian Bridge, Part XI

It sure looks like 12 is flexing significantly. I've been thinking for a while that the concrete was much tougher than I gave it credit for.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7)

which shows that both the canopy and deck were cracked and falling before member 12 was pushed off.

I would argue this conveys imprecise understanding. It is totally expected that canopy and deck would move before you see any movement at base of 12. All it takes for deck and canopy to move as shown is for compression in 11 to no longer counterbalance tension in 10. Member 12 Node 11/12 does not need to be kicked out for this to happen. Member 11 only needs to be displaced or crushed shorter in the order of millimeters, and that is not visible on camera. A computer animation of collapse with millisecond by millisecond forces would make this point clear.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7 (Electrical))

The most interesting frame is 057, which shows that both the canopy and deck were cracked and falling before member 12 was pushed off.

The use of the word "before" in the above may be hazardous if this can be proved incorrect later.

People with structural engineering background are more easily convinced by the following:

(a) It is more difficult to have either the canopy at the joint canopy/10/11 and deck at the joint deck/9/10 to fail prior to the deck/11/12. Each diagonal member of every truss, like 9, 10 or 11, had an axial force internally which in statics could be replaced/substituted by one horizontal component and a vertical component in a ratio according to the triangle it makes with the horizontal. Thus the force in the upper section of 11 had a reaction opponent provided by the upper section of 10 to achieve equilibrium. Similarly the bottom of 10 and 9 could hold each other in equilibrium. The joint at deck/11/12 had nothing horizontally to act against with and logically became the easiest one to fail first among the three.

(b) The canopy section on top of Member 11 is structurally redundant. By that I mean it can be removed without affecting the behaviour of the structure. Indeed a standard Warren truss does not have it.



However the extra canopy section here provided the weather cover and served as anchorages for the longitudinal tendons in the canopy. The lack of structural importance of this section of canopy with Member 12 will become obvious if one examines the equilibrium condition at the joint where both meet but find neither of these two members has an opposite number to balance internal axial force with.
Since both canopy and Member 12 were rigidly joined together their member forces would have a full compliment of axial force, shear and bending moment but the axial force magnitude will be small by comparison. For example the canopy's shear, due to its half of last bay's dead weight, will be balanced by the axial force in Member 12.

(c) Had canopy/10/11 or deck/9/10 failed first then there is a good likelihood the north end could fail in a manner similar if not identical to its south end. The equivalent joint to deck/11/12 at the south end is deck/1/2. It was almost undamaged sitting on top of the pier after the collapse.



(d) There is an absence of photographic records and historical evidence pointing to the deficiency of either canopy/10/11 or deck/9/10. The shortcoming of deck/11/12 however was known as soon as the shoring was removed while the bridge was still at the roadside.

(e) Among the three joints of Deck/9/10, canopy/10/11 and deck/11/12 the joint deflection of the last is most devastating. Prior to the collapse 11/12 had horizontal deflection about 5 to 10mm or 1/4". Whenever Member 11 flattens its angle, by the addition of horizontal deflection to the horizontal dimension, its horizontal pushing force is increased at the expense of reducing the magnitude of the vertical component. The increased horizontal push then feeds more deflection. This would explain why the cracks were reported progressively getting worse daily. Geometrically it can be proved when the 11/12 deflects to clear the 24" wide Diaphragm II the horizontal pushing out force would have increased by about 3.5%. The deflection to the north by 11/12 could lead to a downward spiral impossible to stop once the point of no return had been passed. Once Deck/11/12 fails first the breaking of deck/9/10 and canopy/10/11 will follow as the consequential damages.

(f) I have repeatedly went over between your Frame049 and Frame057 and my observation are (1) bottom of 11 suddenly lost support and (2) Member 12 bent and its movement was similar in magnitude to the deck/9/10 and canopy/10/11. It is not possible with sufficient confidence to say which joint went first. To me the most important piece of information from your captured frames is Member 11 suddenly gave away. The apparent bending of the Member 12 could come from the local yielding near the vicinity of the joint with deck/11 by the concrete crushed/failed first but the embedded rebar were still fighting to hang on their position. Momentarily the local yielding of Member 12 produced a hinge which gave the impression the the Member 12 bent. After the collapse this yielding position or hinge was stripped off concrete leaving the rebar exposed.



We all are interested in how this bridge failed. You have certainly done a great job using your skill to scan every possible clue from the video. I am just trying to make our assessment as watertight as possible. At the end of the day the collapse would be explained by the physics and none of the personal opinions would matter.

Sym P. le (Mechanical),

I also noticed the flexing of Member 12 and explained with Item (f) above. If the joint deck/11/12 failed first the damage joint could have concrete broken out and the remainder crushed resulting a local hinge formed momentarily during the collapse. Not saying my explanation is what had happened but just from the failure mechanism point of view such development should not be a surprise.

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87 (Mechanical) 7 Jul 19 11:10)

Quote (saikee119 (Structural) 7 Jul 19 14:39)

I edited my post of 7 Jul 19 04:44 concerning member 12 and added more material. I understand the need for clarity because "others are watching". Let me know if anything else needs to be fixed while I still have time to edit.

RE: Miami Pedestrian Bridge, Part XI

MikeW7 (Electrical),

I joined this thread 8 days after the FIU bridge collapse. It was already pointed out by many members here including myself that the Deck/11/12 joint was problematic and the collapse was due to a connection failure. This was before the serious cracks were known, construction drawings, NBC timeline, NTSB/OSHA reports/updates were available.

Since then with more information and data available to the public this forum has refined the our common consensus which is based on the members' contribution.

We are all prepared to look into other possibilities causing the collapse as long as evidence is available.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119 (Structural) 7 Jul 19 14:39)

The apparent bending of the Member 12...
EDIT: In case anyone is wondering, the orientation of member 12 appears to be the same in frames 051-061, so the "bend" or "flex" of member 12 in frame 057 (compared to frame 049) that was mentioned by others is not an image-editing artifact.

In case anyone is wondering, frames 051-061 all show the same leftward "bend" or "tilt" of member 12 that is evident in frame 057, so it is not an image-editing artifact.

RE: Miami Pedestrian Bridge, Part XI

Quote (Mike)

In case anyone is wondering, frames 051-061 all show the same leftward "bend" or "tilt" of member 12 that is evident in frame 057, so it is not an image-editing artifact.

Is there a link to these frames?

RE: Miami Pedestrian Bridge, Part XI

I think saikee119 summed it up well. The video doesn't show what failed first. It does show the deck and canopy failing earlier.

In the case of the canopy, it should be under little stress if member 11 is doing its job. In the case of the deck that would require massive and sudden failure of PT member. Highly unlikely and also I don't believe these were shown to have failed. Alternatively if member 11 or its connection failed and no longer was loading bearing the failure as shown is what you would expect.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural) 8 Jul 19 01:04)

Is there a link to these frames?
Here is a ZIP file of those frames, in two folders. One folder contains 640x360 frames cropped directly from the original video. The other folder contains the same frames enlarged to 2560x1440 using the methodology described at the end of my 7 Jul 19 04:44 post.

You probably won't be able to tell the difference between the two sizes if your image viewer automatically upscales and interpolates small images, a problem I discussed in my post dated 13 Jun 19 16:53. If your image viewer can be set to "view original size" or something similar, the larger images will appear as bigger versions of the smaller images with no interpolation artifacts - each pixel of the original image is expanded to 16 identical pixels in a 4x4 block.

RE: Miami Pedestrian Bridge, Part XI

I did an analysis of the dash cam video a while ago, and abandoned it as more info came in. I have made .jpg files of the slides. dashcamanalysedfrompowerpoint.zip
Green outlines original position
Yellow outlines current position
Orange shows position on previous slide
(Before any movement detected)

(angle in canopy at 10,11 angle between canopy and 11 remains constant angle between 10,11 changes)

(continues as 12 appears to remain vertical)

(continues as 12 appears to remain vertical)

(continues as 12 appears to remain vertical)

(Finally 12 tips)

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Thanks for posting these.

By that stage the collapse is well underway. Are there any earlier frames showing excess curvature?

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural))

Are there any earlier frames showing excess curvature?
I was not able to detect any curvature, but the cherry picker arm in the foreground and the building behind make it difficult to make out 12, I admit.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural) 8 Jul 19 02:19)

Are there any earlier frames showing excess curvature?
No. Frame 049 is part of an identical sequence 046-050, which is then followed by the 061-061 sequence. The time stamp in the lower RH corner of the traffic-cam footage was updating about every 0.4 to 0.5 seconds, so the phone was recording sequences of 10-15 frames for each update of the traffic camera. The video metadata says it was shot at 29.97 fps.

RE: Miami Pedestrian Bridge, Part XI

I went back and reviewed my analysis of the traffic cam footage, and the curvature in 12 is constant thru all the frames where 12 is intact, so it is not an artifact of the process, but probably an artifact of a wide angle lens on the traffic camera itself.
MikeW7 (Electrical) your work is awesome.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7 (Electrical))

In my mind, the 7 July comment simply restates the 22 June observations in fewer words. Please help me understand why the pre-edited 7 July comment was criticized but the 22 June observations were not. The only significant difference I see between the two is the more detailed "blown off off the pier" versus "pushed off".

First your work on the video frames is awesome as SFCharlie has put it.

Secondly it wasn't a criticism but using factual information to back up deck/11/12 could have moved "before" deck/9/10 and canopy/10/11. I probably put in more basic information but that was for your benefit as you are not from the structural side.

Your earlier 22 Jun post has the frames from a video captured by a "moving vehicle". It is not a like-to-like comparison to your 7 July frames which were extracted from a camera at a "fixed" position.

I must admit I didn't pay much attention to your 22 June when it first came out because many have already offered various comments on this video. It is funny different people from different background will pick different information off the same photos. Your 22 June frames are the prime example.

While you were preoccupied with Member 11 didn't deflect vertically my attention went to the horizontal extrusion, believed from Member 11,occurred in the 5 photos of Frames0074 to Frame0078 inclusively. This extrusion may have been distorted by the moving camera in the vehicle but it happened exactly when the bridge started to collapse and "disappeared" completely in Frame0079 and later when the angulation of the falling bridge would have pulled north end of the bridge inward to clear the pylon. Thus your photos reinforce my belief Deck/11/12 was the weakest point to fail first. The extrusion was the local breaking-up of concrete from the Deck/11/12 joint from a sudden release of stored energy from the overstressesg of materials. OSHA used the word blow out in the report.

RE: Miami Pedestrian Bridge, Part XI

Quote (saikee119 (Structural))

Member 11 didn't deflect vertically my attention went to the horizontal extrusion

I'm afraid that I don't understand how you're using the word "extrusion" in this context. Could you elaborate please.
I appreciate your experience very much, as I have none.
Thank You

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Mikew7 on June 22 post has posted the following frames which I cut off half for space saving listed below. The purpose of re-posting these photos is to show the side extrusion on the north side of Member 12 at the bottom of Member 11. Mikew7 has already gave a run down of the general events.






The extrusion on the north of Member 12 first appeared in Frame 74, remained visible in Frame 75, 76, 77 and 78. It can be said the extrusion disappeared in Frame 79, 80 and 81. Something similar, possibly from the broken off bottom section of Member 12, were sticking out at nearly the same level in Frame 82, 83, 84 and 85. Member 12 could not be viewed properly as it was partially shielded by the rear crane most of the time. From Frame 82 to 85 the dust and debris from the deck/11/12 joint breaking-up could have darkened some areas.

The deck could be seen resting on top of the pylon in Frame 72 to 82. Its departure progress from the pylon was recorded in Frame 83 and 84. Although the view of the deck was blocked by the front crane in the last Frame 85 it is known the deck final resting position was on the ground.

RE: Miami Pedestrian Bridge, Part XI

SFCharlie (Computer),

Some Forum members have already mentioned something were sticking out at the north side of member 12 during the collapse. It was generally interpreted as the deck/11/12 suffered a blow out at this position or some concrete was broken off but still retained by the rebar momentarily.

At exactly the same location of the extrusion, by that I mean something extruded out from Member 12, there was an evergreen tree at the background and so one can say the extrusion could be from one of its branches. However the size and times it appeared and disappeared in the video does suggest it could also have been a byproduct of the collapse.

The evergreen tree branches might have been trimmed when the Google Street View was recorded later and depicted below. It is between the empty pylon and the steel column for the traffic signs. This tree could be seen in all the frames above.




The evergreen tree at the time of collapse can be seen below.

RE: Miami Pedestrian Bridge, Part XI

I had interpreted the rectangle to the left of member 12 and aligned with the front of the deck as the northwest corner of the deck. I think it appears as the truck moves from west of the deck to southwest of the deck. It does appear attached to the deck as it rises in line with the deck as the deck falls. To me, it appear to move with the deck as they slide off the pier.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

SFCharlie (Computer),

I would agree one possible explanation for the sticking-out object could be the northwest corner of the deck to an approaching vehicle from the west.

On balance the frames show the object more triangular than rectangular although it is harder to tell in the original video.

The bit difficult to explain is as a part of the permanent structure this object was able to disappear when the camera got closer.

It is also slightly thinner than the rest of the deck's edge, visible in different color and seemed too long without the temporary fencing attached.

RE: Miami Pedestrian Bridge, Part XI

I went back and reviewed the frames I had analyzed. One thing I noted is the appearance of dust on deck between the bases of 12 and 11 in the frames that show 12 starting to tilt. (small in the frame where 12 barely tilts and larger in the frame where the tilt becomes definitely visible.)

RE: Miami Pedestrian Bridge, Part XI

I just reviewed the timelapse of the bridge move. the temporary fencing ends at the south face of 12.
As the diaphragm tips south, the corner of the deck would also move south.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

2
I agree the horizontal protrusion is most likely just the corner of the deck.

You can see it earlier in the video, well before the collapse:



Then the corner passes in front of the tree, which camouflages it:



The corner is emerging from the tree at the exact moment the bridge collapses.



And then the corner vanishes as collapse continues and deck falls:



So it tricks you that it suddenly appeared and vanished at the moment of collapse, but really it was just camouflaged by the tree just before the collapse.

RE: Miami Pedestrian Bridge, Part XI

I have focused more on whether debris shows up north of the pylon. There is a dirty spot there but it could just be the algorithm or it could be the algorithm because some debris is there. Either way it's the algorithm.

RE: Miami Pedestrian Bridge, Part XI

End view of the north end of bridge showing the limit of the temporary fencing. No obvious crack was seen then at the Diaphragm II and the deck.

RE: Miami Pedestrian Bridge, Part XI

Quote (Sym P. le (Mechanical))

There is a dirty spot...
Yes, it lingers for a least 0.4 seconds. Could be very fine dust, but no aggregate, as it would have fallen in that time...

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (SYM)

I have focused more on whether debris shows up north of the pylon.

Quote (sfcharlie)

Yes, it lingers for a least 0.4 seconds. Could be very fine dust, but no aggregate, as it would have fallen in that time...

You can see a similar object earlier on:




I think it may be the rag tied to the reo:







RE: Miami Pedestrian Bridge, Part XI

Well, that takes care of that one. I'll strike it off the list. It's looking more like OSHA's easy answer isn't supported by the evidence.

Edit: I guess now I can say it's the algorithm because a rag is there.

Edit 2: This also supports the value of the video, rough as it is.

RE: Miami Pedestrian Bridge, Part XI

Quote (3DDAVE)

I expect what looks like curvature of 12 is an artifact due to the use of a rolling shutter video capture.

Yes, there are similar kinks along the other members too.

RE: Miami Pedestrian Bridge, Part XI

I accept the horizontal protrusion, with a rag hanging underneath, can be just the corner of the deck too. Although the reason why it appeared triangular instead rectangular in the 5 frames of approaching camera still needs to be explained.

Quote (Sym P. le (Mechanical))

It's looking more like OSHA's easy answer isn't supported by the evidence.

Can you point out which part of OSHA's easy answer isn't supported by evidence?

3DDave (Aerospace) and Tomfh (Structural),

The rolling shutter effect in the video is valid. Do you guys have a view on shutter rolling vertically or horizontally?

The curvature effect, easily seen in the shots extracted by MikeW7 from a stationary camera, would support the rolling took place horizontally possibly from south to north. The south face of Member 12 could have been captured slightly earlier than the north face which was already moving when the shutter reached it.

RE: Miami Pedestrian Bridge, Part XI

Saikee,

Regarding the rectangle/triangle/dirt, I’m not sure what it is. I suspect we’re seeing the corner of the bridge and the rag for many of the frames. Could we also be seeing signs of collapse? Or are we just chasing shadows? It’s so hard to say.


I was assuming the rolling shutter was vertical travel, but again couldn’t be sure.

There’s also the possibility of the phone adding rolling shutter? Phones typically have rolling shutters.

Can you tag exactly what you mean by the “curvature effect, easily seen”. I want to make sure I’m on the same page as you guys.

RE: Miami Pedestrian Bridge, Part XI

ROLLING SHUTTER EFFECT

The image sensors of most digital cameras are continuously active, but they can only record a portion of the image at a time because of data transfer limitations (limited bandwidth). The image is "taken" by sequentially reading rows (or columns) of the sensor and transfering the data stream to a storage medium to create a file. When one file is complete, the next read-write cycle begins.

The rolling shutter effect results when there is rapid motion perpendicular to the camera's scanning direction. If the data scanning is by rows, top-to-bottom, the top portion of the image sensor is read before the bottom portion, so any part of an object that actually looks like "|" is recorded as "/" if the object moves rapidly to the left, or as "\" if it moves rapidly to the right.

This phenomenon also causes problems when taking pictures of a video display, since displayed images are created using a similar technology, in reverse fashion - a source data stream is "drawn" onto the screen one line at a time.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural) 9 Jul 19 09:56)

Can you tag exactly what you mean by the “curvature effect, easily seen”.

If your image viewer allows you to view consecutive images using the L-R arrow keys, you can bounce back and forth between Frame 049 and 057 and note that the top of member 12 seems to move to the left (south) in Frame 057, making it appear to "bend" as the canopy collapses.

If the traffic camera is scanning top-to-bottom this cannot be explained by the rolling shutter effect I explained in my post dated 9 Jul 19 15:04 since it is the top of member 12 that appears to move, not the bottom which remains stable against the background. If the scanning is bottom-to-top then the "bend" at the top may be due to rolling shutter if the top of member 12 suddenly moves to the left.

RE: Miami Pedestrian Bridge, Part XI

In what now seems like a long time ago, several of the more experienced members of this forum were discussing and drawing pictures of how much of 11 and 12 were missing. It seemed like there was more missing than was accounted for on the top of the pier and the deck. Someone suggested that NTSB should check the bottom of the canal. I think this may still have merit.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie (Computer) 9 Jul 19 15:34)

It seemed like there was more missing than was accounted for on the top of the pier and the deck.

The missing stuff was probably in the big debris cloud visible in dash cam frames 079-080. I think it was caused when the 12-deck joint literally exploded.

(Apologies in advance if somone figured this out earlier - this thread is getting way too long.)

When the canopy and deck both hinged north of member 10, the bridge was divided into 4 sections:
  • A solid truss between members 1 and 10, rotating counter-clockwise (when viewed from the west) about a pivot point on the south pier.
  • A solid "L" shape formed by member 12 and the north canopy.
  • The deck section north of member 10.
  • The remains of member 11.
EDITED to add analog clock analogy, with truss pivot point located at center of the clock:
The "L" shape and north deck may have initially remained connected in some way, but as the 1-10 truss fell:
  • The north canopy and north deck were pinched together at their southern ends (bending the "L" shape if member 12 is still firmly anchored to the deck).
  • The north deck was pulled southward because its hinge point is below the truss pivot (big clock hand moving from 9 o'clock toward 8).
  • The north canopy was pushed northward because its hinge point is above the truss pivot (big clock hand moving from 10 o'clock toward 9).
These effects spring-loaded the "L" shape, and 12-deck joint ruptured explosively (based on the rapid growth of the debris cloud, visible in the dash-cam video that was running at 5fps) as the north deck was literally pulled out from beneath member 12, and the bent "L" shape straightened itself out.

The spring-loaded "L" theory might also explain the apparent "bent" shape of column 12 seen in traffic cam Frame 057, which was taken at about the same time as dash cam Frames 077-078.

RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie (Computer)9 Jul 19 15:34 Someone suggested that NTSB should check the bottom of the canal. I think this may still have merit. SF Charlie)

I think I recall an excavator dredging the canal at the time of the collapse. ?? Did that activity continue?

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance Wiley (Structural) 9 Jul 19 17:22)

I think I recall an excavator dredging the canal at the time of the collapse. ?? Did that activity continue?

After reviewing the North View timelapse, it appears the excavator parked immediately, remained stationary for a few days, then all the equipment from that area was removed.

RE: Miami Pedestrian Bridge, Part XI

Here are before/after views of the canal from the North View timelapse, enlarged 400% by nearest neighbor algorithm (each original pixel becomes a 4x4 block of pixels). I couldn't crop any further north because the bottom of the images is the actual bottom border of the original video.

I can see some debris on the concrete landing north of pier, but no dust poofs in the dirt or splashes in the canals - plus the canal is still roiled after the excavator lifted a bucket of sludge.


Cropped Frame 26422 from timelapse video "Bridge-109 Mar 8-19 2018 600X-1080.mp4"


Cropped frame 26423 from timelapse video "Bridge-109 Mar 8-19 2018 600X-1080.mp4"

RE: Miami Pedestrian Bridge, Part XI

Quote (Mike)

I can see some debris on the concrete landing north of pier, but no dust poofs in the dirt or splashes in the canals

It may be ejected debris that landed north, or it may not. It could just be shadows


edit: ok, it's probably debris:


RE: Miami Pedestrian Bridge, Part XI

The OSHA investigators evidently never looked at the dash cam or traffic cam videos:

Quote (OSHA report, page 83)

Diaphragm II experienced a blow-out of concrete at the junction of diagonal 11 and column 12 creating a hole shown below in Figure 58 through Figure 70. As a result, column 12 lost support over the pylon and failed with the top tilting approximately 80 degrees towards the south, as shown in Figure 52. The base of column 12 shifted a few feet towards the north but remained on the top of the pylon. Thereafter, collapse of the canopy, diagonal 11 and the deck followed.

Let me rewrite that in simpler terms (with my comments):
  1. Diphragm II suffered a blow-out at the junction of members 11-12, as shown by a bunch of pictures. (Some of the damage shown in the pictures may have been caused when member 12 separated from the deck and fell behind it, *ADD* and when 12 slipped off the deck and sandwiched 11 between the canopy and deck - see dash cam Frame 81.)
  2. As a result, column 12 fell over 80 degrees to the south. (How? The top of 12 was firmly attached to the canopy.)
  3. The base of 12 shifted a few feet north but remained on top of the pylon. (It slid completely off the pier, dropped 3-4 feet, and was dragged back on top by the truss canopy.)
  4. After column 12 fell over, the remainder of the bridge collapsed. (When member 11 failed, the deck hinged north of the member 10, and member 12 began it's wild ride midway through the collapse.)
I wonder how their interpretation of the collapse affected their explanation of its causation?

RE: Miami Pedestrian Bridge, Part XI

Quote (Mike W7)

As a result, column 12 fell over 80 degrees to the south. (How? it was still attached to the canopy!)

There was a plastic hinge created in the deck at the #11/#10 joint. Once the base of #11 can't transfer the horizontal shear, there are large bending forces in the canopy (hinging at the joint) and in the deck where the moment is maximum at the #10/#9 joint. The canopy and deck can not withstand these bending forces (not even close to withstanding the bending moments) which causes the plastic hinging. As the truss collapses, the canopy/#11/#12 triangle rotates the noted 80 degrees. The large bending forces in the canopy can also make it curve (assuming plane sections, the curvature in the non-plastic region is M/EI but you also get curvature and even complete rotation in the plastic region).

RE: Miami Pedestrian Bridge, Part XI

Quote (jrs_87 (Mechanical)2 Jul 19 12:54 This larger fragment of 11/12 node (green) is of interest to me. How it separated with this size and shape should be explored. T)

Since this is back on the table for discussion, the large chunk in the photo of 2 July post looks like it had a formed or finished surface. From my jigsaw puzzle days it would tell us that the piece was from the side or top of something - maybe. I held hopes that NTSB would retain and assemble the larger pieces as being important in defining the mechanism of collapse.
This piece seems quite large to be devoid of reinforcing - particularly at an important zone.

RE: Miami Pedestrian Bridge, Part XI

Quote (Earth314159 (Structural) 10 Jul 19 02:43)

As the truss collapses, the canopy/#11/#12 triangle rotates the noted 80 degrees.

Thanks for the explanation of how the hinging worked, but my point was that the OSHA report states that the blow-out caused column 12 to fall over and assume its final position (with the 11-canopy end of the triangle touching the deck - see picture 52, page 91) BEFORE the bridge collapsed. This is not what happened, at all.

The dash cam video CLEARLY shows that the bottom of member 12 slipped completely off the pier, fell behind it, and was then dragged back on top of the pier by the attached canopy as the mid section of the bridge continued to fall - see dash cam frames 80-83. The triangle assumed its final position (OSHA picture 52) after the deck was pulled off the pier (frames 83-85).

ADD: After 12 slipped off the deck, member 11 was almost completely sandwiched between the canopy and deck (dash cam frame 81), which may explain why so much of 11's bottom end is missing in OSHA picture 52. After this event the "triangle" is just an "L" (member 12 and canopy) with the shredded remains of member 11 attached to the canopy.

RE: Miami Pedestrian Bridge, Part XI

In frame 78, the angle between 12 and the canopy has decreased. Could the concrete have withstood this? Or is the rebar just holding them together????

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie (Computer) 10 Jul 19 04:32)

In frame 78, the angle between 12 and the canopy has decreased.
It's hard to make out 12 behind the light pole and man lift arm, but the debris cloud appears to start in frame 77 so it's possible the base of 12 is already sliding northward, allowing the 12-canopy "L" to tip.

That 12-canopy junction was pretty strong. The "L" was pinched inward as the deck was pulled out from under 12, then it was spread outward as the canopy dragged the "L" back on top of the pier, yet all the pictures I can find show it to be intact after the collapse.

RE: Miami Pedestrian Bridge, Part XI

Quote (Wetlander)

Six more companies are agreeing to settle lawsuits, including FIGG and Bolton Perez & Associates

Smart move on their part. I can't see any good defense for this case.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7)

Thanks for the explanation of how the hinging worked, but my point was that the OSHA report states that the blow-out caused column 12 to fall over and assume its final position (with the 11-canopy end of the triangle touching the deck - see picture 52, page 91) BEFORE the bridge collapsed. This is not what happened, at all.

I don't think the OSHA report explained the collapse very well. It could be because they were trying to write it for laypeople rather than structural engineers. There are more issues with the report besides this one. I also question the judgement of the authors based on some the things they commented on. I wasn't overly impressed.

The reason the bottom of #11 was ripped out was because the anchor for the lower PT bar was anchored into the deck and stayed with the deck. #11 stayed on the abutment. As the deck fell, the rod ripped out the back of #11. The first few ties in #11 may have (likely) been severed during the initial slide to the north. The back face of #11 was weaker than the shear capacity of the rod.

The base of #11 and #12 would have been push north as you suggest. This is consistent with the analysis of the failure mechanism.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh (Structural)9 Jul 19 22:19)


In your post referenced above I see an opportunity to pretty well estimate the remaining length of member 12. Using a ruler and scaling the 6 foot pylon as 10 - 1/8" divisions and the length of 12 as 26 divisions my crude methods suggest the north face of 12 is 16 feet + long. (hard to pick a measuring point for the top at the canopy).
Can any 'vid gurus' improve the accuracy - or is this close enough?
If the (now) north end of 12 was the top of the deck (joint) that should tell us something. Perhaps of no importance, but something.

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance)

Using a ruler and scaling the 6 foot pylon as 10 - 1/8" divisions and the length of 12 as 26 divisions my crude methods suggest the north face of 12 is 16 feet + long. (hard to pick a measuring point for the top at the canopy).

Here is my attempt. The units are pixels.



If the pylon is 6 feet I measure the 205 pixel long arrow as ~205/70 = 17.6 feet


No adjustments made for parallax. Just a simple scaling

According to drawings the top of canopy to deck dimension is 17.2 feet. This would suggest the damaged 12 member we can see is approximately full length?

RE: Miami Pedestrian Bridge, Part XI

Vance Wiley (Structural) Thanks for your probing question and estimate. Tomfh (Structural) Thanks for showing your work. It seems to show that the north face of 12 remained largely intact. I seem to remember that the rebar going up into 12 remained straight up in the top of the diaphragm. I also observed what I interpreted as a cloud of debris on the deck expanding in a couple of frames of the dashcam video (about the time I interpret 12 to have just started to tilt (top of 12 barely visibly south of its previous position)). Photos of the NTSB examining the north deck seem to show both ends of 11 damaged, so I'm having difficulty determining how much of each end of 11 disintegrated. So, I find myself asking, was the compression in 11 enough to cause it to explode, or did it require 11 to gain some momentum, or what?
Was there a cold joint at the top of 11, I think the canopy blisters were cast last.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (Wetlander)

Six more companies are agreeing to settle lawsuits, including FIGG and Bolton Perez & Associates. https://www.nbcmiami.com/news/local/More-Defendant...

So what is next? I wouldn't think a settlement absolves anybody from liability.

RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie (Computer) 11 Jul 19 14:00)

I'm having difficulty determining how much of each end of 11 disintegrated.
Here's a JPEG of OSHA Figure 52 on page 91 of report. (I don't have a link to NTSB pics.) If you can reposition 11 to its original angle with the canopy that will give you a starting point. The pier width can be used a ruler, but parallax is a problem because you have to guesstimate distances from camera position.

RE: Miami Pedestrian Bridge, Part XI

Thanks MikeW7 (Electrical)
I can use the width of 11 or 12 to calculate the remaining length of 11. What I don't know is how much of 11 was destroyed at the 10,11 joint. The first visible movement was the 10,11 joint lowering, yet the top of the blister at that joint seems to remain intact? The angle between the north section of the canopy and 11 seems to remain constant for several frames. The canopy seems to drop more than the deck in the first few frames. How is this possible, the south canopy, 10, and the south deck seem to remain intact? Also, the 12, deck joint seems to remain intact during this first few frames. My thought is that 11 can slide on the deck and the horizontal vector of the compression is focused on the fillit. Wouldn't the compression be highest in the narrower fillit?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

One more thought... What are all the white things sticking out the north end of the deck/diaphragm?

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (SFCharlie (Computer) 11 Jul 19 15:4)

I can use the width of 11 or 12 to calculate the remaining length of 11

Ref. OSHA Figure 52 - The bottom face of 11 appears to be shredded, possibly when it was sandwiched between the deck and canopy - Frame 81, more likely due to PT rod being ripped out, as noted by Earth314159 (Structural) 11 Jul 19 16:36.

When the sandwich occurred, the top section of 11 probably moved south quite a bit (stripping it of concrete) since the end section was jammed in some way against 12. It's possible most of the lost length occured at the TOP end of 11 instead of the bottom end where the initial failure presumably occured.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7)

The bottom face of 11 appears to be shredded, probably when it was sandwiched between the deck and canopy

The bottom of 11 is shredded because the PT rod rip off the bottom of the member as the deck fell to the ground.

The blistered popped off because there was a massive deceleration when the bridge hit the ground and the canopy had a punching failure at the joint.

RE: Miami Pedestrian Bridge, Part XI

Thank you, Tom, Charlie, and Mike. I am having a bit of a problem with my first concept of this failure.
On March 15 of 2018 the first thought was the truss lost a heel joint at the north end. I am still of that opinion.
The actual damage sequence is a bit more elusive to me. From the photo by Tomfh, west side, and the one by MikeW7, east side, the top joint of 12 to the canopy appears basically intact. And it appears from measurements (scaling display and counting pixels - wish I could do that) of the north face of 12 that the face is not blown away at the bottom, as I would expect if the joint 11/12/deck failed and moved north a foot maybe.
Is the geometry such that it 'protected' 12 throughout this bumpy ride?
The splits and separations in the lower part of 11 and across the top of the fillet zone between 11 and 12 are a clue to the condition of 11 just before the collapse.
If the cold joint is moving, that would induce angle changes in 10/11/canopy and 11/deck with subsequent stresses from bending.
Did member 11 simply explode just above the fillet (or did both 11 and the fillet to 12 explode), with the resulting shortening of 11 allowing node 10/11 to begin dropping freely? With the damage at the canopy/12 joint appearing minimal (compared), the dropping of 11/12 pulled the top of 12 to the south, prying it from the bottom joint casting, preserving the north face? There is a lot of concrete missing from the bottom of 11 and the formed joint of 11/12/deck.
If 11 "exploded", that would have been pretty sudden. I am just a bit surprised that the north face of 12 is pretty much intact.
EDIT ADD: With the concrete having departed the lower portion of 11 only the reinforcing remained, limiting any further influence of 11 primarily to tension thru what anchorage of reinforcing remained. Thus 11 was able to pull some of the remains back to the south face of the pylon.

RE: Miami Pedestrian Bridge, Part XI

Earth314159 (Structural) 11 Jul 19 16:36 - Fixed my post of 11 Jul 19 16:22 - THX!

RE: Miami Pedestrian Bridge, Part XI

@SFCharlie - white things are various conduits running the length of the deck (both sides).

RE: Miami Pedestrian Bridge, Part XI

I suspect that because 12 protruded beyond the diaphragm, that the bottom of 12 hit the top of the pier, when the deck fell. this would be consistent with 12's north face remaining intact and the rebar in the diaphragm sticking straight up. This puzzle has many pieces...

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

Quote (FCharlie (Computer) 11 Jul 19 17:15)

This puzzle has many pieces...
Now that the autopsy is reaching its end stage, the CSI work gets more attention....

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7 (Electrical)11 Jul 19 17:12 the description of a Warren Truss by saikee119 (Structural) 7 Jul 19 14:39 implies that the 11-to-12 canopy section as well as the upper section of 12 (i.e. the 12-canopy "L" section) are not a functional part of the bridge at this stage of its construction, so why would the 11-12 blowout cause 12 to move a foot north? It would seem that the base of 12 remained in place until the bridge started to fall, then the combination of the deck-canopy pinch at the member 10 hinge points plus the deck being pulled south and canopy being pushed north caused the "L" to separate from the deck.)

The description of the Warren truss is focused on the actual "truss action" of the structure. The canopy north of joint 10/11 does not support any "truss" forces (except, in this case, residuals from continuety of cast joints vs. pinned joints). True "trusses" support loads by axial forces in members only - those members may have loads applied between their ends, causing bending in some members but that is another matter.
In this case, the canopy is attached to the "warren truss" and extends from node 10/11 to coplumn 12. And the canopy is continuous, complicating matters again. But the "truss" would have been a truss without member 1 or 12 and without the canopy between 1 and node 2/3 or the canopy between node 10/11 and member 12. Other things would have to provided stability to the top joints 2/3 and 10/11 for the structure to be stable but the "truss" action would remain complete.
But the canopy and the column 12 are connected to the structure, and will be pushed or pulled depending on what is happening at their connections to the truss. If joint 11/12/deck is sliding, the bottom of 12 must go with it because they were cast together. At what point they separate and act independently (or react) will depend on the damage and its progression.
The initial loss of "truss action" seems to be due to the loss of the joint of 11 to the deck or the internal failure of 11. Either would render member 11 useless (or so limited as to be so) and result in a loss of vertical component to support node 10/11, causing that node to begin to fall.
Because of the cracking of the deck as photographed before the collapse, the node 11/12/deck is the most likely culprit to have initiated this failure. A failure at the top of 11 would have been just as catastrophic. But that probably would not have left the north end of the deck with the appearance of a blowout as we can now see.
Thank you for your comments. I invite you and others to clarify mine.
EDIT ADD:
The "pinch effect" mentioned is a very real part of this failure - the length of the canopy from its connection to member 1 to node 10/11 is basically fixed until it hits the roadway, and adding that length to the length of diagonal 11 should force the bottom of 11 and 12 maybe 4 feet north as node 10/11 passed thru the elevation of the top of the deck. After that, node 10/11 pulls member 11 and the bottom of 12 into the collapse. The 4 feet would basically allow 12 to remain on top of the pylon. The geometry is a part of the final positioning of the parts.

RE: Miami Pedestrian Bridge, Part XI

Vance Wiley (Structural) 11 Jul 19 17:45 - appreciate the lengthy reply. I deleted my question before you posted your answer because after re-reading saikee119's post I realized there was a lot more going on - canopy tendons were anchored at the end of the canopy above 12, etc.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7)

The truss is kinda basic to structurals - both in concept and historically. A truss can use short pieces to create a long span. After laying a log across a stream, then encountering a stream wider than the length of a log, the world created structural engineers.
Thankfully, not many of the trusses they design are made using concrete.
It was a good question - I only hope my response clarified some concepts.

RE: Miami Pedestrian Bridge, Part XI

Vance Wiley - If the base of 12 was kicked north a foot during the blow-out, that would also bend the attached canopy, based on the apparent rigidity of the 12-canopy joint. Given that the canopy tendons were anchored in that section of canopy, it would require a LOT of energy to bend the canopy, wouldn't it? Seemingly much more than was released during the blow-out. Either that, or the kickout may be what initiated the 10-11 canopy hinge, which was then followed by the 9-10 deck hinge. Saikee119 (Structural) 7 Jul 19 14:39 discussed this, and the possible order of events, and I'm sure many others have as well, but my memory is not that good anymore, and my "reading comprehension" of structural terminology is limited. My contribution to all this is establishing that the "L" appears to be very rigid.

RE: Miami Pedestrian Bridge, Part XI

(Edit: With all due respect), my analysis of 29 Jun 19 17:27 is correct. The bridge gained a degree of freedom when 11/12 snapped free (audibly even) of the upper half of the deck/diaphragm and the bridge truss sagged. The same strain was occurring at the south end but it had yet to break free. 12 bowed slightly to the north as it was pressed on by 11. The greater movement was the deck sagging and moving past 11/12 in the opposite direction. The deck was pulling the heel of 11 down with it and preventing the heel from following the toe and 12. The result is that 11 is split while the illusion is created that it is sliding along the deck, it is not!

The torque action that I see is not one of a wheel spinning on an axis but of a stress field developed between the diaphragm and 12. The diaphragm is twisted. It has to contend with the longitudinal PT cables pulling the wings of the deck to the centre of the span and the truss members pushing the centre of the diaphragm away from the centre of the span. In addition to this, at least for a time, the diaphragm had to contend with distributing the vertical load to the poorly placed shims. The internal stresses in the remaining interface are complex and a mountain of rebar in the diaphragm/12 connection absorbed this punishment.

The cracks in the slab and diaphragm are from the twisting of the diaphragm and the sticky movement of the deck past 12. OSHA drew a beautiful sketch with triangles relating shear between 11 and the first PT cables but this is nonsense. The plastic pipes cannot transfer shear nor could the balance of the ill purposed interface. The members were not contiguous at that level.

It is not reasonable to think that the distressed 11 had the resolve to destroy 12/diaphragm. The video shows the collapse in motion before any substantial vertical descent of 12. Also, if the node blew out first, this would have instantly relieved 11 and it would have remained in tact. This and the fact that crews were tensioning the PT rods leads me to believe that 11 failed next. There are two possibilities. One, the toe of 11 crushed, and two, the bending moment on 11, imparted by the sagging deck, combined with the dead load and increasing compressive load wrought by the PT rods caused a failure towards the center of 11 (note the mangled rebar at the remaining end). I suspect an impulse blew apart the balance of 11 and the node as a pancake failure would mangle the rebar.

12 broke out of its lower rebar when it was yanked to the south and down by the canopy.

RE: Miami Pedestrian Bridge, Part XI

I'm afraid that the time it took me to develop this info left me behind the curve, but here it is anyway.
Member 12 East and South faces: 3' x 10.45' from bottom of canopy to start of crumble.
Member 11 East and South faces: 2' x 17.20' from extrapolated top of deck to start of crumble.
Obviously there is some distortion from the wide camera lens and proximity of the camera to the wreck of the bridge.

SF Charlie
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RE: Miami Pedestrian Bridge, Part XI

MikeW7 (Electrical)11 Jul 19 19:13
If the base of 12 was kicked north a foot during the blow-out, that would also bend the attached canopy, based on the apparent rigidity of the 12-canopy joint.
Correct. If the canopy stays level - but the canopy does not have enough strength to support the truss reaction . If we focus on the top of member 11 at node 10/11, that node must stay in the air with only very small elastic deflections - and it stays there only because it is "propped up" by member 11. Relieve that "propping force" and it will fall. Things will happen instantly unless there is a lot of ductility - so if the bottom of 11 slips, the top (node 10/11) will drop - instantly. If member 11 fails, node 10/11 will drop. Instantly, unless 11 develops capacity after failing and splitting - and that did not happen. Something about the geometry allowed the canopy to fail at the north side of the blister (or north side of the intersection with 11) without cranking much moment into the joint of 11 12 and the canopy.

Given that the canopy tendons were anchored in that section of canopy, it would require a LOT of energy to bend the canopy, wouldn't it? Seemingly much more than was released during the blow-out.
The energy required would be only a small part of the forces in play at the end of a 950 ton structure. The canopy is a curved section with partial PT forces - it is not fully developed at the time of the collapse. And the bending capacity of the canopy is very small compared to the demand for support generated by this structure. The top of 12 is pulled toward 10/11 by the PT tendons, but that only served to maintain the distance between the top of 12 and the break in the canopy.

Either that, or the kickout may be what initiated the 10-11 canopy hinge, which was then followed by the 9-10 deck hinge.
I see the canopy hinge as a result of the vertical dropping of node 10/11. The north end of the canopy was supported by 12 over the pylon. I think the "kick out" was simultaneous with the dropping of node 10/11, perhaps a millisecond or two behind. First, as I see it, was the loss of capacity in the cold joint at the deck - and any horizontal slip immediately released some of the support whick was keeping node 10/11 in place.

the "L" appears to be very rigid.
It was enough to survive a wild ride. But not intended to provide major support to the entire structure.

RE: Miami Pedestrian Bridge, Part XI

Refer to the cut-and-paste mashup below for an clearer explanation of what I am trying to describe. Details after the images. Apologies for the north-south reversal - this was the best diagram I could find.



  • The red diamonds are the fixed pivot points atop the piers.
  • The red slashes are roughly where the hinging occured.
  • The red X indicates where 11 failed (more or less).
When the span started to fall:
  • The hinge points forced the north sections of canopy and deck to "pinch" - move closer together.
  • The north deck section was pulled south because its hinge point was below the plane of the pivot points.
  • The north canopy section (and the upper section of member 12) was pushed north because its hinge point was above the plane of the pivot points.
These effects combined to force 12 off the north side of the pier.
ADD: Estimated maximum distance the bottom of 12 moved was was 4 feet, according to Vance Wiley (Structural) 11 Jul 19 17:45

The lower section of member 12 did not (could not) begin moving south until the truss section between members 1-10 collapsed on impact with the ground (starting at dash cam frame 82) allowing the truss canopy to drag the 12-canopy "L" to its final resting place.

RE: Miami Pedestrian Bridge, Part XI

Quote (Mike W7)

The red slashes are roughly where the hinging occured.

You want to move the upper plastic hinge to the left (centre line of the joint) so that the canopy/#11/#12 remains more or less as a triangle. But that is the rough concept otherwise.

RE: Miami Pedestrian Bridge, Part XI

Quote (Earth314159 (Structural) 11 Jul 19 21:20)

You want to move the upper plastic hinge to the left (centre line of the joint) ... But that is the rough concept otherwise

Yeah, it really doesn't matter exactly where the hinge is. What's important is that there was a hinge in the canopy, and this pushed the 11-12-canopy diamond (or 12-canopy "L") to the north, and popped the upper section of 12 over the north side of the pier. Except for what remained attached to the deck, the 11-12 joint never moved south until the truss canopy collapsed on impact and dragged the diamond/"L" southward.

RE: Miami Pedestrian Bridge, Part XI

The triangle and hinge location is important since it keeps #12 more or less square to the canopy.

RE: Miami Pedestrian Bridge, Part XI

Quote (Vance)

Is the geometry such that it 'protected' 12 throughout this bumpy ride?

Makes sense. The north face of 12 was inside the failure block, well back from the front line, so would have been spared much of the damage.

Quote:

scaling display and counting pixels - wish I could do that

https://www.getpaint.net/

You can draw lines and it tells you their length in pixels.

RE: Miami Pedestrian Bridge, Part XI

The measurement by Tomfh of 17'6" agrees well with the photo documentation.


Another view of #12


Following up on the last post by Sym P. le (Mechanical) - More and more it looks like the failure was purely the 11/12 anchor zone of the Upper PT bar. That the fractured mass of concrete & rebar in front of the anchor plate yielded. It is certainly true that prior to work, to retension the PT bars in #11, that all the rebar in the 11/12 node was resisting the weight of the bridge but when the front of the 11/12 node failed towards the south, #12 was momentarily relieved of performing any work other than self weight & the canopy.

The base of 12 was already hinging upwards & outwards before the collapse. That is why their are cracks in the base of 12 before the collapse. Because it is bending at the deck/diaphragm elevation.

It is hard to perceive from the photos available but I think it is very likely, that the #11 upper PT bar and the concrete surrounding it, had failed completely during detensioning and that the only thing holding the bridge in place was Rebar & mechanical interlock of the broken pieces of concrete. That isn't to say that there wasn't progressive cracking but that the concrete was now little more that a pinned & caged rock formation, holding up a bridge. The more you group the cracks into a whole, the more it becomes clear #12 was lifted slightly during the initial cracking at the time of detensioning.

The deck and diaphragm haven't moved but the failed concrete still homogeneous to 12 has lifted. (Rotation)




It appears more & more, to my my mind, that the front of the 11/12 node failed to the south.




This kind of failure, if accurate, sort of raises the issue as to the procedure used to detension the PT bars. I've managed too many jobs to rule out and "Oh Crap!" moment. Most jobs have a number of them. It is just the nature of the people doing the work. You can give people all the training in the world but there are still instances, where if you don't spell it out for them, they revert to stupid. This is where I go back to Member 11.



Here in California, residential post-tensioned concrete has warning signs posted in the garage & stamped into the concrete at the point were the garage floor meets the driveway. It is just one example of the extreme hazards associated with Prestressed & Post-Tensioned concrete. The FIU Bridge plans & Structural Groups, VSL shop drawings are all loaded with Caution & Danger warnings. The work area hazard & safe zones for PT work are defined. Likewise the majority of Figg's work is Post-tensioning structure, including cantilevered segmental bridge work. Does anybody really believe that Post-Tensioning Institute or PCI would ever suggest that is was OKAY to tension a cracked concrete member? VSL should never have agreed to retension #11 and should have known better. Figg was just completely out of their minds. BPA was out of their depth but should have also understood that retensioning was a harebrained & dangerous idea. It begs the question whether firms offering Inspection services should be required to have one member that has been through some failure analysis course work, so that when things go sideways, they can step up.

RE: Miami Pedestrian Bridge, Part XI

Quote (Tomfh)

.... Paint.net

Wow! I remember trying this out while this was still a student project by a kid at at Wazzu. VirtualDub was also a student project, and IrfanView was released when the author was 22 (as a DOS only program, if I remember), so I assume it was a class project as well. Smart kids!

RE: Miami Pedestrian Bridge, Part XI

Quote (epoxybot (Structural) 12 Jul 19 00:29)

The base of 12 was already hinging upwards & outwards before the collapse.

If the span was starting to sag at all, does this up-and-out lifting of 12 correspond to what I was trying to explain in my cut-and-paste mashup of 11 Jul 19 20:46, or is it just a coincidence? I'm not a structural guy, but once I figured out that concept the whole collapse video started making perfect sense to me.

ADD: Even without hinge points, it seems like the same pull-deck-south and push-canopy-north dynamic should hold true if the span sags, just on a much smaller scale.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7)

The north canopy section (and the upper section of member 12) was pushed north
I like your diagram and agree with your interpretation. I do note that the canopy/12 node was significantly crimped during the collapse before springing back. The damage can be seen on the canopy roof on the post collapse photos, not to mention the damage to 12 at the node. This would have been pushing on the top of 12 from the earliest stages and would participate in opening the vertical crack at the filet of 11/12.



RE: Miami Pedestrian Bridge, Part XI

Quote (epoxybot)

VSL should never have agreed to retension #11 and should have known better. Figg was just completely out of their minds. BPA was out of their depth


I think we all agree it wasn't a wise move to retension!

But I can understand why experienced engineers and stressors would take that decision, even though with the benefit of hindsight it was clearly a bad decision.

RE: Miami Pedestrian Bridge, Part XI

Quote (Epoxybolt)

Does anybody really believe that Post-Tensioning Institute or PCI would ever suggest that is was OKAY to tension a cracked concrete member?

Can be the correct thing to do depending on the circumstances.

RE: Miami Pedestrian Bridge, Part XI

Epoxybolt,

I don't think that the diagonal cracks in 11 and the vertical crack on the south side of #12 are the same as indicated in the note in the marked up photo. I think you are correct in stating that the base of 12 was rotating which caused the vertical crack on the south side of 12.

There was a large increase in vertical steel in 12 compared to the rest of the joint. The south three feet or so was sliding. The base of #12 was not going to move so easily. #11 is essentially pushing the bottom of #12 which is being resisted and anchored by the #12 vertical bars. This is why there is also some rotation. The base of 12 is firmly anchored so the chunk of concrete at the base of 12 starts to rotate outward.

#12 had 3#11 vertical bars on the south side and a total of 9#7 bars on the other faces. Those bars essentially stopped the chunk of concrete at the base of 12 from sliding.

RE: Miami Pedestrian Bridge, Part XI

I think I cover the concept of using PT in fractured conditions with Rock Anchors. None of it is done without some fairly well defined conditions of the substrate, which is absent in the FIU Bridge circumstance.

MikeW7 (Electrical) - I went back and watched the video. The South Diaphragm is pulled to the North during the collapse.


I think the tortured nature of the rebar in the base of 12 suggests that it momentarily stood independent of 11, dropped ~straight~ down and slid out the back of the north diaphragm, before being dragged back to the south by the diaphragm and the deck.



RE: Miami Pedestrian Bridge, Part XI

Earth314159 (Structural) - I'm a bit confused by your 1st paragraph and photo references.

The 2nd paragraph agrees with my thinking. The concrete has failed and the rebar is ballooning out the back of 12 to the north and vertically. The rebar in the back of 12 is also more securely developed in the diaphragm.

RE: Miami Pedestrian Bridge, Part XI

Quote (Epoxybolt)

The 2nd paragraph agrees with my thinking. The concrete has failed and the rebar is ballooning out the back of 12 to the north and vertically. The rebar in the back of 12 is also more securely developed in the diaphragm.

I speaking of the cracks prior to failure. It took more stress to move the base of #12 than the base of #11. Hence you get the vertical crack on the south side of 12 and rotation prior to the final failure.

I am not too sure which side of 12 is the "back" side. You mention "A1 crack" and the vertical crack as being the same crack. I am not too sure why you are doing that. The diagonal cracks in #11 are due to shear friction and the vertical crack in #12 is due to rotation at the base of 12.

RE: Miami Pedestrian Bridge, Part XI

Quote (epoxybot)

The base of 12 was already hinging upwards & outwards before the collapse
I'm not sure I follow the ascension theory. It's been raised before but I've always seen this as a descent into the depths... Seriously though, we don't have a solid baseline to reference the distortions and I think the engineers were at a disadvantage as a result. I also tend to believe that the 11/12 node cradled in the deck/diaphragm was largely intact. The surface flexure cracks could be picked up by the rebar. My idea of 11/12 snapping free was with regards to the surfaces as indicated in the following diagram or adjacent areas where 11/12 could easily be detached from the deck/diaphragm. Consider the chevron areas as gaps with no rebar and the deck lowered at the red ARROW as a quick example of the freedom of movement that would be allowed.


RE: Miami Pedestrian Bridge, Part XI

Earth314159 (Structural) I reference (A1) as one singular crack formation that travels down the west vertical face of 11, then up and across the south vertical face, at the base of 12, that is, above & across the 11/12 node/fillet and then up the east vertical face of 11. The backside of 12 being the northerly face.

I refer to failure (Concrete) as something that has taken place prior to collapse.

I'll try and explain myself further tomorrow.

RE: Miami Pedestrian Bridge, Part XI

Quote (epoxybot (Structural) 12 Jul 19 02:26)

The South Diaphragm is pulled to the North during the collapse.

As the 1-10 truss section begins to fall it appears to mostly just tilt at the pivot point formed by the south pier bearing pads. The south diaphragm isn't significantly pulled north until the deck hinge hits the pavement and the 1-10 truss flattens out.

From the dash cam video: ZIP file of cropped frames 074-088 and AVI video of the south end, all "enlarged" with nearest neighbor algorithm (each pixel replaced by 4x4 block of identical pixels) to minimize artifacts.

Relevant frames 076, 080, 081 and 087:

South end - Frame076


South end - Frame080


South end - Frame081


South end - Frame087



Meanwhile at the north end, when the deck hinge hits the pavement at Frame081, the north diaphragm is almost completely off its pier and member 12 has fallen behind the pier. A fifth of a second later, at Frame082, the north diaphragm is off its pier:

North end - Frame081


North end - Frame082


Note that in Frame082, the "falling man" is still in the air. What an incredibly fast sequence of events....

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7)

What an incredibly fast sequence of events....
It sure is. The most important piece providing support had signaled for days that it was in trouble.
One of the Prime Directives for designing Reinforced Concrete is to NOT over-reinforce it. The codes provide limits to the amount of reinforcing which can be used. Specifically, to ensure the steel fails before the concrete fails. The reason ? Steel has a long yield curve and can stretch while yielding but retain strength until failure, and therefore undergo a lot of strain. Which usually translates to deflections and cracking and somebody being alerted to a problem.
Not so with concrete - concrete fails abruptly. But it does not disappear - so a column can be highly loaded and still provide support - IF the concrete remains confined with steel ties well detailed. Such a structure in this condition will need serious repairs or even demolition, but it may not fall abruptly.
Taking advantage of the time provided can save lives.
Great work with the images.

RE: Miami Pedestrian Bridge, Part XI

Quote (MikeW7)

Note that in Frame082, the "falling man" is still in the air. What an incredibly fast sequence of events....

The workers were attached to a line strung center-line across the 174 foot canopy. This worker was held aloft either by being entangled with something during collapse or because he intentionally attached his harness to crane cable at some point. In any case your image is just after his harness failed. Read OSHA citation for that violation (Sep 9 2018, pages 6-7): https://www.dol.gov/sites/dolgov/files/OPA/newsrel...

RE: Miami Pedestrian Bridge, Part XI

The falling guy - take a yardstick, set one end on a support and hold the other level. Put a coin onto the held end and then let the yardstick go. The coin will fall more slowly than the end of the yardstick.

RE: Miami Pedestrian Bridge, Part XI

jrs_87 (Mechanical) 12 Jul 19 22:00 - The canopy first deflected in frame 077, and the deck hinge point hit the pavement at frame 81, 0.8 seconds later. The falling man didn't have his crotch straps buckled and the only resistance his harness offered was the time it took to yank his arms up to allow him to drop out. His feet were never more than 2-3 feet above the canopy all the way down. What an incredibly fast sequence of events....

RE: Miami Pedestrian Bridge, Part XI

Not sure how the harness could see any resistance. The safety cable was attached between the two ends of the canopy which were coming together and a fast pace; the cable would have gone slack almost instantly.

RE: Miami Pedestrian Bridge, Part XI

Quote (3DDave)

The falling guy - take a yardstick, set one end on a support and hold the other level. Put a coin onto the held end and then let the yardstick go. The coin will fall more slowly than the end of the yardstick.

It is counter intuitive but true. The south bridge support adds a rotation force (a couple between the CG and the south support) that allow the tip of the yard stick or north end of the "remaining bridge" (for lack of a better description of the south portion of the bridge) to accelerate faster than G. The CG of the remaining bridge is accelerating slower than G but the north end of the remaining bridge is accelerating faster than G.

RE: Miami Pedestrian Bridge, Part XI

Quote (3DDave (Aerospace) 13 Jul 19 01:57)

Not sure how the harness could see any resistance.

He had his lanyard attached to the crane lifting hook, so he's probably part of the mobile crane crew that positioned the jack equipment for VSL. See the images posted by saikee119 (Structural)8 Jul 19 16:38 - he's yanked up in frames 78-79 as the canopy drops out from under him, then falls out of his harness (with his arms over his head) in frame 80. After that he's in free fall and stays within a couple of feet of the canopy. Frames were taken every 0.2 seconds (5 fps).

Midsection of the canopy was 12.5 feet wide with a raised lip on each side, so it's no surprise he had his crotch straps undone. That's like a basketball court for guys used to working at elevated heights. I know because I was one of them - drove a mill crane and worked with millwrights and iron workers...

EDIT ADD: I think the six people on the bridge were part of different crews, so each crew may have had their own safety protocals.

RE: Miami Pedestrian Bridge, Part XI

Quote (JRS 87)

Read OSHA citation for that violation (Sep 9 2018, pages 6-7): https://www.dol.gov/sites/dolgov/files/OPA/newsrel...

The citation appears to say that the worker was attached to the horizontal life line in top of the canopy. I don't think that the citation is necessarily issued for contributing to injuries. It was just that proper safety protocol was not followed.

RE: Miami Pedestrian Bridge, Part XI

EDIT: added more info and details on citations

OSHA cited five companies for fall protection violations. I check up on this from time to time to see if the violations are ever closed. The two other companies at the end aren't part of the FIU inspection - they got intertwined with the search somehow.
  • Details for the specific citation to Structural Technologies are in the PDF document cited by jrs_87 (Mechanical)12 Jul 19 22:00
  • Basic information about the two citations (01001 and 01002) mentioned in the link above are as follows:
Citation 01001 refers to Standard 5A0001 OSH Act General Duty Paragraph:
(a) Each employer --
(1) shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees;
(2) shall comply with occupational safety and health standards promulgated under this Act.
(b) Each employee shall comply with occupational safety and health standards and all rules, regulations, and orders issued pursuant to this Act which are applicable to his own actions and conduct.

Citation 01002 refers to Standard 1926.502(d)(16)(iii)
  • 1926.502 - Fall protection systems criteria and practices.
  • (d) - "Personal fall arrest systems." Personal fall arrest systems and their use shall comply with the provisions set forth below. Effective January 1, 1998, body belts are not acceptable as part of a personal fall arrest system. Note: The use of a body belt in a positioning device system is acceptable and is regulated under paragraph (e) of this section.
  • (16) - Personal fall arrest systems, when stopping a fall, shall:
  • (iii) - be rigged such that an employee can neither free fall more than 6 feet (1.8 m), nor contact any lower level;

RE: Miami Pedestrian Bridge, Part XI

Has anybody noticed the jbar in the youtube video > FIU Bridge Collapse Lift off of 12 explains J-bar pristine condition < does not match up to the drawing?
Fig 63 in the OSHA report does lead you to believe jbar was not embedded into deck.
In fact by the pics the jbar (in pristine condition) appears to have been bent to fit the deck after pour and cure.
It's bent to fit and 180 degrees out from the print.
It was supposed to be right next to lower rebar embedded in concrete.
Fig 63 certainly shows it was not.
It's pristine alright and tells a tale.
Keep comparing fig 63 and the video jbar pic and the print.
Just a little detail.
It's just amazing how such a small rebar can go thru all that destruction and come out unscathed........
just how is that possible?

RE: Miami Pedestrian Bridge, Part XI

Quote (RAB678)

It's just amazing how such a small rebar can go thru all that destruction and come out unscathed........
just how is that possible?

I commented on the J-bar in Part X 21 Jun 19 23:22. I came to the same conclusion that the rebar was not in accordance with the drawing.

As far as how the rebar can be unscathed, check my comments in this thread 29 Jun 19 17:27, 11 Jul 19 19:23. In the second I propose that an impluse extending from a smaller initial sudden failure shattered the lower end of 11. I'm interested in hearing others thoughts on this.

RE: Miami Pedestrian Bridge, Part XI

Quote (Sym P. le (Mechanical))

sudden failure shattered the lower end of 11
That is consistent with the dramatic shortening of 11.
Please see my posts:
SFCharlie (Computer)11 Jul 19 19:55
and
SFCharlie (Computer)11 Jul 19 20:11

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XI

Quote (RAB678)

Has anybody noticed the jbar in the youtube video > FIU Bridge Collapse Lift off of 12 explains J-bar pristine condition < does not match up to the drawing?

I believe the hooks were above the shear plan at the base of #12. The shear plane dips down below the pour joint at #12. The concrete above the shear plane was shattered rather than pulled over. It is very interesting that the bar came through relatively unscathed.

I believe the shear plan dips below the pour joint at this location since there is some heavy vertical reinforcing in #12.

RE: Miami Pedestrian Bridge, Part XI

Regarding the clean rebar, another thought. My initial thought was that a hammering impulse from an initial crushing of 11 (mid-length) could be the cause but in addition, this would instantly release the tension in the PT rod which could reverberate and clean the adjacent J-bars. I can't imagine how violent that failure was. I don't have experience in this field.
[Post Deleted]

RE: Miami Pedestrian Bridge, Part XI

Re: Construction Engineering, & EQUILIBRIUM
His web presence has included several channel names. One of his websites was for wedding photography.
He is very enterprising, searching out info on the collapse, including feedback from his subscribers, monitoring this site... He refuses to provides links to his sources. That's how I found this site, search for a link to one of the photos he used in a clip. He has long held that the collapse started somewhere else than 11.12. He holds the participants on this forum in disdain.

SF Charlie
Eng-Tips.com Forum Policies

RE: Miami Pedestrian Bridge, Part XI

I found an active link for the dash-cam video, in case anyone is interested. It's on the FaceBook page for WSVN-TV in Miami, a Fox affiliate.

It has the same resolution (1280x720) as the one I've been using, but it has a larger file size (5.3MB versus 3.3MB) because it isn't as compressed, hence there is more detail in certain areas. You can see more of the safety netting fence posts, for example. The improvements are small, but may be significant is you are trying to glean information from a small cluster of pixels.

I made an enlarged, cropped video for frames 72-90 using the same procedures as before. The video, and frames, are included in this ZIP file.

RE: Miami Pedestrian Bridge, Part XI

So how about this one......
FIU? Bridge photogrammetry 17 by Zac Doyle also on youtube.
On my monitor, I can see the end of the crane, vibrate what looks like twice.
Can anyone back this up? about at 22 seconds in.

This is exactly when the start of fall begins.
You should see vibration and then the magical unexplained mystery bulge at the North end of 12.

If the crane is maybe supporting the tensioning cylinder and the worker.
What makes the crane vibrate twice?
Certainly the weight of 400? lbs would not make it do this?
Also notice a black lifting strap? suddenly shoot upwards out of the picture.
Given the weight of the cable and hook?.......
What would you be doing to load this cable, cause that strap to shoot upwards,
vibrate the crane twice and at the same time the bridge falls?

At the the time they were tensioning 11's rods.
Given the OSHA picture of 11a top? rod was spun in about 2 inches.
Since they claimed they were incrementally tensioning. unknown if incremental or fully before second one.
It would lead one to believe tensioning the lower one dropped it.

Either thru concrete laxation or rebar snapping. or both in reverse order.
Now while 11a (top) rebar appears intact, we could assume......the concrete failed around it at the bottom.
And 11b appears sheared but its bottom still embedded in the deck.

So..........did tensioning make it snap? I would think if the foot moved over the protruding end of 11b rod while intact, then the end would be bent somewhat. But it does appear straight.
The longer portion of the rod itself appears bent near the base end. Which could have happened when 11 slid North, it became trapped and got bent then.

Note: While in the pictures it still appears connected. It could have simply fell back into its previous position. OSHA pictures show some powdered concrete on the bar end.
I wouldn't think they would have cut it right there for examination.
Leading me to believe, it snapped right there.

SO......THE QUESTIONING PART...........
IF you were lifting the canopy to reverse the sagging and close the cracks.
You would be lessening the friction of 11 base area. Meanwhile.... tensioning the bars. NOW the rods would be under apparently MAXIMUM tension. Basically cocking a gun. Then SNAPPED 11b. The force unloaded would decimate the end. The crane becomes more loaded, cable lift device lets go. Crane vibrates. 11 sends the end outward in pieces. Falls down. Simple conclusion really.

Any thoughts on all this?

Sorry I am not a decent writer.
Analyzing mechanical systems and why they failed or how to make them better, I would be better at.


RE: Miami Pedestrian Bridge, Part XI

Please discontinue posting in this thread. Go to thread815-455746: Miami Pedestrian Bridge, Part XII: Miami Pedestrian Bridge, Part XII: Miami Pedestrian Bridge, Part XII for any further posts.

This topic is broken into multiple threads due to the long length and many images creating longer load times for some. If you are NEW to this discussion, please read the following threads prior to posting to avoid rehashing old discussions.

Part I
thread815-436595: Miami Pedestrian Bridge, Part I: Miami Pedestrian Bridge, Part I: Miami Pedestrian Bridge, Part I

Part II
thread815-436699: Miami Pedestrian Bridge, Part II: Miami Pedestrian Bridge, Part II: Miami Pedestrian Bridge, Part II

Part III
thread815-436802: Miami Pedestrian Bridge, Part III: Miami Pedestrian Bridge, Part III: Miami Pedestrian Bridge, Part III

Part IV
thread815-436924: Miami Pedestrian Bridge, Part IV: Miami Pedestrian Bridge, Part IV: Miami Pedestrian Bridge, Part IV

Part V
thread815-437029: Miami Pedestrian Bridge, Part V: Miami Pedestrian Bridge, Part V: Miami Pedestrian Bridge, Part V

Part VI
thread815-438451: Miami Pedestrian Bridge, Part VI: Miami Pedestrian Bridge, Part VI: Miami Pedestrian Bridge, Part VI

Part VII
thread815-438966: Miami Pedestrian Bridge, Part VII: Miami Pedestrian Bridge, Part VII: Miami Pedestrian Bridge, Part VII

Part VIII
thread815-440072: Miami Pedestrian Bridge, Part VIII: Miami Pedestrian Bridge, Part VIII: Miami Pedestrian Bridge, Part VIII

Part IX
thread815-451175: Miami Pedestrian Bridge, Part IX: Miami Pedestrian Bridge, Part IX: Miami Pedestrian Bridge, Part IX

Part X
thread815-454618: Miami Pedestrian Bridge, Part X: Miami Pedestrian Bridge, Part X: Miami Pedestrian Bridge, Part X

Part XI
thread815-454998: Miami Pedestrian Bridge, Part XI: Miami Pedestrian Bridge, Part XI: Miami Pedestrian Bridge, Part XI

Part XII
thread815-455746: Miami Pedestrian Bridge, Part XII: Miami Pedestrian Bridge, Part XII: Miami Pedestrian Bridge, Part XII

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