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Miami Pedestrian Bridge, Part VI
31

Miami Pedestrian Bridge, Part VI

Miami Pedestrian Bridge, Part VI

(OP)
A continuation of our discussion of this failure. Best to read the other threads first.

Member jasm linked the "For Construction" plans of the bridge here, thus the reopening of discussion.
https://cdn2.fdot.gov/fiu/13-Denney-Pate-signed-an...

Part I
thread815-436595: Miami Pedestrian Bridge, Part I
Part II
thread815-436699: Miami Pedestrian Bridge, Part II
Part III
thread815-436802: Miami Pedestrian Bridge, Part III
Part IV
thread815-436924: Miami Pedestrian Bridge, Part IV
Part V
thread815-437029: Miami Pedestrian Bridge, Part V

RE: Miami Pedestrian Bridge, Part VI

hokie66 et al.,

Let's presume what you say is true, that the #4 @ 12" (typical) are not shear reinforcement, but are "column ties". That thought, of using column ties for structural members within a bridge span, makes the hair on the back of my neck stand up! Should we expect a full A), B), C), accounting of the design assumptions made when the NTSB issues its final report?

RE: Miami Pedestrian Bridge, Part VI

(OP)
Yes, we should expect that. But we will have to wait a long time, I think a year or more.

I believe the philosophy was to design a typical truss bridge, but using concrete instead of the usual choice of steel. No doubt that philosophy is now regretted, along with the chosen structural form of using wide plates rather than compact members as chords.

RE: Miami Pedestrian Bridge, Part VI

3
I believe the horizontal reinforcing in diagonal 11 are just column ties since if they were shear reinforcing they would be spaced at d/2 where d when the p t is in place can be taken at no less that .8 x t =19.2 inches making d/2 9.6 inches. Without the pt force, d is approximately 21 inches calling for spacing of 10.5 inches center. The d/2 requirement is to make sure a potential crack is intersected by at least one tie.
In a two force truss, there would be no shear in diagonals since the ends are considered pinned. This is not a true two force member since the ends have fixity. An analysis of the truss with ends fixed would give some moments in the diagonal and therefore shear, though I doubt it would be much. Equal to about the difference in moment at the end divided by the length.

RE: Miami Pedestrian Bridge, Part VI

Interesting videos showing span fully suspended from deck ends on temporary shorings before move. Interesting mainly because span may have had an opportunity to collapse at this stage. I'm curious if PT rods for 11 and 2 were stressed while span was suspended like this.

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

Just a tidbit I would like to throw out there.

(To fellow posters, thank you for your great posts, I feel better after studying them.)

RE: Miami Pedestrian Bridge, Part VI

Correct me if I'm wrong, but after reviewing photos and videos here, it appears that both faces of member 11 are completely wrapped in that banner, "City of Sweetwater, FIU, Florida International University." This banner would obscure any shear cracks that would've developed in member 11 warning of a potential failure.

RE: Miami Pedestrian Bridge, Part VI


jrs87 (Mechanical)30 Apr 18 13:12
"Interesting videos showing span fully suspended from deck ends on temporary shorings before move. Interesting mainly because span may have had an opportunity to collapse at this stage."

I thought about about this a while back. When the truss columns were poured, there would have been a low level concrete bonding from deck to diagonals/columns. Since there was little stress on said construction joint at that time, it most likely stayed bonded. That all changed during the seismic type twisting that occurred in transit, while one corner of deck was on sidewalk, & the other was rising over the street curb. My best guess is about 10" rise at each corner. It would have debonded for sure at that time.

RE: Miami Pedestrian Bridge, Part VI

TheGreenLama
"Correct me if I'm wrong, but after reviewing photos and videos here, it appears that both faces of member 11 are completely wrapped in that banner, "City of Sweetwater, FIU, Florida International University." This banner would obscure any shear cracks that would've developed in member 11 warning of a potential failure."

I'll go one better....
During transport, there was a stoppage for an extended period of time, while 3-4 workers inspected the south end @ #1/#2. They did this 2x, that is to say...on deck, down, & back up again. There was no evidence of inspection on the north end. Interestingly enough, one of the drone videos shows a rapidly flashing light on north end #11 bottom/fillet during this time. After wondering about all this for a while, I have to wonder if they got sensor wires crossed fromm north to south. It would explain a lot, because the south end was very robust by comparison to north, & likely didn't have any issues.

RE: Miami Pedestrian Bridge, Part VI

To TheGreenlama:

What temporary supports is Step 3 talking about?


Nice catch. It appears that the construction sequence drawings are "a little" "incomplete" because they do not mention all the needed info:

For instance, a "good" erection drawing should mention the concrete strength needed to move the bridge. Otherwise, it will be assumed that 6000 psi is enough. So, we are missing a line saying : "After the concrete reaches 8500 psi, you can move the bridge"

Also, It is possible that the reference to the "temporary supports" is something left from a preliminary erection sequence. I did not see any other depiction of temporary supports in the plans. It is also possible that a set of updates after 4/4/17 exists that we do not know about.

Another thing, the shims supporting the span at pier 2, how do they look like?, how thick? How many? where are they? Because that affects the forces at the 11/12 node. No info in the Erection plans.

Finally, the removal of the PT bars in members 2 and 11 is not in the erection drawings. It is in the drawing that shows the PT bars in one of the notes. That is not really "acceptable".

So, are these the final plans existing before the sad day?

Thanks

Live long and prosper.

RE: Miami Pedestrian Bridge, Part VI

Reply to The Mad Spaniard (Structural) 30 Apr 18 17:09

I believe is was Retiredat46 that questioned "Step 3". Very good, Retiredat46. The beams connecting the transports is probably why middle of the cross section support is not seen.

I know this thread is way too long, but conversely, the more it continues, the more it uncovers a load a apparent weaknesses in this project.

Reply to TheGreenLama about banners

Banners being installed March 9, 2018 Video 1 Link

Video 2 (Please excuse worker for temporarily not wearing hardhat.) Link

Some type of sensor may be attached to member 11 under banner. March 9 photo of member 2 below. (Not a camera, cameras are on sticks in corner of orange mesh)



Reply to Cutterhead (Industrial) 30 Apr 18 15:39

Perhaps chains attached to span were meant to address "seismic" stress during move. How tight were those chains tensioned? Could they have caused cracks at the 90 degree intersection between member and deck?

RE: Miami Pedestrian Bridge, Part VI

2

Quote (hokie66)

As with many structures, the key is in the connections.
A good discussion of general connection failures is in the Appendix of the Latvian Maxima collapse analysis, starting on page 23.
https://www.dropbox.com/s/2e1lvbe3709baxg/Causes%2...
An excerpt: "Connections are mostly guessed, leading to fatal flaws in the design."
Interesting to see discussion on this now.

The engineer (Toomas Kaljas) said computer programs are good for designing loads and members, but many don't analyze connection strength. He immediately saw the the cause as a weak 11,12,deck connection. His 3D analysis of the span is here.
https://www.dropbox.com/s/uz3jmqjiyl238t5/Cause
He said a 2D analysis wouldn't show the problem.

=> I don't understand why the stress at #12 is so much different from #1 (must be from the asymmetrical design, but it's drastic compared to the difference in X/Y load forces).

RE: Miami Pedestrian Bridge, Part VI

A question that has been in my mind, about the grouting of #2 and #11. It might be possible to see from the sequence of photos whether the work being done on the day of the failure was a late modification instead of the planned de-stressing.

The construction specifications say that the PT bars in #2 and #11 "will not be grouted", and it's clear in the videos of the span being moved that #2 and #11 were left ungrouted (while other blisters have been filled in). On the day of the bridge failure both #2 and #11 are obviously open. But some photos seem to suggest that in the intervening time, #2 and #11 were grouted, as though the planned work was already complete, and only later chiseled out again. (Also there is a chisel clearly visible in post-failure photos, as many commenters pointed out). The photos below were posted by LittleInch on the V thread at 29 Mar 18 09:13. They seem to show the span set in place and with all blisters grouted, including 2 and 11. Can others judge whether this is just an optical illusion? It seems important to figure out which.

RE: Miami Pedestrian Bridge, Part VI

Quote (jasm)

"Will not be grouted"

Grouting in plans refers to bar duct, not blister. The bars are also not be be removed. Which brings up a point, what will prevent bars from corroding if they are not grouted? Does it matter that non stressed bars will corrode over time?

RE: Miami Pedestrian Bridge, Part VI


Nice work by Toomas Kaljas. It gives a big hint of the magnitude of the self-weight forces in the structure.

However, it may not reflect the effect of the top and bottom PT which may add bending moments to the intersection between 11 and 12. Maybe he will add those effects in the future.

In addition, I expect a large (tranverse to the deck and in the plane of the deck) internal tensile force in the end diaphragm due to the forces from the deck longitudinal tendon anchors. That tensile force is added to whatever happens at the node. All this is critical for the shear friction analysis of the node.

All these forces can only be really obtained by a 3-D analysis whether be FEM or three-dimensional grillage. Also, additional secondary moments, due to non-linear geometric behaviour (buckling) may be of importance. Who knows? But this is not a "simple" structure for sure.

Also, we should all remember that the node concrete side faces are compromised by the vertical PT holes and the drainage pipe hole. There may be not much concrete "meat" there or crossing rebar to accommodate all the forces applied by members 11 and 12.

If after all of this, node 11-12 is ok, time to focus on top node 11-10. But not yet...

Good hunting....

RE: Miami Pedestrian Bridge, Part VI

Quote (jasm)

They seem to show the span set in place and with all blisters grouted, including 2 and 11. Can others judge whether this is just an optical illusion?
The best photo I found is this (full res attached). It appears 2 and 11 blisters are open - which would be necessary to tighten the rods before transport (other blisters appear sealed).


This has audio interview with investigative reporter Tony Pipitone at beginning. It specifies if "post-tensioning" is 'loosening' or 'tightening' (this has been confusing all along). He says the span was set in place and "tension was released on those outer diagonal trusses" soon after (~3:30), then about cracks and "adding tension" to the rods. At ~4:30 he says this was NOT a stress test, but actual work to address something that had been discussed at the meeting that morning.
http://wlrn.org/post/week-later-agencies-pursue-in...

This is a good video by him saying the south end was adjusted then the north 3/4 done when it failed. There's a "puff" mentioned but think consensus here is that's the northwest corner of the deck, but after that a dark triangle below it appears which consensus seems is #12 kicking out (you can see this better in Part V - video loop gwideman 28 Mar 18 07:00, and frame grabs TheGreenLama 28 Mar 18 16:55)
https://www.nbcmiami.com/news/local/NTSB-Focuses-C...

This article specifically says the south side was adjusted (both rods), one rod of north side adjusted, and then failure when remaining north rod adjusted (upper with jack)
https://www.equipmentworld.com/ntsb-confirms-worke...

RE: Miami Pedestrian Bridge, Part VI

Where are these so-called "full res" photos coming from? Even if someone had used a six year old iPhone they would have gotten a higher-resolution image than the one that was just posted above (and I do mean the attached "full res" image).

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

RE: Miami Pedestrian Bridge, Part VI

Quote (John R. Baker, P.E.)

Where are these so-called "full res" photos coming from?
LOL... I (and assume others) pull them from news articles. To find photos, I search a topic on Google images, select photo, "visit" site, then "save image" to check res and hope for best. Eng-tips reduces res on thread, so attached is higher (though maybe not much). So you takes what you can get...

I looked a lot for blister photos before and there aren't many. Maybe someone here has better, but I can [barely] see the open blisters (I wanted to confirm this too).
I thought someone mentioned a drone video, but maybe not (and not sure if it was before/after transport or collapse.

RE: Miami Pedestrian Bridge, Part VI

The attached "full res" is 1140x723 pixels across, with no metadata, suggesting a screen-capture from a news website that's been cropped a little. I don't think the intent was to imply it's an original, although, for the photo-savvy, the term "full res" is normally taken to mean the original photo without cropping. A forgivable misuse of terminology, if we accept in this context that not everyone knows or cares how cameras work. I think Chris is giving us the best he's got, that's all. Not enough pixels for me to make out grouting on any of the blister, however.

STF

RE: Miami Pedestrian Bridge, Part VI

My question was intended to be a bit rhetorical winky smile

That being said, the context in which I asked it was that of being a photographer. My point is that my first digital camera, a Canon PowerShot S10, which I purchased 18 years ago, had a resolution that was more than TWICE that of your attached "full res" image. I know you had no say in this, just that it amuses me that it appears to be difficult to find truly high-res images out there.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

RE: Miami Pedestrian Bridge, Part VI

See the image linked to in the following:

http://www.eng-tips.com/viewthread.cfm?qid=436595 epoxybot (Structural) 17 Mar 18 04:22

Chris - most all investigation has been done in the previous pages; most on the first page. I would have linked to the photo directly, but this topic was shut off for a while because of redundant posting. There is a lot to read, but it's worth while.

RE: Miami Pedestrian Bridge, Part VI

Apologies on last post (1 May 18 01:00), photo resolution us upscaled, so it is no better. Pedro Portal is still good source of photos.

RE: Miami Pedestrian Bridge, Part VI

Yes, 2 and 11 blisters were open during transport. My question is whether they were filled in (and then opened again) between March 10 when the span was set in place and March 15 when the bridge collapsed.

RE: Miami Pedestrian Bridge, Part VI

My posts were meant to summarize everything here after getting new specs showing PT anchors and reinforcement in deck, and to add the 3D stress analysis. I got sidetracked with the blisters... I'd been through all posts a couple times, but went back to beginning as 3DDave suggested and found where Rubio, on the first day (and had inside info), said the rods were being tightened when the span collapsed.. there still seemed some question about this.
------------------------------------------------------------------------------------------------

Quote (The Mad Spaniard)

the node concrete side faces are compromised by the vertical PT holes and the drainage pipe hole.

This has been discussed a bit, along with a suspected cold joint at the deck/member junction - which was confirmed in the latest spec.


A "summary collage" (high res attached) shows the bottom of post-collapse #12 on top of pier.
It's fairly flat with rebar hanging out, and appears to have sheared at the cold joint.
The small insert shows intact highlighted area that was pulverized (Meerkat, IV 23 Mar 18 13:43).

Bottom right shows large PVC pipes on sides of #12 intact (gwideman, IV 23 Mar 18 10:54).
Top right shows post-collapse with pipes still in deck. I'd assume #11 and #12 moved straight back - noted in video loop and frame grab sequence mentioned above (30 Apr 18 23:18).
A larger PVC pipe was below the cold joint, and #11 diagonal PT rod ducts crossed through it as larger ovals.
Spec shows no PT rods in vertical #12.
The upper PT rod anchor plate was within the "overhang", and has little concrete behind or to sides of it. Plate spec for 1.75" PT rod is 8"x12", but it doesn't look that big (exposed on top of pier).
This area was weak.

Lastly, the top center photo is from gwideman, IV 23 Mar 18 17:09. I suspect the #11,12,deck joint was stressed during the move and the cold joint/edge of the deck was affected. The upper rod had been tightened, and the lower rod was being tightened which would add to the dead load force pushing #11/12 north (total ~1927 kips/Peter's calcs; ~1400 kips horizontal with 30 degree member).
As Toomas noted, the edge and bottom of #12 were highly stressed, with only part of 12 attached to the deck. It's possible the workers were told to "give it a little more" on #11's PT rods because of the cracks - the rods were spec'ed at 280 kips, with typical (Williams) Min Ultimate Strength at 390. There appears to be some spalling (epoxybot, V, 29 Mar18 05:33) where the "overhang" sheared, as the deck fell.

RE: Miami Pedestrian Bridge, Part VI

Right before posting was temporarily halted on this thread, there were several individuals, Gwideman and MikeW7, who appeared to be working towards tracking down the original (or close to original) dashcam video and creating a zoomed-in video loop of the north end of the bridge. It became apparent that previous video loops posted herein of the collapsing bridge were created from video of unknown history (i.e. poor quality), or had been digitally manipulated to create super-slow motion with zero vehicle movement.

Am I the only one who thinks that analysis of the original dashcam video would prove exceedingly interesting and useful to the general public? Particularly, a zoomed-in loop of members 10-12, beginning at least one second prior to any noticeable disturbance, along with the still images from this video loop. Maybe nothing new will be learned from investigating the video further, but you don't know until you try.

If anyone has the ability to create such a video and post here I think that in addition to myself the general public would find it extremely valuable. This is not my area of expertise. MikeW7 provided this link on 29 Mar 18 03:56 to a relatively high-quality video, but I don't know if this is the original. Link

RE: Miami Pedestrian Bridge, Part VI

TheGreenLama: I did ok with gwideman's loop and your frame grabs, but this one appears to show the northwest corner better (thought to be a "puff", but clearly isn't on this), and the tree's out of the background so #11/12 kick out is easier to see, but the bucket lift is still in the way. It's better with the span further from the edge. I put it on a 32" monitor and it's still very clear, so if someone could blow it up bigger maybe could see something better. It'd be the best quality video.

I read somewhere here that you can Pause, then single step Youtube fwd with "." and back with "," - it works and can see #11/12 end in 5 frames (can also move 5 sec fwd and back with horiz arrows).
=====================
SFCharlie: The video looks like same angle/light, but maybe not as high as the photo. Thanks - I'll look at it more later and try some screengrabs (just learned how to do this).
=====================
John R. Baker: I thought you were joking around... I still use an old 2MB slide phone so don't ask much from photos, but detail IS important here.
=====================
jasm: Not sure why you wanted to know if blisters had been cleared after placement. I saw the jackhammer there and thought maybe hammering triggered this (transmitting impact to lower end), but after seeing the hyd jack on end of rod, figured they weren't jackhammering around it.

RE: Miami Pedestrian Bridge, Part VI

chris snyder - Peter Dow's analysis seemed to assume that the work done on Mar 15th was the planned destressing. The reason I'm asking about the blisters is that if the blisters were first filled in, then opened again on the 15th, then the destressing had almost certainly already occurred. In that scenario the Mar 15 work would instead be a late change of plan, likely in response to perceived problems with the bridge. This seemed to me an answerable question given all the footage that was shot between the 10th-15th.

RE: Miami Pedestrian Bridge, Part VI

To see minimaly degreated video zoomed in, download MP4 from above link, install VLC Media Player on your PC. https://www.videolan.org/vlc/index.html
Then play in interactive zoom mode and use [ ] keys to adjust playback speed. Sharpen image and adjust zoom to your liking. I cannot prove it at this point, but I was able to clearly differentiate falling concrete rubble from tree in background by using sharpen function. Please try yourself.

RE: Miami Pedestrian Bridge, Part VI

Quote (jasm)

This seemed to me an answerable question given all the footage that was shot between the 10th-15th.
That's a valid question. I didn't see much video from the 10th-15th - only a few ones celebrating and PR'ing after the span was set.

The sequence of post-tensioning was:
1) Rods were tightened to prevent ends of span from sagging during transport
2) Span was set in place, and then rods loosened (on their own) as the #2 and #11 changed from tensile to compressive, and had to be tightened. Tony Pipitone said in video "the original design stress adjustments to those diagonal trusses were completed before the road was reopened last Monday morning" (spec sheet shows 280 kips for all these rods).
3) After the meeting on morning of the 15th, the engineer had them "add tension to the rods", which now (come to think of it) sounds like it would have to be more than 280 kips on spec sheet.

If someone has notes or a link where it says the rods were being destressed/loosened, I'd like to know... Peter Dow must have picked it up somewhere (though it almost seems the term "destressing" is being used for "post-tensioning").

They COULD have sealed the blisters after adjusting rods after span was set (before the road was re-opened), and opened them before adding tension on the 15th, but I don't know and it wouldn't affect the sequence or direction of post-tensioning.

RE: Miami Pedestrian Bridge, Part VI

jrs87: I had VLC Media player, but maybe my computer's too old. It doesn't play right consistently, but after doing what you said (zoom and slow), I think I can see a 'pointy' end on #12 lay on top of the pier. I tried 'screen shooting' it but it was blurry if I increased size. Will see what others say.

RE: Miami Pedestrian Bridge, Part VI

Quote (chris snyder)

"computer too old"

Chris, transcode the mp4 to avi. MP4 compression codec gets in the way. AVI is much better for stopping and moving about timeline, MP4 depends heavely on data stream. How to transcode: https://youtu.be/qYYTpAKEOHc

Keep in mind member 12, the tree, and the lift boom all line up at time of collapse. So it's difficult to rule out artifacts here. Sharpening helps.

I was able to "cast" video to 55" 4k television. It is clear the span dropped and held for a moment (cannot rule out video artifact), then events bounce between 12/11, 11/10, and 10/9. The next major event is 9 punching through folding deck. then 12/11 explodes, next 11/10 angle opens up and 11 can be seen going under top of 10. Finally 11 is shoved hard and the canopy and deck pancake. What I saw made me suspect something in the deck gave way first, dropped the span, and then geometry change was too much for the truss to bear.

RE: Miami Pedestrian Bridge, Part VI

Thanks jrs87 (Mechanical)
Can you point me to more info on how to zoom (how do I get the little window that lets me pan the zoom area) and sharpening ( What is the "right" setting for the slider?"
Thank you,
Charlie

RE: Miami Pedestrian Bridge, Part VI

SF Charlie,

VLC Media Player Zoom: Ctrl+E > Video Effects Tab > Geometry Tab > Check Box Interactive Zoom > Save > Close

A zoom box will appear on playback and a zoom level slider will be near it. Speed of playback: every press of [ is slower, ] faster.

RE: Miami Pedestrian Bridge, Part VI

jrs87,

How do I extract? Are these JPG files? What App to use?

RE: Miami Pedestrian Bridge, Part VI

As far as whether or not the "puff" is a bridge corner, it is not. The reason I say this is that early in the video, you can see the bridge corner to the left of the vertical man lift mast. This is at a point in truck travel, that you can compare the left, & right bridge corners. As the truck moves forward, the corner disappears. It would be impossible for bridge corner to pop back out on the left of mast as the truck moves forward.

RE: Miami Pedestrian Bridge, Part VI

Quote (Cutterhead)

"How do I extract?"

This is 7 Zip file of 111 .jpg files. Needed extra compression due to forum limit. Extract with the Windows program 7Zip. This is a very common mainstream program.

RE: Miami Pedestrian Bridge, Part VI

Link to app to extract 111 .jpg files. https://www.google.com/search?q=7zip&oq=7zip&a...
My apologies for not including this before.

From what I see, there is NO puff, any puff was an artifact. BTW, the dashcam was recorded at 4.6 frames per second and played back at 30. 111 frames covers 24 seconds. I ran 24 seconds because of dust cloud.

RE: Miami Pedestrian Bridge, Part VI

Update on post jrs87 (Mechanical) 1 May 18 23:35

Upon closer inspection of linked document, these are cracks that were found after shoring was removed, and before bridge was moved.

RE: Miami Pedestrian Bridge, Part VI

Quote (3DDave)

"...expunged"

Please explain, still works for me. We now have reports of cracking when forms were removed, when 2 and 11 were first stressed, and when shoring was removed. Do I got that right?

RE: Miami Pedestrian Bridge, Part VI


jrs87, It works now. It previously reported that the content had been removed with a great big graphic that was quite unmistakable. It's Sharepoint, so who knows.

RE: Miami Pedestrian Bridge, Part VI

Pictures post shoring removal, pre move #4, #5, #6 tells me the story.
The cracking evidence is that the bottom of #11 slid across the deck, while the lower (4" tall?) side was being held back by a stirrup. Then they prestressed #11 to 560kips, & transported. The next step would place the bridge on pier with full 2,000 kips load at small 1/2 size #11 fillet block. #12 held tight, & #11 slid to crush small undersized fillet. The #4 pict crack is probably 2x the .024" travel I figured was needed to explode fillet.
https://res.cloudinary.com/engineering-com/image/upload/v1525232520/tips/CRACKS_REPORT_AFTER_SHORING_REMOVAL_tcq1tk.pdf

RE: Miami Pedestrian Bridge, Part VI

jrs87 -- agreed, odd premise. While the author ponders the merits of slow engineering, I'm happy to snap up jobs relying on quality technical and project engineering instead. Not bending to the perpetual time crunch pressures, but acknowledging the value of timeliness (to the client, end users, and the traveling public).

Also, yet another academic who missed the aesthetic nature of the "stays".

----
The name is a long story -- just call me Lo.

RE: Miami Pedestrian Bridge, Part VI

I tried to correlate the photo numbers to span locations - 1,2 match page 4 diagram - 3,4,5,6 don't.

Just throwing this out there..
A) Photo 3 at west #12 base? (does #1/2/base have PVC pipe sticking out?)
B) Photo 4 definitely looks like #11 (good input Cutterhead/0.024"), but there appears to be a hole in deck that I can't see elsewhere
C) Photo 5 IS #11/12 (can see the rod sticking out elsewhere)
D) Photo 6 is what I'd expect to see on the opposite side of Photo 4 for #11, but the angle looks steeper - both in crack and bit of diagonal member at top. Angle ~like #7

Attached is an enhanced photo 4. There are parallel hairline cracks that match PT rod angle and angle of large crack that appears to continue along deck. With rods in ducts, there's not a lot of surrounding concrete in these rodded members (as there'd be in #2).

RE: Miami Pedestrian Bridge, Part VI

#4, #5, #6 are all at base of #11. The diagram shows them at the bottom of deck, but it isn't so.
One thing that I take from picts, is that some cracks were surfaced over, & some cracks were after surfacing. In other words, there was a lot of movement from #11 to deck.
It was stated by Pate that they were going to tighten the tendons to close the crack. That might have worked if the bottom #11 anchor was above the deck, but they weren't. Applying more tension on tendons only added to the slide, & hence short fillet failure.
I'm guessing the hole is to aid grouting in #11 tendons.

RE: Miami Pedestrian Bridge, Part VI

Number 11 diagonal was never planned to be grouted.

RE: Miami Pedestrian Bridge, Part VI

Has all work on the bridge project at FIU come to a halt pending the outcome of the NTSB investigation?

Did the support structures on the ROWs get demolished as part of the cleanup of the accident scene? If not, are they usable or repairable?

Obviously, they still need a pedestrian bridge. Have FIU, Florida DOT, and community leaders publicly discussed or decided on a path forward?

If the NTSB finds that the design was fatally flawed, what effect will that have on Denny Pate, FIGG, and the others involved?

What is the likelihood that the concept bridge can/will be redesigned to overcome its flaws, and eventually be built?

RE: Miami Pedestrian Bridge, Part VI

If the anchor for #11 were above the deck it could not have carried the necessary tension to the deck during the bridge movement when it was a tension member. In place, on the piers, it became a compression member as it was originally planned. Why did this cracked compression member fail? I have concerns for the anchor design but we do no have those as they are assigned to the post tension company and not part of the plans.

A lot of good thoughts and information here. Thanks to all.

RE: Miami Pedestrian Bridge, Part VI

To Mr. MOJOJOHN

If you are talking about the PT bars (2) stressed in member 11, for me they make sense to accommodate the tensile force in member 11 during bridge movement. That member only has 8 #7 bars and whatever compression it gets from the tensioning of the deck tendons (minus, probably, some tension from the stressing of the canopy tendons.

After the span is set on the piers (and we, BTW, do not clearly know from the plans the layout of the shims at the piers), that strut gets into real compression (and some additional bending from the stressing of the longitudinal tendons in the canopy and the deck). Now, what?

My interpretation of the contract plans is that the 2 PT bars, which are not needed now for tensile effects, are requested to be removed . AND THIS IS THE OPERATION BEING DONE THE DAY OF THE EVENT (my guess). It does not make sense to add more force to a member that is in compression already.

But, guess what, these PT bars are anchored at the deck, and, therefore, are probably clamping the shear horizontal interface between member 11(and 12) and the deck. If there is lack of shear interface capacity (shear friction), provided by the concrete area or the rebar crossing that plane from member 11 and member 12, now we have a potential failure mode. This has been pondered by members of this group before.

One has to see the EOR calculations to see how that potential failure mode was evaluated. With that, we have to remmember that the constants in the AASHTO code depend on the roughness of the shear plane (about 76"x21", I am guesing). The constant of 1.0 applies to 0.25" "roughness". Has anybody seen in the plans a note requesting that those "cold joints" be roughen to 0.25"? Otherwise, we may have to assume a 0.6 factor. Are the FDOT specifications regarding cold joints ensure a 1.0 factor or a 0.6 factor?

Good hunting

Live long and prosper.

RE: Miami Pedestrian Bridge, Part VI

Answers to Retiredat46 Aeronautics - 3 May 18 08-25
Has all work on the bridge project at FIU come to a halt pending the outcome of the NTSB investigation?
Yes, it is halted. A video on YouTube shows the the site devoid of activity (Published on Apr 25, 2018).
https://youtu.be/qcy7dFugf3M?t=543
Besides, the contractors are not getting paid, so do not want to “run-up” their payroll.

Did the support structures on the ROWs get demolished as part of the cleanup of the accident scene? If not, are they usable or repairable?
They where dismantled.

Obviously, they still need a pedestrian bridge. Have FIU, Florida DOT, and community leaders publicly discussed or decided on a path forward?
Yes, they have had meetings and the community still wants a bridge.

If the NTSB finds that the design was fatally flawed, what effect will that have on Denny Pate, FIGG, and the others involved?
Denney Pate (2nd e in Denney) will have to repair his image or retire.

What is the likelihood that the concept bridge can/will be redesigned to overcome its flaws, and eventually be built?
That image will be too painful to the community and a detriment to FIU's image as “We Build Bridges”
I would build a steel bridge with curving members and soaring design as a memorial (That's what Figg pitched after the twin cities bridge disaster).
Thanks for your questions,
Charlie

RE: Miami Pedestrian Bridge, Part VI

Quote:

If the NTSB finds that the design was fatally flawed, what effect will that have on Denny Pate, FIGG, and the others involved?
Denney Pate (2nd e in Denney) will have to repair his image or retire.

Well, if the DESIGN was flawed then yes. But after over 1000 posts on this set of threads there is still a lot unknowns and the investigation could possibly result in a sequence, stressing, construction, contractor error vs. an engineer error. We'll see I guess.

If it was a construction error (vs. design) then perhaps Mr. Pate will be in better shape career-wise.

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RE: Miami Pedestrian Bridge, Part VI

Greetings to all

Our fellow members JAE SFChralie and Retiredat46 (and may be others) have commented in something that could be relevant to all.

IF , and lets say for now without any proof at all in this case and in a general hypotetic case, that an engineer has signed and sealed a set of plans that causes a collapse with deaths of an structure based on a mistake in the calculations/plans, or anything to do with design, what are the cconsequences:

Loss of license forever after being found guilty by the Board after the Board gets a complain by a member of the public (annonimous or not)????
Jail, if a criminal case is brought???
Civil penalties, if a case is brought by the families of those that were affected?

These are probably easy to be determined based on previous cases (do you guys can present any similar cases in the state of Floridaor elsewhere?)

But, what about the P.E's that designed the critical members or oversaw or checked the project without signing and sealing? Can a case be brough against them similar to that against the EOR? What happens to the "peons" that do not get the "big bucks" given to the Project Managers?

The interesting thing is that in the case of an owner/s without a P.E. they may only loose the company (or maybe not) or just some money...

These are scary thoughts for all that have a P.E.

Food for thought..

RE: Miami Pedestrian Bridge, Part VI

Well, there is the Hyatt Regency walkway collapse in Kansas City in 1981.

Here's a link to an Engineering.com article on it.
https://www.engineering.com/Library/ArticlesPage/t...

A relevant quote:
Due to evidence supplied at the Hearings, a number of principals involved lost their engineering licenses, a number of firms went bankrupt, and many expensive legal suits were settled out of court. The case serves as an excellent example of the importance of meeting professional responsibilities, and what the consequences are for professionals who fail to."

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RE: Miami Pedestrian Bridge, Part VI

Quote (JAE)

Well, if the DESIGN was flawed then yes. But after over 1000 posts on this set of threads there is still a lot unknowns and the investigation could possibly result in a sequence, stressing, construction, contractor error vs. an engineer error. We'll see I guess...

It might be within the realm of possibility that a construction fault caused the collapse. However, for a construction fault to cause the bridge to so completely underperform its specification (and I'm pretty sure they specified "doesn't fall down under its own weight"), it would have to be simultaneously very serious so as to use up all available factor of safety, but also very subtle so as to escape notice in all of the various layers of inspection. And construction codes and inspection regimes are specifically designed to catch or at least mitigate such errors. Note for example the emails observing that the lateral PT members were not exactly located per plan. I think that the far more likely scenario is the failure of imagination that we've pretty exhaustively illuminated in Parts I through V on this topic: the failure to anticipate the need for a robust shear connection between the 11/12 junction and the deck.

--Bob K.

RE: Miami Pedestrian Bridge, Part VI

2
Bob
In other words... if the structure strength/stability is that sensitive to contractor induced variations then there’s simply an inherent design flaw.

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RE: Miami Pedestrian Bridge, Part VI

As JAE notes, there are several issues the NTSB has to examine; they include:
Construction Contract,
Construction Documents including Drawings and Specification,
Change Orders and other changes to the Contract,
Any drawings or sketches prepared for clarification or intent,
Any Field Orders,
Minutes of all meetings between the Engineer and the Contractor,
Any unrecorded changes to the Work,
Any QA/QC undertaken by the Contractor and the Engineer,
Any shop drawings submitted, and any shop drawings reviewed by the Contractor,
Any shop drawings submitted, and any shop drawings reviewed by the Engineer,
Any eMails and correspondence between the Owner, Contractor, and Engineer,
Any Field Notes made by the Contractor or the Engineer or Other parties,
A mandated peer review,

Any Site Construction Reports prepared by the Contractor and Engineer or Other parties, and
All visits by the FDOT guys (a government agency may play down the role of these guys).

I've probably missed a few... but, it's a good start.


RE: Miami Pedestrian Bridge, Part VI

I suspect strongly that there was a serious design flaw at one or more of the 'nodes', but, I don't have enough information to go beyond 'suspect strongly'. There has not been enough information, of the right kind, to go beyond. There could be a couple of other plausible reasons for the collapse.

I'm glad that Tesla released their information to the public; I'm not thrilled that the government is controlling all information released. Anyone reporting to the NTSA should do so under the condition that any information provided to them may be released to the public (off my apple box).

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (The Spaniard)

Loss of license forever after being found guilty by the Board after the Board gets a complain by a member of the public (annonimous or not)????
Jail, if a criminal case is brought???
Civil penalties, if a case is brought by the families of those that were affected?

I think 'Mad' is being redundant... We'll have to wait and see what charges can/will be filed. If the engineer that made the site visit and viewed the crack and followed it up with a statement that it was nothing to be concerned about is incorrect. If the crack was a serious precursor to the failure, he could be in serious trouble. We'll have to wait, and, hopefully the Statutes of Limitations doesn't pass before the NTSB releases its report.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (The Spaniard)

These are scary thoughts for all that have a P.E.

Not scary, but, sobering... after a while you get used to it, and, it just becomes a part of a way of life. I often see structural issues without consciously looking for them... as, I assume most structural guys do.

Dik

RE: Miami Pedestrian Bridge, Part VI

(OP)
dik,
Your list should include the mandated design peer review.

RE: Miami Pedestrian Bridge, Part VI

Do we know for sure that there are pictures of the cracks that were the subject of Denney Pate's voice mail on March 13? Of course, we don't know exactly where (other than "the north end of the span") or how serious the cracks were, either. It seems like any pictures would have been released by now if they exist.

RE: Miami Pedestrian Bridge, Part VI

Retiredat46 (Aeronautics)
NTSB has embargoed release of anything since Feb 2018.

RE: Miami Pedestrian Bridge, Part VI

The very end of video 00560MTS from those posted by jrs87 (Mechanical)30 Apr 18 13:12 briefly shows the bottom of the 11/12 area before the move. Is that worth zooming in on for a closer look?

RE: Miami Pedestrian Bridge, Part VI

Looks like a fight between Florida and Federal laws on release of documents is under way.

http://www.miamiherald.com/news/local/community/mi...

Given that we seem mainly to be looking at the member 11/12 connection to the deck issue I still wonder how much design went into the short term single span design compared to what the bridge would look like and act when both spans were present and apparently locked together to give support and resistance to movement / stress.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part VI

Quote (hpaircraft)

"failure of imagination"

I would add it seems likely something never encountered before arose regarding shear between 11 and deck. I don't know if member 12 really had any structural significance except for canopy end support. Hinges were placed on member 1 to allow for different thermal expansion of deck and canopy, so member 1 was not structurally significant.

I, like a couple of other posters, would like to know more about the shims on north landing. There were 4, and spaced such that the diaphragm area under 12 was floating. What if (big if) the shims were wrong and two inner shims did not fully make contact? The span did seem to rock a little hard when lowered.

If it turns out design anticipated other section of bridge was required to fully stabilize 11 and they just said to themselves "don't allow live load until entire structure is complete", I will be horrified.

If this bridge was innovative and first-of-its-kind, it would have been prudent to make test models in the lab first. Is it industry practice to make the bridge itself the prototype? Did they have plans for this concept to be implemented all over the world? I don't think so.

I don't think we have to wait for NTSB report to conclude the allure of public funding lead to untested decisions and design constraints. Add to that a little social engineering.... If FIU/Sweetwater was using their own money, this overpass would have been completely different.

Quote (Retiredat46)

"zoom in"

The additional application of a white finish to members would probably obscure cracks except at close focus.

RE: Miami Pedestrian Bridge, Part VI

Quote (Hokie)

Your list should include the mandated design peer review.

I've added it, but, I'm not sure I agree with this requirement. Two eyes are always better, but, my experience with peer reviews is not great.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (Retiredat46)

Do we know for sure that there are pictures of the cracks that were the subject of Denney Pate's voice mail on March 13? Of course, we don't know exactly where (other than "the north end of the span") or how serious the cracks were, either.

We don't, just my speculation... I know if I were on site and saw them, first thing out of my pocket would be my camera. It's possible, but, I cannot imagine an engineer seeing cracks that were worthy of mention (if only to say they were of no consequence) not taking photos. Photos may not have been taken, but if they were consequential, the Engineer may be in for a pile of heartburn.

Dik

RE: Miami Pedestrian Bridge, Part VI

At least in our region, state DOT's typically require a complete, independent, second design check on all calculations and drawings for bridges.

In addition, the state DOT would also have their own engineers review.

In this case, with the primary client being a university, I'm not sure how intensely involved was the Florida DOT.

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RE: Miami Pedestrian Bridge, Part VI

Quote (LittleInch)

Looks like a fight between Florida and Federal laws on release of documents is under way.

I hope they are successful... we seem to have lost sight of what 'public servant' means.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (jrs87)

The additional application of a white finish to members would probably obscure cracks except at close focus.

My experience is that adding a coating (and, not a heavy high build), makes cracks more visible.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (JAE)

I'm not sure how intensely involved was the Florida DOT.

This will be an interesting facet of this collapse. For their sake, I hope they were not involved, but, you know how governments work.

Dik

RE: Miami Pedestrian Bridge, Part VI

Please expect that the NTSB preliminary report will be out in a least 2 to 3 weeks.
SFCharlie

RE: Miami Pedestrian Bridge, Part VI

Quote (hpaircraft)

I'm pretty sure they specified "doesn't fall down under its own weight"

I'm not sure that I would have included that line in a spec... but, I will in future.

Dik

RE: Miami Pedestrian Bridge, Part VI

To Dik, JRS87, JAE and SFChralie (and others)

Regarding independent review of the bridge:

This is a Designg-Build project

The "peer review" activities are not very clear in the State of Florida (Plans Preparation Manual). Because they do not specify every step on the peer review so that a truly inpependent analysis of all the elements of the bridge is done. It is open to interpretation. It is something that may have to be evaluated by FDOT.

The peer review is made usually in FL at the 60%, 90% and 100%. BUT!!!! It is potentially made "under pressure" to meet the schedule. You may have the EOR and the contractor breathing in your neck. And under these, it is potentially done very fast and without paying enough attention to details. It is an issue that needs to be resolved and clarified in the FDOT PPM. It also involves how much the "peer reviewer" is going to be paid...

We know for sure that the Structures Design Office in Tallahasse reviewed the project and provided comments. But we do not know, at what level the calculations or the plans were at that time. We have to wait and see for that report. Maybe the Miami Herald will get that in their request for documents. It should have been done before February 18 2018 and therefore obtainable.

And finally, if the shims at the pier are not directly undet members 11 and 12, OOOOPS!!! The transfer of vertical force at that node it is done thru vertical face shear friction and strut-and tie which potentially creates a horizontal force in the diaphragm (in the plane of the diaphragm) to mess up more the force situation at the node 11-12-deck-diaphragm.

Furthermore, you guys are great. I enjoy your insights.

Good hunting.

RE: Miami Pedestrian Bridge, Part VI

Quote (The Mad Spainard)

"OOOOPS"

FIU Bridge north landing shims (repeat post from another user):



Quote (dik)

makes cracks more visable. [sic]

Okay, it's just I don't see any cracks on images from March 10 to March 14.

Retiredat46, about photos of cracks, they are from February 2018 after shoring was removed and span was dead loaded from ends and BEFORE move. If you examine the record you will find separate crack development reports after forms were removed, after shoring was removed, after PT bars and 2 and 11 were stressed, and after bridge was placed on permanent landings. Some of the reports were done by consulting firm. NTSB has locked down everything after Feb 19 2018.

I want to reexamine term "Stress test". It has been said this was misnomer, what if it was not? Perhaps the cracks lead them to test stress and strain using the PT bars. I'm grasping at straws now.

RE: Miami Pedestrian Bridge, Part VI

jrs: [sic] thanks, corrected.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (The Mad Spaniard (Structural) )

To Dik, JRS87, JAE and SFChralie (and others)

Regarding independent review of the bridge:

This is a Designg-Build project

The "peer review" activities are not very clear in the State of Florida (Plans Preparation Manual). Because they do not specify every step on the peer review so that a truly inpependent analysis of all the elements of the bridge is done. It is open to interpretation. It is something that may have to be evaluated by FDOT.

The peer review is made usually in FL at the 60%, 90% and 100%. BUT!!!! It is potentially made "under pressure" to meet the schedule. You may have the EOR and the contractor breathing in your neck. And under these, it is potentially done very fast and without paying enough attention to details. It is an issue that needs to be resolved and clarified in the FDOT PPM. It also involves how much the "peer reviewer" is going to be paid...

Each professional design firm should have a Quality Control Plan included as part of their Project Management procedures. This is not something that the State should be telling you what to do, it should be a normal part of the engineering business and of project management.

The Federal Highway Administration (FHWA) created a guidance document on QC/QA for bridge design in response to NTSB Recommendations after the catastrophic failure of the I-35W highway bridge over the Mississippi River in Minneapolis, Minnesota. The following paragraph is in that guidance document.

"For major projects involving unusual, complex, and innovative features, a peer review may be desirable to raise the level of confidence in the quality of design and construction. A peer review is generally a high-level QA review by a special panel of professionals specifically appointed by the bridge owner to meet the demands for quality and accuracy, recognizing the complexity of the design. Peer review is an effective way to improve quality and to reduce the risk of errors and omissions. The need for such peer reviews is at the discretion of the bridge owner."

fhwa

In light of this particular bridge's features including design build contract, unusual complex concrete design, supposedly innovative construction techniques, etc., a peer review would have been desirable. Don't believe any information on peer review has been released.


RE: Miami Pedestrian Bridge, Part VI

Quote (bimr)

"Don't believe any information on peer review has been released"

I understand Louis Berger reviewed the design. Does that count as peer review? In the future, designs will be reviewed by cloud-sourced engineering talent, so get ready.

RE: Miami Pedestrian Bridge, Part VI

(OP)
jrs87,
This bridge was the first, and hopefully the last, of its kind. Your suggestion that lab testing should have been done is good, but they had a full scale prototype. They could have load tested it on the ground beside the road where it was built, by using something like sand bags or water. Rather, they erected it, and it fell under its own weight. "If your foresight were as good as your hindsight, you would have a devil of a sight."

The Louis Berger review was supposed to be the project mandated peer review, but nothing about that review has been released to the public.

RE: Miami Pedestrian Bridge, Part VI

The FDOT On Thursday, the agency released a statement of “preliminary findings” suggesting that Louis Berger, the company hired by FIGG Bridge to conduct the independent peer review on the concrete construction was not pre-qualified for the work as required.
Read more here: Link

According to my reading, the FIU Bridge would have been categorized as a Major Bridge due to its complexity & truss design. The consulting criteria is here: Link

Based on the stringent Major/Concrete/Complex Bridge unique experience & number of personnel, Louis Berger may not have locally met the requirements for this particular area of design consulting.

MCM+FIGG's submittal mimics the FIU request for proposals, nearly word for word.



Just an observation of the postensioning schedule (Sheet B-69). In truss #2 the A tendon is the longer (Top) of the two, while in truss #11 "B" is the longer (Top) tendon.

RE: Miami Pedestrian Bridge, Part VI

I'm trying to understand the timeline.

From TheMadSpaniard VI 1 May 21:35 pdfs
The "FIU Pedestrian Bridge Crack Inspection" email (shows only hairline cracks) says photos are after 6 Feb 18 inspection..
"The members showing these small cracks are truss members that share the same blister at the canopy of the already stressed members No 2 (stressed 1/30/18) & No 11 (stressed 1/29/18) . We believe, this first stressing operation has temporarily created tension on members No 3 & No 10;... No other truss members within span 1 show any cracks similar to these shown on members No 3 & No 10."

The six photos of other cracks have a created date of 28 Feb 18, and (assumed? I saw no description) before the span was moved into place. These could relate to jrs87 videos showing the span supported only at the ends after all other shoring was removed.

Quote (jrs87)

I'm curious if PT rods for 11 and 2 were stressed while span was suspended like this.
These videos are dated 9 Mar 18 (heading at top of page) - if accurate, the span was moved the next day but someone had already seen these larger cracks. I looked at all (nice hi-res) videos - #561 gets close to bottom of #11... starts a little above the crack, pans down the span, then comes back lower, and ends just before it might show the crack.

All reports have said the cracks were noticed after the span was moved so it's odd if the photos were taken before. #11 had already moved back almost 1/16", and I'd think cracks would get worse after moving the span (jostling as it traveled, set in place, etc).

From VI 2 May 15:24

Quote (Cutterhead)

It was stated by Pate that they were going to tighten the tendons to close the crack.
Was there an article about this, or is a logical deduction? (the closest I read was ~"work was ordered by Pate to deal with something discussed at that morning's meeting")

I'm curious that the media hasn't mentioned these photos/cracks since MiamiHerald(?) noted this site in an article.

All this, and my question is does anyone know when the shoring was removed and SPMTs moved into position?

RE: Miami Pedestrian Bridge, Part VI

Quote (chris snyder)

"...mentioned these photos/cracks since MiamiHerald(?)"

No sign the media knows about other crack reports. Youtuber Construction Engineering & Photography knows. He seems to resent members of this forum, although I find him to be doing his best with what he has. Your mileage may vary.

Quote (chris snyder)

"timeline"

A limited timeline would be a good idea. I would like us to piece one together. To start, Google street view shows formwork in August 2017.

Side note: Why do so many experts and academics think titanium oxide was mixed into 8,500 PSI concrete mix? I have trouble trusting people that fall for marketing hype. Nanoparticle titanium oxide smog scrubbing concrete indeed. I would like to see line item charge for coating.

One nice side-effect of this thread for me is that I have seen dozens of other interesting footbridges in my causal research. Some are inspiring.

Out

RE: Miami Pedestrian Bridge, Part VI

Time Lapse FIU-Sweetwater Bridge for February 2018 - Stripping Forms, Post-Tensioning
Stuart Grant
Published on Mar 6, 2018
https://youtu.be/qBXnr0FoqA8
PG6 Camera Time Lapse FIU-Sweetwater Bridge for February 2018 - Stripping Forms - PG6 Feb 2018 1800X 1080P with Music
*Post-tensioning activities can be seen occurring over several days on the canopy in the second half of the video*
Bridge design is by Figg Bridge Engineers and it is being built by MCM.
Music is "Grasshopper" by Quincas Moreira from the YouTube Audio Library

Please also see other bridge build and move time-lapses Stuart Grant's channel.
Stuart Grant is the Facilities Planning Coordinator at Florida International University in the Miami/Fort Lauderdale Area.
https://www.linkedin.com/in/stuart-grant-7b01b214

Thanks SFCharlie

RE: Miami Pedestrian Bridge, Part VI

FWIW, this is the very end (22/23) of #560 showing the base of #11 before the move. The progress bar hides part of the joint area for several seconds after pausing the video. I couldn't step the video frame-by-frame, so there may be more to see.

RE: Miami Pedestrian Bridge, Part VI

Quote (epoxybot)

The FDOT On Thursday, the agency released a statement of “preliminary findings” suggesting that Louis Berger, the company hired by FIGG Bridge to conduct the independent peer review on the concrete construction was not pre-qualified for the work as required.

That's just part of the legal quagmire coming up... If the FDOT knew Berger was not pre-qualified, and, that they should have been, and that the project was proceeding... The FDOT should have all their press releases 'screened' by their legal department, if they don't already. FDOT has to be very careful in what they release; maybe best for them not to say anything.

If they have been busy throughout the project, they may have difficulty removing themselves from it.

Dik

RE: Miami Pedestrian Bridge, Part VI

But "not pre-qualified" does not necessarily mean "unqualified".

RE: Miami Pedestrian Bridge, Part VI

Retired... agreed and edited.

Dik

RE: Miami Pedestrian Bridge, Part VI

jrs, Thank you.
SFCharlie

RE: Miami Pedestrian Bridge, Part VI

chris snyder
"Was there an article about this, or is a logical deduction? (the closest I read was ~"work was ordered by Pate to deal with something discussed at that morning's meeting"

I can't remember exactly where I heard it, but this comes close. The issue at hand was cracks, & tightening tendons was meant to strengthen member. I know it was stated somewhere, but now I can't find.
It was repeated several times, but look at minute 12:00.
https://www.youtube.com/watch?v=8lN1hpMbMxs

RE: Miami Pedestrian Bridge, Part VI

Not sure if anyone has noted this before, but our friend Juan Browne (AKA Blancolirio), of Oroville Dam fame, has also started to post a series of videos covering the bridge failure. Now he won't be giving us aerial views of the bridge from 'The Mighty Luscombe' but he is providing commentary and opinions.

Anyway, here's the first of those videos and from this link you should be able to find the rest is you wish to:

https://www.youtube.com/watch?v=sBDDQLcp6iI

John R. Baker, P.E. (ret)
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Irvine, CA
Siemens PLM:
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It's finding someone you can't live without

RE: Miami Pedestrian Bridge, Part VI

dik - Legal quagmire indeed but FIU signed a Local Agency Agreement with FDOT, essentially taking on the responsibilities of being a Transportation Dept, while also having control of permitting (Building Dept) & being the "Owner/Purchaser" and possibly Project Management organization.

RE: Miami Pedestrian Bridge, Part VI

Quote (epoxybot)

but FIU signed a Local Agency Agreement with FDOT,

I suspect that if the FDOT was actively involved with the project, as government agencies tend to be, that they are in for a pile of heartburn, agreement or not. If FIU was taking over, the FDOT had no place on the site.

Dik

RE: Miami Pedestrian Bridge, Part VI

We'd been discussing what might have caused the collapse, but the tragedy was the loss of life. I'm curious if all responsibility will fall on the one person (Denney Pate) who said the cracks weren't a safety concern.

From the Tony Pipitone radio interview:
"A.Z.: It certainly was a tragic error. We don't know why that decision was made to keep the road open, right?
T.P.: We don't, but we do know about accelerated bridge construction, which is what this technology is being highlighted there. FIU is a national leader in it. One of the big selling points is that you only have to close the road for the positioning of the bridge - sometimes only one day. In this case, it was about two days. So, it's a point of pride for these people that they don't have to close the road. There's a reluctance to close the road because it's not called for. But clearly it was called for in this case, and whether that came into play in the decision-making, I don't know. But it was determined it wasn't a safety issue."

There was pressure/pride to keep the road open. When this first happened, people (FIU, FDOT, etc) were quick to say closing the road wasn’t their responsibility. Someone (FDOT?) said the permit for a road closure was there for the asking. I think about the worker bees (like myself) who might have thought it would be been good to stop traffic, but just did what they were told.

RE: Miami Pedestrian Bridge, Part VI

I hope the judge has his 'bullsh*t baffle' on...

Allen's letter notes that making the information public will cause people to 'clam up' and destroy evidence. Allen thinks that by announcing that several federal agencies are looking at charges as well as the Florida cops that everyone will become more talkative and will freely present all the evidence they can locate.

Some days you just have to shake the sawdust out from between your ears...

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (chris snyder)

I'm curious if all responsibility will fall on the one person (Denney Pate) who said the cracks weren't a safety concern.

We have no idea of what the cracking was... no one has come forth with photos or any information from the meeting about the cracks. We don't even know if cracks were discussed. If they were anything to be a concern, Pate, could be criminally negligent or worse.

Dik

RE: Miami Pedestrian Bridge, Part VI

ABC... Always Be Careful?

Dik

RE: Miami Pedestrian Bridge, Part VI

Jaun Brown made a comment about tensioning the web members would affect the camber; I'm not familiar with concrete trusses that are post-tensioned. Is this correct? I would have thought that it would have minimal effect.

Dik

RE: Miami Pedestrian Bridge, Part VI

I don't think they're actually looking to bring criminal charges against anyone, but they're just being very careful in the way they're conducting the investigation so that anything they find can be used in court if warranted. It would be a real shame to find wrongdoing and not be able to prosecute because of a sloppy investigation. You only get one chance to get it right the first time.

RE: Miami Pedestrian Bridge, Part VI

(OP)
dik,
I don't think anyone is "familiar with concrete trusses that are post-tensioned".

Back to the peer review. They should be able to release that, as just like the construction drawings, the peer review was certainly completed prior to February 19, if it was done at all.

RE: Miami Pedestrian Bridge, Part VI

dik,
If you look at it as a beam, the horizontal component of force of the PT in the web would create a couple about the c.g. of the cross-section. This in turn would generate a moment and impact camber.

RE: Miami Pedestrian Bridge, Part VI

dik,
Please forgive my ignorance of bridge structures, but, to my eyes, most of the bridges seen in the links using those search terms look like steel bridges with stucco or some other "cladding". Some of them even have steel neighbours visible in the backgrounds, rivets and all...
Rue Denis Lecoq, Liege, Belgium for an example, the McMillin Bridge in Pierce Count WA for another.
I can't see how those bridges could work without a lot of steel hiding inside.

As for your PDF file. That is the only behaviour you can expect when uploading a PDF file to the Engineering.com server and including it in your post. I was able to access the file from the first try and your second. PDF's are not "pictures" that can be displayed like JPG, GIF, and PNG image files. They can only be opened by Adobe Acrobat and the associated plug-ins in some browsers, and Adobe wants it that way. If you'd like more assistance with image/document/file uploads, don't hesitate to ask.

STF

RE: Miami Pedestrian Bridge, Part VI

(OP)
SparWeb,

You are correct, most of those bridges linked by dik are steel. There is one concrete footbridge in Austria, trusses each side, which is interesting architecturally. And there is one now abandoned footbridge in California, which is really an arch bridge. Concrete trusses are rare, for good reasons.

RE: Miami Pedestrian Bridge, Part VI

...post-tensioned truss bridge from Australia that I previously posted in Miami Bridge Collapse Part III (or maybe IV) - The Rip Bridge, north of Sydney:



Cantilever of 240' each side of the Rip crossing, that supports a 120' drop-in central-span. Constructed in the late [EDIT] mid 1970's.

RE: Miami Pedestrian Bridge, Part VI

Quote (sparweb)

most of the bridges seen in the links using those search terms look like steel bridges

It's used and I've seen a couple... my preference is steel... I like ductility without cracking.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (sparweb)

If you'd like more assistance with image/document/file uploads, don't hesitate to ask.

Thanks for the offer. The website or my browser didn't give me an alternative method other than I can copy the URL and paste it into the reply. I converted the pdf to a png hoping to just paste the image into the reply but after waiting 5 minutes for the png to load, I gave up.

Dik

RE: Miami Pedestrian Bridge, Part VI

dik: "I converted the pdf to a png hoping to just paste the image into the reply"
I've used this pdf to bmp converter for years
https://www.tipard.com/free-pdf-to-bmp-converter/
Converted your doc, opened .bmp w/MS Paint (~6000x5000 pixels, 98MB), saved it as .jpg (after resizing to ~600x500 =>160KB) and it pasted quick/ok in reply (did not post, but used "preview" to chk)

RE: Miami Pedestrian Bridge, Part VI

Here's a 40k a day traffic, 70mph double span bridge, that I traversed 4 times a week for 6 years while being built, while 50 year old one was being torn down. I was very impressed with how it all was coordinated on such a busy I-71 interstate highway. The new one is basically a concrete arch, built formed and poured in place. I don't see us going back to all steel bridges as a standard any time soon.

Truss bridges are another matter. The problem as I see it is in the gusset system. Concrete doesn't play nice with so many opposing forces in the nodes. The only way I see them being predictably safe is to have pre-designed, & tested/certified steel nodes, with concrete cover. The top & deck in concrete is not the issue.
Note the comparison shots of old vs new....
https://www.youtube.com/watch?v=pizGda1_x4M

RE: Miami Pedestrian Bridge, Part VI

There were at least a dozen heavy chains and load binders installed between the middle of the canopy and the outer edges of the deck during the move. There were two on each side at each end with more mid-span. I didn't see anything about them on the drawings.



Would that rigging been capable of developing enough bending loads on the truss-to-deck joints to be of concern? Do we know anything about how tight they were and what procedure was followed for their installation?

RE: Miami Pedestrian Bridge, Part VI

Wanted to learn about concrete hinges and found this.
http://happypontist.blogspot.com/2014/07/yorkshire...
http://www.tallbridgeguy.com/2013/02/19/bridges-on...
It doesn't look very strong, but gets inspections "regularly". 158' long over 6 lanes of busy highway, out in the middle of nowhere (with a road overpass ~1200' south). Built in the 1960s, it was "designed as concrete Wichert trusses, possibly the first such designs in concrete". This design was used as it's in a mining area where foundation settlement is probable.
https://www.google.com/maps/place/Smithy+Wood+Dr,+...
Comments are interesting, esp this:
"the average engineers nowadays use suites of computer programs that “do everything” for them without the need to understand what is actually going on inside the concrete, or how to use Mohr’s circle". There may be be other issues, but it will be interesting if we learn how much attention was given to the 11/12/deck connection.



RE: Miami Pedestrian Bridge, Part VI

Quote (chris snyder)

dik: "I converted the pdf to a png hoping to just paste the image into the reply" I've used this pdf to bmp converter for years

Thanks Chris... use Paint.net for graphics, and, can covert to many different formats, including *.pdf. and can do transparancies (sp?)... often *.dwg -> *.pdf -> transparent *.png for loading into excel and smath programs. I'll figure out some way to get around this hiccup. The smaller sized format may be one of them.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (TheGreenLama)

If you look at it as a beam, the horizontal component of force of the PT in the web would create a couple about the c.g. of the cross-section. This in turn would generate a moment and impact camber.

I have to think about this for a bit... stressing would occur to the tension web members. At the top, this would impart a horizontal force towards midspan, increasing the deflection. At the bottom, a horizontal force component towards the support, also, increasing the deflection.

Have to give this a greater thought...

Dik

RE: Miami Pedestrian Bridge, Part VI

Chris: tried Tipard and it works well...

thanks, Dik

RE: Miami Pedestrian Bridge, Part VI

Yesterday there was discussion of establishing a timeline. I don't know if this video has been posted but it shows the percentage of completion up to Dec, 17 2017. Stars are on offer to anyone who can explain the need for the transparent quonset hut in the last third of the video. It just looks like a big waste of money. Link

Here is the best image I've seen of the PT Tensioning Ram. It gives a good since of scale. I'm not sure how much it weighs but I wonder if the work on #2 & #11 wasn't done on the day of the move because not enough forethought was given to fall protection and doing fall protection as poorly as they did on the day of the collapse, when so many species of officialdom where present results in a deferment. Link

It turns out that Sweetwater is a politically contentious little town. Link Lots of mud slinging, a mayor the is going up the river for some serious corruption. Link Property theft by the Police Dept. The ex-mayor is supposedly the shadow owner of a towing company that received a no-bid contract as Sweetwater's tow company. The city was charging well over $250 per tow. The police were targeting newer cars that had been fully paid off and when the owners failed to claim their cars in an allotted time, auctioning the cars off. There seems to have been either kick backs AND/OR skimming going on with this. An FBI investigation & an audit of the Police Evidence room found large amounts of impounded money missing and a subsequent investigation found that the 7 member PD had a "secret evidence room" filled with confiscated items, counterfeit merchandise & cash. 7 police officers for a population of less than 20K and a city of only 2 square miles. Towing seems to be a big source of cash for Sweetwater, so when a developer proposed building two multi-unit student housing projects with little to no parking, of course the city said YES!

Now, add to this that the Sweetwater student housing parking problem was going to be addressed by FIU, by allowing students living in Sweetwater to park their cars in the university's parking garage/s and the fee would just be a part of their rent paid by the landlord; this scheme doesn't really seem to actually fit the means vs need aspect of the TIGER program. The already built 109 Tower across the street from the university only has 25 parking spots for over 500 residence & half are reserved for staff. Still the much touted danger of crossing HWY 41 was totally conflated. Even the time lapse video prove that there is "Currently" no need for a pedestrian bridge. The developer that built the 109 Tower & the 4th Street Commons sold the 109 Tower but retains the mixed occupancy 4th Street Commons and based on Yelp it is rapidly becoming Student Slum housing.

I'm not against these kinds of partnerships but I object to Federal money contributing more than 50% to a scheme were the university profits long-term from dedicated parking funds that come to them through little to no investment of their own. When you look at projects like the 200 ft span Martin Olav Sabo bicycle bridge Link, including 2000 feet of of bicycle path earthwork for far less money that the FIU bridge, you have to ask some hard questions.

RE: Miami Pedestrian Bridge, Part VI

Dik,
The suggestion from Chris is good for starters but I'd avoid BMP graphics if at all possible.
(1)
When viewing the NTSB letter in PDF, nothing was stopping you from posting the URL. To do that you click on the address bar of your browser which usually highlights the address so that you can copy to clipboard (temporary memory in Windows or Mac). You can then navigate your browser to Eng-Tips and start your post. Paste (CTRL-V) in the body of the posting once you place the cursor in text where you want the address to be placed. If you want no more nuance or fancy business than that, then you're done. If you want it more fancy, well, you can use the "Link" button instead.

(2)
When you have a graphic on your screen and you want to share it, nothing is stopping you from hitting the PRINT-SCREEN button on your keyboard. That puts a screen-shot into your clipboard and you can then open PAINT or whatever equivalent there is on a Mac, then trim that down to only what you want.


(3)
I would have sworn that I've seen you posting various stuff in the past, and assumed you knew already.
Now that I've said my piece, be warned that there are on-line file conversion websites that offer lots of convenience, but in the background the server is keeping a copy. Word-search algorithms and humans can "review the contents" at will. I recommend you keep your work on your own computer, rather than in miscellaneous single-purpose downloads.

STF

RE: Miami Pedestrian Bridge, Part VI

epoxybot - I've looked at the video several time and see no Quonset hut. Any hint on where in the video it is?

RE: Miami Pedestrian Bridge, Part VI

The green house is in the lower right just north (edit: toward the viewer) of the canal:
(link will start at the correct time to see the greenhouse)
https://youtu.be/JiWhw9BZgII?t=307

RE: Miami Pedestrian Bridge, Part VI

Quote (sparweb)

I would have sworn that I've seen you posting various stuff in the past, and assumed you knew already.
Now that I've said my piece, be warned that there are on-line file conversion websites that offer lots of convenience, but in the background the server is keeping a copy. Word-search algorithms and humans can "review the contents" at will. I recommend you keep your work on your own computer, rather than in miscellaneous single-purpose downloads.

I've posted lots of stuff with nary a glitch... something with my browser or the site is causing a little heartburn. I don't often use web based conversions... I sometimes use ClipConverter fot youtube clips. I can convert various file types on my home machine. I try to stay away from sites I'm not familiar with .

Should have noted that I don't use bitmaps... if any graphics format, usually use *.png.

Dik

RE: Miami Pedestrian Bridge, Part VI

Thanks - the original video pops up a "next video" over that area for me. I need to find a way to shut those off; it's irritating; a number of videos are not quite over when those block the view.

RE: Miami Pedestrian Bridge, Part VI

Epoxybot, I know it wasn't intentional, but the picture of the ram also shows the body of the deceased worker to the right of the man in the striped shirt. You may want to do some cropping.

I'd say the hut is for whatever project involved removing the houses.



RE: Miami Pedestrian Bridge, Part VI

Retiredat46 - Yeah, originally I was going to crop the photo and after having done so, I got to researching the weights of the PT rams on William's Forms & Engineering, when I returned to typing the post, I decided to just post the photo link having forgotten why I cropped it to begin with.

RE: Miami Pedestrian Bridge, Part VI

Epoxybot, thank you for your great post. Do you have any information on how the ram works for PT bars? I know how they work for cables, but not solid bars. Is the plate and nut shown part of the bridge? Is the long tube a trumpet or part of the jack?

RE: Miami Pedestrian Bridge, Part VI

Quote (expoxybot)

"Currently" no need for a pedestrian bridge

New take on the bridge to nowhere: bridge for no one.

https://news.fiu.edu/2016/03/fiu-sweetwater-break-...

Quote from linked page: Though the exact number of pedestrians crossing SW 8th Street at 109th Avenue is unknown, it is estimated that more than 4,000 FIU students currently live in Sweetwater,

RE: Miami Pedestrian Bridge, Part VI

I think this image answers my own question about jack.

RE: Miami Pedestrian Bridge, Part VI

3

Quote (jrs87)

Do you have any information on how the ram works for PT bars? I know how they work for cables, but not solid bars. Is the plate and nut shown part of the bridge? Is the long tube a trumpet or part of the jack?

The one used on the Miami project was similar to the graphic you posted above (from Williams Form), except it was an enclosed-type (similar to the photo below), where the offset plate at the base of the jack has an internal racket/chain wrench and socket that enables the PT bar nut of the STRUCTURE to the secured/loosened. There is usually a short pull-bar and coupler within the jack/trumpet length to 'grip' the assembly. The nut/washer/bearing plate assembly at the top of the jack is to transfer load between the jack and the STRUCTURE bearing plate/s. Sometimes a load cell is placed at the top of the jack assembly to cross-check gauge pressure/force to an independent reference.

RE: Miami Pedestrian Bridge, Part VI

Hi All,
I forget which PT video I was watching, but it showed putting a connector on the end of the PT rod, and then a short piece of rod to accommodate the ram. Oh yea, just like the excellent diagram above. The video showed a round tube on the end of the ram with a hole to loosen or tighten the nut.
SFCharlie

RE: Miami Pedestrian Bridge, Part VI

A little more about Sweetwater, it floods all the time, with just the least amount of rainfall. The easy flooding of Sweetwater (3ft above sea level)is in fact, preventing the USACE from pumping greater quantities of swamp water seaward to prevent coastal flooding in large portions of Miami Beach. Every storm drain in Sweetwater always has water just below the drain. One would think that any project in Sweetwater involving Federal funds would have some component of future proofing flooding. Miamians are greatly concerned about the ever increasing coastal flooding and while sea-level rise is always pushed to the front, it turns out large portions of Miami are sinking by -3mm/year. I'm surprised there isn't a telenovela about Miami-Dade politics. This bridge collapse is quite an eye opener.

RE: Miami Pedestrian Bridge, Part VI

With climate change and the rising of sealevel, Sweetwater could be inundated in a decade or so... Why would anyone develop in that environment?

Dik

RE: Miami Pedestrian Bridge, Part VI

It'll be the new Venice. Just think of the civil engineering challenges and opportunities.

RE: Miami Pedestrian Bridge, Part VI

Retiredat46... there will be lots of those... in particular any water treatment plants and sewers for a start...

Dik

RE: Miami Pedestrian Bridge, Part VI

(OP)
Nasty looking cracks. Not sure exactly where those two photos were taken, but those are not off form finishes, which may indicate some attempt to repair or conceal has been done previously. Regardless of whether release of these photos was an accident by FIU, the newspaper is giving us information which the engineering community needed. The NTSB should get cracking with releasing the preliminary report.

RE: Miami Pedestrian Bridge, Part VI

When I looked at the long list of major bridge projects Figg has completed successfully, I asked myself why they even bothered with something like this. I'd like to know if Denney Pate made a special trip to Miami just to look at this bridge before the move, or if it was a brief side trip since he was already there for something more important. It's hard to imagine a minor project like this got his full attention at any time before the collapse.

From the MH article: "Outside experts have zeroed in on that truss member, identified in plans as No. 11..." The angle of the member suggests it's #11, too. It's just inches away from the area shown at the end of video 560.

Oh great...another signature bridge, this one for I395.

RE: Miami Pedestrian Bridge, Part VI

The photos in the Miami Herald are at the base of member 11. They are supposed to have been taken following the application of tension to the bars in member 11, and before the lift, which must have taken place after the white coating, which is not apparent in the cracks.

I think the photos support the theory that they thought that applying tension in the bars would pull the crack(s) closed without understanding they were levering the member off the deck instead. The part breaking off is not in the main load path of member 11 and shows what looks to me like lateral movement. There is little doubt in my mind that, if the cracks were caused by lateral slip induced by the tension bars, the additional load from the bridge installation would make that displacement greater, possibly by a lot, as the rebar between the members and the deck bent and the concrete it was embedded in gave way. Would their discussion then be about what the greatest load the bars could sustain in order to close the cracks?

I don't recall any evidence that there was a spotter at the base of member 11 to see that the crack was being closed.

A somewhat related article http://www.miamiherald.com/news/local/community/mi... "The firms behind the failed FIU bridge give up the fight to rebuild I-395. So now what?"

Part of the reason they were fighting is they lost a bid, in part, because "the FAM team's bridge design violated one of the competition's rules because it included decorative elements that served no structural function. The FAM bridge, designed by FIGG and another subcontractor, featured two giant support pylons meant to resemble dancers that incorporated spinning discs at the top with no structural role."

RE: Miami Pedestrian Bridge, Part VI

(OP)
A lot has been made of the joints at the ends of member 11. I agree that these joints had little hope of surviving, but I think focus on the stress bars and axial capacity of the diagonals is too narrow. To me, shortening of the top and bottom deck due to shrinkage and post-tensioning, resisted by the diagonals, made cracking at these joints inevitable.

RE: Miami Pedestrian Bridge, Part VI

hokie66,

If you are referring to my statement, it has nothing to do with axial capacity of #11 and only to do with the shear capacity between #11 and the deck.

RE: Miami Pedestrian Bridge, Part VI

It looks like someone made a vertical pencil mark across the larger crack so they could tell if got worse. The larger crack appears to turn horizontal to follow the joint with the deck toward #12.

Is there any way they could have added reinforcement around that area to save the bridge at that point in time?

RE: Miami Pedestrian Bridge, Part VI

To be fair to the Miami Herald, that (link by SF Charlie) is a pretty fair and accurate report for a non technical journal.

One question for those who are able to decipher the released drawings - Is there a clear picture of what the reinforcement looks like in the 11/12/deck interface?

What was actually connecting the compressive forces on no 11 to the lower deck? I've not seen in any of the drawings posted to date re-inforcement details that explain that.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part VI

LittleInch, while this link is not direct answer to your question, it may help guide on where to look. I believe "collector" is the applicable term.

https://www.nehrp.gov/pdf/nistgcr10-917-4.pdf

RE: Miami Pedestrian Bridge, Part VI

(OP)
3DDave,

I wasn't referring to your post. I agree with you, I think. Was just pointing out that axial shortening of the decks would be a big contributor to stress in the joints.

LittleInch,

From the information in the drawings, I doubt that I could draw details of exactly what was intended in that joint. The 8-#7 bars in the web member would seem to be woefully inadequate, no matter how they were developed into the deck. And I saw no indication of extra confinement reinforcement in addition to the column ties.

RE: Miami Pedestrian Bridge, Part VI

Quote (SFCharlie)

We were wondering what the Miami Herald knew about the cracks. Well...
Cracks where FIU bridge buckled may have signaled 'imminent failure'

I sent the following eMail to the reporter of the article:

Since the NTSB has an embargo on information beyond February of this year, you might want to obtain copies of approved/reviewed reinforcing steel shop drawings; this would have occurred at an earlier date.

These drawings would show the location, placement and quantity of reinforcing steel in the 'truss'.

Great factual article, thanks,


Dik

RE: Miami Pedestrian Bridge, Part VI

LitleInch: "Is there a clear picture of what the reinforcement looks like in the 11/12/deck interface?"

I think the info is on sheets B39 and B40 which show up out of sequence in the group of drawings posted by jasm (Civil/Environmental)21 Apr 18 02:37. The areas of interest are:

The bottom of #11/#12


The section showing typical rebar placement


If you step through this time lapse video from 35 seconds to 1:35, you can see the form work for the truss was in place before the deck was poured. That makes me wonder if there really was a cold joint between the deck and truss members. I couldn't tell for sure from the video. Maybe you can.
https://www.youtube.com/watch?v=JiWhw9BZgII


RE: Miami Pedestrian Bridge, Part VI

This has better detail from above referenced doc page 86/110 B-61 comparing #1/2 to #11/12. There seemed to be more rebar in collapse photos than in #11/12 here - I looked for it elsewhere in prints but didn't see it... like what's sticking out the bottom of #12 where it's resting on top of pier.

On page 64/110 B-39, note 8 says "Secondary pour shall take place 180 days after casting of main span". Pour sequence elsewhere said 1) Deck, 2) members, 3) canopy, so I wondered if this meant 180 days between deck and members being poured (probably a dumb question, but time lapse messed up my perception).

RE: Miami Pedestrian Bridge, Part VI

Thanks for the better detail.

I think Note 8 goes along with Note 7, and only applies to the concrete for the hinge locations. I remember seeing pictures after the collapse that showed the hinge locations hadn't been filled yet. They had slings through the gaps.



RE: Miami Pedestrian Bridge, Part VI

This might be a silly question, but:

When I'm designing a fiberglass or carbon fiber structure, I tailor the local lamination schedule, particularly the fiber orientation, to accommodate local loads. Specifically, I use bias (usually +/-45-degree) fibers to distribute shear stresses into the rest of the structure. Based on that practice, I would have expected to see a set of angled "chevron" rebar reinforcements in the deck as shown below to distribute the thrust applied by #11 into the rest of the deck. The chevrons could go under or over the PT tendons, or maybe both.



Would that have been an unworkable approach? The argument might be made that the deck is too shallow to accommodate these, but that might just be another way of saying the deck is too shallow to function properly.

--Bob K.

RE: Miami Pedestrian Bridge, Part VI

(OP)
That's not a silly question, you are just thinking like a structural engineer should.

RE: Miami Pedestrian Bridge, Part VI

hokie66 (Structural)
The NTSB has been diligently doing the fact gathering. The Preliminary Report is being review, and might be released in 2 or 3 weeks provided no major concerns are found. Please remember that the Preliminary Report will only contain evidence, not conjecture nor speculation.
SFCharlie

RE: Miami Pedestrian Bridge, Part VI

(OP)
I know that. Hopefully, the raw evidence will be helpful, but maybe not, without the analysis to follow. The fact of the embargo of evidence after the February date makes me think maybe there are some revelations.

RE: Miami Pedestrian Bridge, Part VI

Bearing shims. (uncropped photo, circled in red) Note one is broken and the other one ended up on deck.

RE: Miami Pedestrian Bridge, Part VI

Thanks for the diagrams. Is there one in plan or do we just assume that rectangular box of re-bar is only as wide as the no 11 column?

given that the 6507 bars are higher than the rather slim deck it would look like that to me.

I'm with hpaircraft here, but them I'm not a concrete structure person However, I would have anticipated either a significant end beam at 90 degrees to the column which is then essentially held in by the long PT rods in the base structure or something spreading the end column load more effectively into the lower slab structure as per the hpaircraft post.

It would be interesting to see where that cage of reinforcement ended up - in the deck or ripped apart

Any thoughts from the concrete structural experts?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part VI

Quote (hpaircraft)

I would have expected to see a set of angled "chevron" rebar reinforcements in the deck as shown below to distribute the thrust applied by #11 into the rest of the deck.

Good post.

I too am not a concrete structure expert... hoping someone will chime in on your post, as it seems sensible.

What I'm still wondering (apologies to the group if this has already been answered elsewhere) is if that end thrust from the #11/#12 node was designed to be counteracted by the #1 and #2 node of the other half of the bridge, or if the two spans were intended to act with relative independence.

We haven't discussed the bottom node at #1/#2 much in our discussion so far.. on that end of the span, is there a shear lug or other feature which transfers thrust into the top of the column at that end, which differentiates it from the design used at #11/#12? Or is the span truly free floating at both ends as designed?

If the intent was that #11/#12 thrust would be counteracted by the other span, then not having any temporary method of bracing against that thrust seems like a massive oversight.

RE: Miami Pedestrian Bridge, Part VI

I don't have enough information, but, it may be that the loading from the web diagonals was not properly distributed into the deck... waiting for info... I'll then be able to see what failed catastrophically. The cracking observed may be related to the collapse.

Dik

RE: Miami Pedestrian Bridge, Part VI

hpaircaft is correct about the benefits of these chevron ties to bring back the compression effects of the tendonds to the location of the struts. Also, that can be accomplished by the reinforcing of the end diaphragm.

As somebody that have done this kind of things many times in concrete structures, I can tell you that typically you would put a big tendon in the same vertical plane of the struts to control the tension tie of the strut-and-tie system that happens that the node 11-12-deck. In this case it appears that the system relies on shear friction to transfer the tension transversally to the tendons. This is a akward way to do it, but it is possible to do it provided that you are careful and you have your forces correct, including any secondary effects. These effects can only be known if you do a tridimensional analysis. I wonder if only a plane frame analysis was done.

This is why it is important to get the signed and sealed calculations from FIGG. Although may be a brave soul out there can do the analysis and give us the forces in the node at the time of collapse using both methods.

But do not forget that node 11-10-canopy is also worthy of inspection.

However if the critical plane is that at the interface with the deck (or the canopy at below the canopy), the chevron ties are useless. Take a look at these planes and you will see that they do not have much rebar crossing them.

Good hunting

RE: Miami Pedestrian Bridge, Part VI

jgKRI,

I've been wondering that thought before (temporary single bridge versus final two bridges) but no way to find out so far.

It's not clear how the two bridges were going to be attached to each other, but there is a lot of spare re-bar floating around and possible connections to the pier.

I reality I don't think they would do it that way as it is not clear that the two structures would be tied together securely enough to allow forces to be balanced between the two structures.

The mad Spaniard I think hits the right note - to separate significant loads being generated into the deck and then having the tie rods which resist those loads not in the same plane on the face of it appears a little odd, though the nearest PT rod is fairly close.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Miami Pedestrian Bridge, Part VI

The decorative 'cables' would tend to pull the two halves of the bridge together. The force on the abutment would be both vertical and horizontal.

Dik

RE: Miami Pedestrian Bridge, Part VI

I wonder if FIGG ever considered load testing the 174' span before transporting.

There are photos that show evidence of surface-mounted strain gauges on some diagonals, so some instrumentation was thought about. With FIU and its BSCE students/professors it could have been used as a research project.

It would have been relatively simple to load test, at least to part simulate the design vertical loading. It would have required some (significant?) temporary foundations to support the imposed test loads, but given the "pioneering" design, the cracking that occurred 10 days before transporting, and where we are now (collapse and loss of life), it may have been a prudent thing to do - granted, with 20/20 vision.

RE: Miami Pedestrian Bridge, Part VI

(OP)
Ingenuity,
Figg may not have thought of that. Several of us here have suggested that they had a full scale prototype of this unique structure to test. Yes, they would have needed some footings at the ends. But the load test would have been simple, perhaps using sandbags or water bladders for load.

RE: Miami Pedestrian Bridge, Part VI

If we assume the available bridge construction as a three-plate girder, I would still expect some kind of lateral torsional buckling with large span length before attaching the tension cables. The upper compression plate has the small with, and the weight of the bridge is high since it is a concrete bridge. Additionally there is nothing to stop the end against rotation especially for the upper plate. I guess that the crash initiated with the buckling with the end rotation, and failed soon after.

The buckling might have been initiated by the application of tension in the cables cables one side only on the upper plate or unequal tension forces between cables located symmetrically. The failure occurred since there was not any balancing restraint at the end.

This are speculative guess-works only. I hope the final report will come out soon and give us better explanation about the failure.

RE: Miami Pedestrian Bridge, Part VI

Please read "The upper compression plate has the small width,...." instead of "The upper compression plate has the small with,..." in the my post.

RE: Miami Pedestrian Bridge, Part VI

I had a chance to see the video for the time of collapse, I saw that the last bay was acted as a mechanism.

I have two scenarios in my mind:

1. It seemed to me that the column-brace connection to the upper plate could not transfer the forces created, and sheared first, and the column (not the end column) pushed the lower plate and caused bending which probably was not designed for, and the entire bridge has collapsed. This requires a close up views/photos to understand the failure type at the connection mentioned here, and the reason.

2. There was not sufficient cable tension forces in the upper and lower plates to handle the forces created during the construction, and therefore the system acted as a mechanism at the weakest (or concentrated larges forces at this bay) location.

These may not be the case, but acting like a mechanism was clearly observed at the last bay. I hope the authorities can provide better explanation.

RE: Miami Pedestrian Bridge, Part VI

Quote (Ingenuity)

I wonder if FIGG ever considered load testing the 174' span before transporting.

Not common unless included in the specifications... and, with design build this would not likely be included in the QA/QC... it's an added cost.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (saplanti)

I would still expect some kind of lateral torsional buckling with large span length before attaching the tension cables.

It failed because of ?, something beyond its inherent shape... an improperly designed tube could fare no better. It's not necessarily the shape. When I did the large freestanding stair at the Cornwall Centre in Regina, the contractor didn't want to remove the forming because he was afraid it wouldn't stand up.

Dik

RE: Miami Pedestrian Bridge, Part VI

(OP)
dik,
I disagree that load testing of a new product is not common. This bridge was a completely new concept, not just a routine concrete structure. When I have done designs of building elements to be used as prototypes, I have required load testing.

RE: Miami Pedestrian Bridge, Part VI

If it's not spec'd, don't expect it.

Unless spec'd, load testing of new construction is not common, and, the owner would not want to pick up the ticket for the load testing. It's not uncommon to spec it. Load testing costs many thousands of dollars and affects schedule. This is particularly true with design build; they won't do it on their 'dime'.

Dik

RE: Miami Pedestrian Bridge, Part VI

I never really believed they could be stressing the PT rods at the time of collapse, but after seeing those cracks prior to the move, both in member 11 AND at the deck interface, it's now in the realm of possibility that stressing operations were being performed at the time of collapse. But to be "potentially" stressing member 11, in an effort to essentially SAVE the bridge, without first installing temporary falsework and closing traffic is beyond comprehension.

One set of data that will be interesting to see, once NTSB releases its report, is how the structure was behaving during the stressing of all post-tensioning prior to the move. Were they getting the expected camber? How did this construction stage deflected shape match calculations? If it wasn't behaving as expected, what changes, if any, were performed? I hope all this comes out.

RE: Miami Pedestrian Bridge, Part VI

Quote (TheGreenLama)

But to be "potentially" stressing member 11, in an effort to essentially SAVE the bridge, without first installing temporary falsework and closing traffic is beyond comprehension.

...and, not running up a 'flag' when the cracks were first noticed... the engineer could be in for a heap of heartburn.

Dik

RE: Miami Pedestrian Bridge, Part VI

I'm suffering from "Too Much Information". I felt certain that I had read, within the FIU drawings, that the temporary shoring of the diaphragms shall support the structure only where the shims & bearing pads would contact the pylon or bent. I've been through the drawings twice now and can't find the note. Perhaps I read something similar in a ABC bridge power-point. The south end deck/diaphragm had plinths but the north end did not. Even with the plinths the top of the shoring would have required some kind of plate to evenly support & spread the load. In the time-lapse, when the bridge is moved off the temporary shoring, there isn't any thing to suggest any final bearing footprint had been mimicked prior to the move. Unless the bridge was supported on the temporary shoring in the same manner it would be supported on the pylon & south bent, there isn't any conclusion or hypothesis one can project on to the performance of the cracked 11/12 node.

I did find a note that might clear up some confusion regarding the temporary supports that shall remain in the middle of the bridge during SPMT transport. A later note states that temporary supports of the deck shall remain inside the diaphragm. I believe this is in regards to the SPMT blocking, since the diaphragm is the portion or the deck with longitudinal PT reinforcing.

RE: Miami Pedestrian Bridge, Part VI

My take...
1 Deck was poured.
2 Truss was poured.
3 Canopy was poured.
4 Blisters were poured. At his point, all concrete, & steel is at equilibrium, with no forces to cause cracks.
5 Once concrete was aged, post tensioning was started while still in supported mode.
6 Then shoring was removed, leaving only end bearing supports. I believe this is when #11 cracks 1st showed up, due to the flexible #11 rebar in shear
interface to deck. It cracked, & jammed #11 into #12. #12 was more solidly restrained in place due to #12's to deck rebar in tension formation.
You can still see #12 to deck rebar at the end of the deck, with the j-bends on them that turned upward into the column. These, combined with the visible
Box out area under #12, made for a more rigid deck connection. The rebar ran in a more parallel interface to deck, which put it in tension, vs #11 rebar
in shear.

6 Once it was time to move, diagonal/post tendons were pre-stressed to allow inward load bearing on transporters. At this point, the crack most likely
stayed open.
7 Bridge was traversed across uneven road ( curb on south end , plus divider curb at mid/northern side) with several moments of inward load bearing, while
they shuffled bridge off center. It's evidenced in one of the time lapse videos. In effect, this may actually have closed the cracks. Due to the extreme
cantilever, it resulted in more weight than the tendons could bear. The tendon calc. was base on an outer bearing point.
8 Bridge is set in place, with crack still closed up, due to additional surface friction from #11 tendons at 560 kips.
9 I'm thinking at this point that once bridge was set, the next step was to de-tension #11 tendons. If this was not the case, why else would have #11's
blisters have been grouted? It was the planned course of action by design.
Remember, they had to jack hammer the opening to get to the tendons for the tensioning, that Rubio later said they were going to do to close the
cracks.
10 Jack hammered out grout, & reapplied tension to close the crack.
11 Bridged collapses while applying tension to last #11 tendon.

The logic behind applying tension to #11 to close cracks, may have come from the likelihood that it probably did just that when it was moved, & set. So,
it worked before, why not again!
The basic push/pull loads from shoring, to transport, to being set, to releasing tendons, to applying tendon tension, at #11's base contributed to bridge
collapse. In the end, I believe that #11 short fillet bore the brunt of horizontal loads, exploded.

RE: Miami Pedestrian Bridge, Part VI

Quote (dik)

not running up a 'flag' when the cracks were first noticed... the engineer could be in for a heap of heartburn.

Is what I was thinking when I first saw those photos - there would have been a lot of people who knew about it. Four independent engineers gave opinions about the photos.. three said cracks were a sign of imminent failure - from one, "There's nothing they could have done short of starting over and redesigning the structure,' said Howell, whose firm conducts bridge inspections for the state of Texas."
No one would have wanted to do this, or even suggest it, but two said they could have tried repairs. The worst was not closing the road while they were working on it, and I'd assume the cracks got worse after the move.

Does anyone think a repair could have been made?

RE: Miami Pedestrian Bridge, Part VI

Chris Snyder: If all else is 'up to snuff' then an epoxy injection is possible; it appears that this is not the case. I think the flaws were too deeply seated for a simple repair to be made. It would have been a messy redesign and reconstruction job. If you are trying to develop a large bar in compression, bends are not effective and the section depth did not look like you can develop a large bar... but, I don't know. I need more information.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (epoxybot)

I'm suffering from "Too Much Information".

Agreed... too much and not enough of the right kind...

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (chris snyder)

Does anyone think a repair could have been made?

That's just it. Nothing is obvious from the pictures included here. One possibility could be to make the temporary tendons within 11 permanent. [Edit: Only as a means of attacking the cracks in the short term. There still would exist serious design issues with the overall design.] But it's hard for me to fathom how an engineer could sign off on a change order like that. I won't even speculate on what that hypothetical wording might be.

RE: Miami Pedestrian Bridge, Part VI

I'm hoping that once the data is released, that, because of the type of collapse. The cause of the failure will 'stick out' like a sore thumb... Need information: 'design forces in the web members, including shears and moments. Require how much reinforcing and how it was placed. Need information on how it was constructed. Need information on the 'strand/bar' and sequence of post-tensioning. Not much information, and, may have missed a couple, but essential to run some numbers through. I don't know how ACI or AASHTO handles the 'triaxial' mess at the nodes.

Dik

RE: Miami Pedestrian Bridge, Part VI

(OP)
dik
I think the cause already stands out like a sore thumb...your "triaxial mess at the nodes". Trusses need axial forces to be coplanar and connect, which wasn't the case here. Neither did the nodes cope with the inevitable bending moments.

RE: Miami Pedestrian Bridge, Part VI

(OP)
Cutterhead,

As to your point 4 above, you forgot shrinkage and temperature forces.

RE: Miami Pedestrian Bridge, Part VI

Quote (Hokie)

your "triaxial mess at the nodes"

I've thought that was the case since the beginning... I need member forces (axial, shear, and moment) to just confirm. My suspicions have been that there has been an improper development of these... and need data to confirm. I don't have a lot of confidence in the NTSB coming to the correct cause... I've seen how Ministry of Labour works in Canada...

Dik

RE: Miami Pedestrian Bridge, Part VI

(OP)
Not sure what capabilities the NTSB has in house for analyzing this. I think it more likely that they will use a consultant like WJE, SJH, or somebody, then take credit for the report.

RE: Miami Pedestrian Bridge, Part VI

Hokie, I'd think that they would use an external consultant, too.

Dik

RE: Miami Pedestrian Bridge, Part VI

dik,
I've had nothing to contribute technically, but I have read with interest all along.
What is stopping you from carrying out a truss analysis to get the loads to a rough order of magnitude?
(Again I am likely exposing my profound ignorance of bridge building).
If FIU intended the bridge to function structurally like a truss, and oriented the members to behave that way, can it not be worked out what the member loads were expected to be?
One would have to disregard any failure on anyone's part to actually accomplish a true truss with pin-joints, members aligned with nodes, no secondary moments, etc. etc.
The design report details all cross-section dimension, member lengths, and material densities, therefore is this enough data to carry out a "back of the envelope" analysis of the loads?

STF

RE: Miami Pedestrian Bridge, Part VI

I don't have the time, or, the applicable codes for loading combinations. It would be handy to have a listing of the applicable loads for analysis. I can work them out, but, it would take a week of solid work. It would be easier to be provided with the loading and to do a quick check to see if the loading is appropriate. This information should be available since it occurred before the NTSB embargo.

Should have added... I don't have real experience with bridges, either.

Dik

RE: Miami Pedestrian Bridge, Part VI

Quote (Hokie)

Trusses need axial forces to be coplanar and connect, which wasn't the case here.

It's nice to have, but, if it's not that way, it's possible to design to incorporate this. I had a 18 or 20 storey building that the owner wanted the one building column moved about 6' on the second floor... had to transfer the loading over 3 floors, but, it's still standing.

Just so much information not available, yet.

Dik

RE: Miami Pedestrian Bridge, Part VI

Just realised it... I'm surprised that Figg or Berger has not called in their own independent consultant to review the design.

Dik

RE: Miami Pedestrian Bridge, Part VI

Why don't y'all take a' look at
Chris Snyder (Electrical)'s great post (many stars) above at 30 Apr 18 21:20
,in it there's a link to the full analysis?
SFCharlie

RE: Miami Pedestrian Bridge, Part VI

SFCharlie: Saw that way back... I could not determine where the forces shown came from. I'll take another peek... maybe there's a link to the actual load generation.

Dik

RE: Miami Pedestrian Bridge, Part VI

SFCharlie: In any analysis you also need to account for post-tensioning forces, anchorage forces, creep, shrinkage, and temperature forces over time. Not sure if anyone has included these in earlier attempts. As has been stated many times before, this is a complex concrete frame, not a steel truss.

RE: Miami Pedestrian Bridge, Part VI

Thank you Charlie, for bringing that back to my attention. It deserved a more detailed look. I did not discover the loads diagram on my first pass through the graphic. Lots to take in, there. Gotta zoom/pan around and around to take it all in. Not sure why it was presented that way.

"in it there's a link to the full analysis?"
Still can't find links to analysis, though. Just the pictures.

STF

RE: Miami Pedestrian Bridge, Part VI

What's missing in the Toomas analysis are loads - I didn't ask him what numbers he used (he'd asked if I had centerlines at one point, but he soon sent the analysis), but may get back to him after I triple check mine (using new specs).
My question is why the stress is much different for #1/#2 and #11/#12 - is from asymmetric truss pattern, because #12 hangs off the deck, both, or ???. But he'd said 2D analysis wouldn't show this, and he's right - #1/#2 diag force shows ~40% higher than #11/#12 (they beefed up #1/2 but not #11/12). In final configuration, #1/2 would be floating on bearing pads (for expansion), while the pylon ends of main/backspans would be fixed, with some strengthening by being tied together and help from stabilizing pipes (but not as much as if this was a true suspension bridge).

Would be good to analyze the strength of the #11/#12/deck connection (is simpler) - even within 10%. Estimated diagonal force is ~1350-1500 kips plus 560 kips PT tension (totals ~1400 kips horizontal) - the rebar is in the specs. This area has weak boundaries with the ~3" PVC pipes (two on each side of #12), rebar/other rods (which strengthens concrete, but would seem to create a "parallel sheer plane?", e.g. if many spaced 2" apart parallel to sheer force), and the ~8"/large drain pipe at the bottom through the diaphragm - plus the cold joint itself, which photos show had cracked. The #4 photo showed another hole near #11 in deck (that was later filled?).

Someone asked what may have happened to the rebar cage under #11/12. That's a good question (in the canal?) because the concrete was pulverized at the base of #11/#12 members, but may have held within the "cage". A chunk is missing from deck (seen in photo), but deck held the lower #11 PT rod. Photo shows the deck concrete broke apart at the east side of vertical PVC pipes as #11/#12 sheered.

Interesting how hpaircraft's idea above (9 May 01:08) is similar to gwideman IV 24 Mar 18 13:55. I can't find good photos that show if the #1/2/deck area had PVC pipes, etc.

I've noticed in a few concrete truss bridges (very limited knowledge) that the connections aren't rigid concrete - hinges, pivots, and steel were used (two of these expected soil shifting). The FIU span was only concrete and rebar - cracks showed after tensioning (adding 560 kips?) with support only on ends even before it was moved. As independent engineers noted, the connection should have been analyzed instead of just saying it 'looked okay', moving the span, and working over traffic. I'm curious how cracks changed after it was moved.


RE: Miami Pedestrian Bridge, Part VI

I do not think complete analysis are needed to pinpoint the problem with this structure, but could be done using available information.
AASHTO Guideline for the design of Pedestrian Bridges was used, supplemented by AASHTO LRFD for Bridges. Applicable load combinations were posted before, the only unknown is if Strength V was used as this is required only for major bridges. So, the bridge had likely SF=1.5 at the time of collapse, or less.
Judging on the location and magnitude of the crack at the base of #11, and the fact that the span was still fully supported, the crack was caused by post-tensioning and shrinkage alone.
So, the sequence of the post-tensioning is critical in establishing the magnitude of the forces generated in the nodes.
AASHTO and AREMA do not have explicit requirements for triaxial stresses, and how to handle sizing of the reinforcement for shear and tension in the areas where such 3-D loading occurs.
As I posted before, the bridge was a failure at the design stage, as nobody question the overall safety factor, and the design of the connections, which were dramatically weak for shear.
It is also worth noticing that the crack presented in the "accidentally" leaked photograph extend horizontally at the intersection of #11 and the deck, clearly indicating shear failure of concrete in between this two elements.


RE: Miami Pedestrian Bridge, Part VI

FWIW, Have you seen this video published May 12 by Construction Engineering & Photography?

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

There are some screen shots from this thread in the video.

RE: Miami Pedestrian Bridge, Part VI

Great video... just a couple of comments:

The crane couldn't have been in a worse location; it obscures the initiation of the failure.

The second comment of maybe a little more merit. He places a fair amount of emphasis in the break in the canopy to the LHS of the intersection of Member 11 and the canopy. It appeared that way, and the reason I hadn't sorted out my reason for the collapse was that this is a lightly loaded member and the crack shouldn't have occurred there. In an ideal pin connected truss, it would normally be assumed to be a zero force member. I saw the 'crack' but couldn't explain it unless it was a consequence of the node (blister) failing. I still haven't 'wrapped my ears' around this. His explanation of the 'hinge' was well done... just in looking at the failure mechanism, it appears to be a hinge, and if any construction joint was beneath the support, it would likely have created this hinge. I still think the failure occurred at initiated with the node, and, not at Member 11.

Dik

RE: Miami Pedestrian Bridge, Part VI

For dik and Retiredat46

At this moment I have turn my sight to the node 10-11-canopy. If you notice, the potential failure area above the struts and below the canopy is about 21x58 inches (1218 square inches). If you use the self weight values in the struts from Mr. Toomas Kaljas (with angles of 27.4 and 51.7 degrees), you get a 1700 kip horizontal interface force applied at the bottom face of the canopy. If we use the AASHTO section 5.8.4.3 coefficients for surfaces not intentionally roughened (K2 = 0.8) , it results in a maximum interface shear friction resistance of 1218x0.8=974 kips. If we reduce it by 0.9 we get 877 kips with is much less than 1700 kips (unfactored).

Now, I know that there are threedimensional effects that may change Mr. Kaljas forces if Mr. Kaljas model is aplane frame. Also the PT may produce forces contrary to the DL shearing load. But, this item is interesting.

Again, has anybody found in the drawings a note that says to roughen the surface to 0.25 inches? Because following the FDOT standard Specicications I believe you only clean the laitance and therefore you get K2=0.8.

With the limited info at this moment, I can not say more. I would appreciate if anybody out there check my analysis. Thanks in advance.

Good hunting.

Live long and prosper

RE: Miami Pedestrian Bridge, Part VI

Too soon to do any serious calcs, but, having a roughness of 1/4" would likely mean that you would have a 'hold down' force of approx the sum of the horiz components in Mem 10 and 11 x roughly sqrt(2) assuming a rough 45 degree plane. The vertical load would be small, so, friction could be roughly ignored. What may have been missed is that the horizontal connection would be the summation of the web diagonal horizontal loads because one is in tension and the other in compression... don't know yet.

Dik

RE: Miami Pedestrian Bridge, Part VI

TheSpaniard: Have you located tablular data that shows the type of loading, and member axial, shear and moment forces for Members 10, 11, 12, and the canopy RHS and LHS of member 10 and 11 intersection. I've looked through the postings and have been unable to locate this. Apparently, it was posted.

Dik

RE: Miami Pedestrian Bridge, Part VI

I think sheet B-24 Note 4 only applies to the back span side of the pylon base. Since the forms for the truss on the main span were in place before the deck was poured, it would have been difficult to control or roughen the surface at the joint locations. Also, I couldn't find a similar note for those areas, but maybe I missed it.

RE: Miami Pedestrian Bridge, Part VI

Quote (Retiredat46)

"Note 4 only applies to the back span side of the pylon base"

I don't think you missed similar notes, no one else has found any. I mentioned B-24 note just to hint that no call out for roughing exists elsewhere for superstructure.

I linked to a LRFD document in my last post as it helped me get into the mindset of the designers of this overpass, and I feel others may share this sentiment.

This thread as concentrated on the span being a concrete truss, and of course it is, however, it is also a bulb I-beam or girder with an open web. As such I believe the following diagram to be enlightening. This thinking may be wrong, but if you go with it, it makes some sense out of what happened to bridge. (In my experience mechanical engineers often simplify the parameters of a problem and then take it the extreme when analysing, similar to Occam's razor. https://www.amazon.com/Thinkers-Guide-Analytic-Thi...)

US Department of Transportation Publication No. FHWA-NHI-15-047 5.105



Any comments on this diagram?


RE: Miami Pedestrian Bridge, Part VI

Thanks dik, retiredadat46 and jr87 for the responses.

The 1/4" roughness may be an important omission in the plans. But we have to remember that the plans have a date of 4/7/17. There may be another more recent set with revisions or the inspectors asked the question and there is a memo requesting to roughen the joints. We have to wait to get more info from FIU via the Miami Herald.

And responding to dik question, I haven't seen any open posted or published mumbers other that Mr. Kaljas numbers above in this thread or what is in the plans. I have not done a structural analysis of the system. I will try to send an e-mail to Mr. Kaljas to see in he can implement the PT in his model and share the info will all of us.

Best regards

RE: Miami Pedestrian Bridge, Part VI

The design data predated the NTSB embargo, and it may/should be available. Give us real forces and design loading.

Dik

RE: Miami Pedestrian Bridge, Part VI

Generally, post-tensioned spans have a solid web whereby the c.g. of the PT force can move up and down depending on the loading and support conditions. An I-beam or a concrete box girder are common examples. In our instance we have neither (I've removed web from drawing):


All PT is straight and located outside the centerline of the bridge. And as far a "web" goes, we don't have an open web, we have no web. Particularly in the end regions of the bridge, how is the PT developed across the cross-section? What is the cross-section? So not only are there issues with shear lag at the supports--all PT is located outside the centerline of the bridge, but there's no web tieing the flanges together. And why is there even PT in the top canopy at this stage? If it's a "truss" you wouldn't expect the need. Is it there to partially balance the bottom (rough calcs show PT stress in top canopy roughly 50% that of bottom slab)? Is it there to help in some way with shear? Is it needed because there's bending in the top canopy?

I'm still not clear exactly how this bridge behaves, other than to call it an ill-conceived frame.

RE: Miami Pedestrian Bridge, Part VI

TheGreenLama - In the original proposal, the only portion of the bridge that was to be post tensioned prior to the move was the deck. At least that is my take. The repositioning of the SPMTs seems to have resulted in a lot of addition PT decisions.

Surface Prep:
Here is one concrete surface note I came across from the Deck Pour drawing.



RE: Miami Pedestrian Bridge, Part VI

epoxybot, so it's conceivable that some portion of top PT is required for the temporary load conditions during the move.

RE: Miami Pedestrian Bridge, Part VI

Quote (TheGreenLama)

"...we have no web."

TheGreenLama, very well put, I see your point. It seems also a whole section of engineering discipline was missing in this project.

RE: Miami Pedestrian Bridge, Part VI

TheGreenLama - Assuming PT Rod became necessary in members #2 & #11, as a result of new locations of the SPMT (Cantilever Condition), yes. Stress the deck, stress the canopy, stress #2 & #11 ...,

If the canopy isn't stressed before #2 & #11 are stressed, wouldn't the canopy crack somewhere between the 2/3 & or 10/11 blisters?

RE: Miami Pedestrian Bridge, Part VI

Time to go back to traffic camera video. Only 2 or 3 frames are useful. Clearest version: https://www.youtube.com/watch?time_continue=7&...

No obvious sign to me of 11/10 failure starting collapse, but this time I notice interesting ripple motion of banner on 11. It appears member 11 is shortening and bursting behind the banner early in the collapse. Or perhaps it's just being blasted by air? I still theorize a shear force found an unexpected path and destroyed 11 at the base precipitating collapse.

RE: Miami Pedestrian Bridge, Part VI

I've come to suspect that #11 was released at the bottom, allowing rotation of the main span segments, which put a side load on #11 at the base, breaking it at the top, because of the semi-parallelogram action of the canopy and deck.

RE: Miami Pedestrian Bridge, Part VI

With all the talk of tensionnig of cables and fancy graphics of stresses in the members and what not, there is one aspect of this bridge I have yet to see anyone address.

This was supposed to be a cable stayed bridge.
http://www.mcm-us.com/projects/road-bridges/road-a...

Now, I'm not a structural engineer, but it seems to me that this span wasn't designed to be supported at just 2 points. Lacking the supporting cables along the length for a long period of time was simply unreasonable. It is also clear in the video of the collapse, there is no temporary support in the middle of the span.

A fan of engineering documentary TV shows, I've seen numerous examples of bridge construction projects. In every example, the supporting towers of suspended and cable stayed bridges are ALWAYS the first things to go up. This bridge's supporting tower had not been erected yet. Seems the cause of the collapse is more in the erection process than the actual "finalized" design or lack of tensioning and such.

Just saying. If this span wasn't designed to be self supporting, it never should have been left up as it was without additional support.

RE: Miami Pedestrian Bridge, Part VI

Quote (dbill74)

This was supposed to be a cable stayed bridge.

No, it wasn't. Did you really think you were the first one to point that out after approximately 1000 posts have been make on the subject of the bridge collapse?

RE: Miami Pedestrian Bridge, Part VI

Please discontinue posting in this thread. Go to Part VII 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.

Part I
thread815-436595: Miami Pedestrian Bridge, Part I
Part II
thread815-436699: Miami Pedestrian Bridge, Part II
Part III
thread815-436802: Miami Pedestrian Bridge, Part III
Part IV
thread815-436924: Miami Pedestrian Bridge, Part IV
Part V
thread815-437029: Miami Pedestrian Bridge, Part V
Part VI
thread815-438451: Miami Pedestrian Bridge, Part VI
Part VII
thread815-438966: Miami Pedestrian Bridge, Part VII




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