New interesting post processed dashcam video at https://youtu.be/D6pGzgm4zZ4
Compression of member #11, before deck fall, could be pushed out PT rod, as someone suggested already.

Quote (dik)

Were you able to determine if the failure occurred at the ends of the web members or at the panel point?

dik....not sure. In one of the NTSB photos, it shows the anchor "blister" relatively intact but broken through the top chord. This is what led me to suggest perhaps a punching shear of the entire panel point through the top chord. There did not seem to be much, if any, lateral mild steel reinforcement across this interface.

Quote (winelandv)

A defense of the design:

To the structurals here, who hasn't, at the behest of an architect, worked their tail off to make something superflous work? Is that not part of our job? Ok, so the "cable stays" really aren't. So?

...

Drawoh's rules of style...

1. Style costs more. There is a way to do it that is cheap and functional.
2. Keep it simple. We are techies, not artists.
3. Bad styling looks a lot worse than un-styled functional.

Okay, I am mechanical, not civil or structural. If management is determined to make it cool, ask for a budget. Maybe you will get a big one! I perceive rule 2 as an artistic issue, but maybe there is a structural component too.

What if they had designed a simple, well proportioned bridge, and added twenty tons capacity for artists to play with?

--
JHG

Drawoh,

I agree with your rules of style. That being said:

1) The people who decided to lay out the money for the bridge wanted something snazzy. Without actually looking in to it (I'll save that for epoxybot), I'm willing to guess that the style of the bridge contributed to the DB team getting the project. So yes, the city/state/university most-assuredly paid more for this bridge than a "standard" (whatever that is) long span bridge.

2) Unfortunetely, with bridges, as you adjust how it looks, you're adjusting how it works. You've essentially got a line with 2 or more supports. The load has to get to those somehow and there's just not much play with it. *shrug*

3) The style of this bridge was interesting enough (to me) to bring it out of the "bad styling" category. YMMV

4) A simple well-proportioned bridge with additional capacity just wouldn't have had the WOW factor that (apparently) everyone wanted.

Long story short: municipalities like fancy bridges in high-visibility areas. You'll notice that interstate bridges over no-name river in the middle of nowhere very rarely deviate from girders and a deck slab.

I worked for VSL as an engineer for two years several years ago, mostly monostrand commercial building design work. My understanding from some former coworkers was that they were in the process of de-stressing one of the threaded bars when it collapsed. So they were attempting to reverse the stresses within the no. 11 diagonal when the collapse occurred.

Quote (drawoh)

Okay, I am mechanical, not civil or structural. If management is determined to make it cool, ask for a budget. Maybe you will get a big one! I perceive rule 2 as an artistic issue, but maybe there is a structural component too.

Like this ones:

Well if they were actually destrssing it maybe the PT rod was the only thing keeping that column straight and once they loosened it it just bent and failed in bending?

I've also been wondering if the bottom slab got damaged/ cracked during the lift due to reverse of anticipated loads / sheer.

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

How about form follows function?
Due the "simplicity" of the truss, one might be able to run a 2-D analysis on SAP80, which would run on a Radio Shack Model II which would run the CPM system. $600 program at the time. About that time we were moving over from VisaCalc to Lotus 123.

In the attached utube video posted today, there may be a crack, or a construction wire/cable, several feet south of where #9 and #10 connect to the lower deck. The crack or wire appears to past through, or over, the floor drain.
www.youtube.com/watch?v=SBoE34-WZoE

Everything about the FIU TIGER-2013 award is here: Link
It is all part of a bigger scheme to expand into Sweetwater aka University City

Quote (LittleInch)

I've also been wondering if the bottom slab got damaged/ cracked during the lift due to reverse of anticipated loads / sheer.

I also thought about this. Specially since in the transit video it seems like 2 of the 4 support points were between the nodes instead of directly underneath the nodes and there were no load distributing elements between the bridge deck and the supports that moved the bridge into place. However, even if that was a screwup and the concrete on the deck was cracked as a consequence (which may be what that engineer's voicemail message was about), the bottom deck only would have tension forces going through it once the bridge was placed in it's final position, which (I guess) would mean that the concrete would not be taking much stress. The bottom slab is in tension so the steel tendons would be doing all the work. So a crack on the bottom deck should not have been an issue, right?... Unless perhaps, if the crack was not between two nodes but rather quite close to one of the nodes where the diagonal elements meet.

PE2015, I'm sure that's just a cable / wire. There seem to be some wrapped around the chord member, and some further along the bridge too. Seems it connects into some little grey box either side of the deck. Not sure what those might be though...

Quote (Ron)

There did not seem to be much, if any, lateral mild steel reinforcement across this interface.

Thanks, that's what made me wonder if the failure had occurred at the panel point and not the actual member.

Dik

PE2015 - That looks to be electrical extension cord but looking at those drains directly at the base of the truss is unnerving.

Are those strain gauges on the members shown on the video posted by PE2015 on 20 Mar 18 18:57?

At 720P the lines leading to the drain terminate at little boxes near the edge of the platform. There is a similar configuration farther along the bridge where the guy is standing.

I know we now know that the bridge failed, but in that video posted by pe2015, does it just feel like the struts are too small and at bad angles?

When you see it like that the width of the bottom flange just feels huge compared to the struts.

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

In the safety section of the proposal document, there’s a brief statement that all work on the main span, after being set, would be performed over closed lanes during off peak hours.

My point is: that entire document shows an impressive amount of diligence and forethought put into the safe implementation of this design. Yet, when things apparently didn’t go as planned, those conservative safety principles somehow didn’t carry through and they ended up working over live traffic. Either the risks of this work weren’t appreciated by the team, or there was some sort of decision made based on risk vs cost/schedule/hassle/whatever.

Besides the technical aspects of this disaster, the lesson to be learned concerning conservative decision making when unexpected conditions are encountered is one we should all take to heart and keep in mind when we encounter similar situations our future endeavors. I feel that we have an obligation to get the full story on the decision making process in this case, then learn from it and reinforce that aspect of our profession going forward, just like we learn from the technical aspects of disasters.

Having worked in the field on multi-billion dollar projects for a good part of my career, I’ve taken serious flack for sticking to my guns on safety issues. I remember feeling like I was on an island with very little backup from senior engineers, who were more interested in not rocking the boat and having a smooth sail into retirement. There was always an implicit pressure to be a “team player”. Probably because of those personal experiences, the decision making aspect of this disaster has resonated with me more than anything else. Even if they thought the crack was harmless, the written safety plan was no work over traffic.

https://www.cbsnews.com/news/florida-bridge-collap...

TME, very gratuitous response on Part II of this topic.

Quote (bones206)

Either the risks of this work weren’t appreciated by the team, or there was some sort of decision made based on risk vs cost/schedule/hassle/whatever.

This is exactly why an engineering firm should NEVER be a subconsultant to a contractor in design-build.

Quote (Meerkat 007)

Compression of member #11, before deck fall, could be pushed out PT rod, as someone suggested already. (Meerkat 007 20 Mar 18 18:02)

Numbering nodes 1 through 6 on the bottom chord beginning with the bottom of Members 11 & 12 as node 1 and the bottom of Members 1 & 2 as node 6, the distance between all nodes on the video Meerkat 007 posted seemingly remain constant before impact with the ground, except for the section between nodes 1 and 2. Therefore, it seems reasonable that the failure was in the bottom chord between nodes 1 and 2. After collapse, what is left of the bottom of Member 11 is pointing at the top of the pier and the deck portion is resting at the bottom of the pier. Thus, it seems reasonable that the point of failure would have been in horizontal shear between the deck and the bottom of Member 11 at node 1.

There does not seem to be any shear reinforcement between nodes 1 through 6 and the deck. Nodes 2 through 5 appear to be confined horizontally, but not vertically. Nodes 1 and 6 appear to be confined vertically and transversely, but not longitudinally in the direction away from the deck. The PT tendons in the deck begin about 8 inches outside of the width of the concrete "truss" and node 1 portrudes about 10 or so inches beyond the anchors for the deck PT tendons. Was the horizontal shear between node 1 and the PT tendons in the deck being carried in (great?) part by the PT rods in Member 11? Did the concrete fail at the anchor in the deck for one of the Member 11 PT rods when tightening, thus leading to shear failure at that point, followed by the deck falling off the pier? I wish I understood post-tensioned structures better.

Has anyone seen Juan Brown's video?
He is talking about the failure of truss #22 but pointing to truss #11.
A bit of a hit to the credibility.
Link
Bill
--------------------
"Why not the best?"
Jimmy Carter

Now my head hurts. Numbering nodes backwards to members.

How about deck between #10 and #11, canopy between #11 and #12. node common to #11 and #12, node common to #10 and #11? No added numbering to confuse people.

waross - he needed reading glasses, and the ability to count left to right.

Quote ((Ron) This is exactly why an engineering firm should NEVER be a subconsultant to a contractor in design-build. )

Unfortunately DB is the way of the future.

Ron, what you said reminds me of something the chief engineer at a bridge firm I worked for in the 80"s said "engineers are like prostitutes fighting over a customer".

I hope he never has to land on runway 11

Bill
--------------------
"Why not the best?"
Jimmy Carter

Quote (bones206 (Civil/Environmental))

Either the risks of this work weren’t appreciated by the team, or there was some sort of decision made based on risk vs cost/schedule/hassle/whatever.

Maybe the early completion bonus was too large to resist?

EDIT - Guess not. See below.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

This is interesting. Something I thought about at the very start.

http://www.foxnews.com/us/2018/03/20/key-design-ch...

Quote (waross)

Has anyone seen Juan Brown's video?

made my tummy hurt... not a good presentation, IMHO.

Dik

What was the span and depth of the bridge? In metres if possible. :)

Quote (Tomfh)

What was the span and depth of the bridge? In metres if possible. :)

In 'pagen' units: 175' span, 15.5' depth ==> L/D = 11

In 'gods' units: 53.3m span, 4.72m depth ==> L/D = 11

Some real crazies sure come out with engineering mumbo jumbo after major failures.
This one seems to take the prize: FIU Bridge Collapse: Why Müller-Breslau Matters

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@ dik, EPCI-Steel and Ingenuity: I really appreciate your answers to my questions about the tensioning process and equipment.
At the office, we were discussing about the fact that one of the tensioning bars of member 11 remained in place while the one having the hydraulic cylinder attached to it came out several feet.
If over-tensioning of that bar up to the point of its violent rupture is not probable, what other thing could have caused that different reaction of both bars during the collapse?
Could under-tensioning and subsequent buckling of member 11 have had a similar result?
Is it possible that the combined loads of compression, bending at nodes and shear endured by member 11 were so high that additional increasing compression from the tensioning bar fractured the concrete of the member?

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

BB....that Chief Engineer was probably right!

Details that stand out most for me: From various photos, and also NTSB footage on YouTube (links below), the following observations seem fairly solid (and some have been pointed out be others previously -- I'm consolidating here):

1. Member 11's two tensioning rods do not appeared to have broken, but rather remain connected in some way to their intended end points:

-- The upper rod remains connected at top end, and at the bottom to the base of vertical member 12.

-- The lower rod remains connected at the bottom to the deck, and at the top is protruding several feet from its intended attachment point, with the blue hydraulic jack attached.

2. Member 11 appears to be substantially in its normal relationship to member 12 at the base, and at the other end, relative to the last canopy section where the canopy-11-12 joint would be. However, it is not connected by concrete at either end, and now rests in that position primarily by virtue of the upper tensioning rod, and maybe some rebar. The lower end of 11 is disconnected from the (lower) deck despite the lower tensioning rod, which zippered out of the underside of 11.

3. The tensioning rods or cables in the (lower) deck appear to be substantially intact, probably being responsible for dragging the northernmost segment of deck off the supporting pillar during the final milliseconds of the collapse (as shown in the dashcam videos). So, I'm dissuaded that this was a failure of the lower deck at some intermediate location.

3. At both ends of member 11, the concrete is broken up. But is this a cause or effect of the overall failure? Certainly, if crushing of one end or the other of #11 occured, it would have resulted in a collapse as seen. But did it?

The purpose of member 11 is to transfer the very high horizontal compression force in the top canopy, into a diagonal compression in member 11 (and tension in #10). Then #11's compression vector translates into a downward compression in the pillar, and a large horizontal tension in the deck.

I think it is very telling that the entire #11 tore away from the deck, except for one of its tension rods. The rest of its rebar and the upper tension rod remained attached to the vertical member #12. Even if there was some other cause for the overall failure, I would have expected the base of 11 to remain attached to the deck. Does this not indicate that the connection of #11 to the deck, specifically to the deck's tensioners, was inadequate?

The base of #11 has to deal with a high level of compression, and evidently some shear forces, for both its vertical and horizontal duties. If the rebar is not judiciously arranged to maintain the concrete's integrity and ability to sustain the compression load, then the diagonal compression will not be transferred to the deck tensioners and the pillar, and #11 fails.

A similar, though perhaps less severe, situation occurs at the top of #11, though here, the top of the member, while crushed, did not split in two different directions (so far as we know).

A further factor: The joint of #11 to the deck and the end wall #12 looks to be a joint that would suffer the most flex between its role as the furthest cantilevered point during transportation, and the reversal of all forces as the bridge was placed into position. As others have mentioned, concrete joints aren't pins. Such flexing could disrupt the integrity of the concrete, and make it less capable of withstanding the compression it is normally so good at.

In theory, the tensioning of the rods prior to transport, and detensioning during placement, should have compensated for that. But one wonders how precisely that was done.

In short, my speculation is that the joint at the bottom of member 11 to the deck will receive a lot of attention (as a couple of others here also favor).

NTSB:
https://www.youtube.com/watch?v=B1PPqa9cS-k,
https://www.youtube.com/watch?v=aeJKqojmHgY

CBS some detailed helicopter footage:
https://www.cbsnews.com/news/florida-bridge-collap...

It appears to me that Ron pointed to the right mode of failure - punching shear dislodging entire blister out of the canopy. Lack of mild reinforcement is quite apparent and entire blister cracked out intact. On one side of the blister there are few #4 rebars visible..
As an oldtimer, I'm sticking to the classical rule - and not relying on the shear capacity of the concrete in this situation. Mild steel reinforcement well anchored in the canopy and sized to handle the total shear would save the bridge.
Another contributing factor, which should be discussed, is apparently low overall safety factor for the span. Assuming that the bridge was designed using AASHTO LRFD, DL=11 kips/ft will have a factor of 1.25, while the LL, which is only 2.9 kips/ft, has a factor of 1.75. Adding it together we are ending with the safety factor of 1.35 for a fully loaded pedestrian bridge with no other loads. Fake stays will add some extra capacity, but not to the blisters.
STRENGTH I: 1.25(DC1 & DC2) + 1.75(PL) + 0(WS)
STRENGTH III: 1.25(DC1 & DC2) + 0(PL) + 1.40(WS)
SERVICE I: 1.00(DC1 & DC2) + 1.00(PL) + 0.30(WS)
DC = dead load of structural components
PL = pedestrian live load
WS = wind load on structure

Most people are in agreement about member #11. Those that are suggesting other alternatives I would suggest you look at a simple analysis of the hinge line that the bridge breaks around. Gross failure in the deck or the canopy would very likely lead to different breaking points and hinge points.

Quote (gwideman)

In short, my speculation is that the joint at the bottom of member 11 to the deck will receive a lot of attention (as a couple of others here also favor).

I initially more persuaded by punching shear of the canopy. However what has caught my attention from the dash cam video and is otherwise hard to explain is the rapid failure of member #12. Member #12 would be under minimal load during both static and during failure. Yet it very quickly collapses. This ties member #11 failing at the deck as failure here would compromise the connection of #12.

I am sure that the compression forces on member 11 by the tensioning rods, during travel and placement had an adequate safety factor.
I am sure that the compression forces on member 11 due to the weight of the bridge had an adequate safety factor.
However, how much would the safety factor be reduced when both the weight of the bridge and the PT rods were acting together on member 11 when the bridge was placed and before the rods were de-tensioned?
Any comments?

Bill
--------------------
"Why not the best?"
Jimmy Carter

JAE That's an interesting article.
The gentleman claims, more than once, that due to the lack of a center support, the bridge failed in the center.
When someone points out that the bridge failed at the end he again claims that the bridge failed in the center.
One feels pity for his students.
He may be suffering from a defect in his central nervous system.
It may be that impulses from his brain do not go directly to his voice box, but are somehow detoured through his bowels on the way to his lips.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Amazing ignorance displayed there. My favorite quote was on of the comments:

"Speaking as a Mechanical Engineer with a Master’s degree on the vibrations of three-dimensional structures, there is no such thing as a bridge without central support"

Here's a look at the connection of 11 to the deck and to 12. First a couple of images for orientation showing where 11 and 12 would have been located relative to the lower deck, that is now resting on the ground.:

and

[and added later, from the link epoxybot posted 17 Mar 18 04:22.]

Now side view of deck at 11-12 after collapse:

... and finally, a closer look at the connection point where 12 and 11 are no longer connected to the deck.

One thing that strikes me is the likely difficulty of transmitting compression from 11 to the deck right in front of the openings for all the services in the vertical 12. And where's the rebar that would tie member 11 to the deck? Perhaps obscured by the rubble, and/or torn out and still attached to #11. But maybe engineers who actually know something about this topic would see something different.
The last two images are from the NTSB videos, linked in my previous post.

Looks like someone is clamming up, From Fox News:

“The city attorney is advising us not to speak about anything to do with the bridge,” said Sandra Antonio, a spokeswoman for the city of Sweetwater,

...National Transportation Safety Board had forbidden engineers or contractors from talking about the project pending its investigation.

A FIGG spokeswoman, Cheryl Stopnick, said it would be “inappropriate to discuss specific detail about bridge design” during the investigation.

A spokeswoman for FIU did not immediately respond Tuesday to questions about the pylon relocation in the bridge design.

Happy as a clam...

Dik

Does anyone know the size and load of those two PT rods in #11?

According to the page 115 (Drawing sheet B17, 2015 NOT FOR CONSTRUCTION DRAWINGS), there is no PT rods in #11. But reality doesn't seem to fit those drawings.

Those drawings are hardly rough sketches either. There are quite specifica about the placement and loads of the PT rods including orientation within the beams so they don't interfere with each other at the nodes. (AKA some PT bars in cross section are in the XZ center plane, others in the YZ centre plane while are distributed to the corners.) One would suspect that member #11 wouldn't need PT once the bridge is in place as there is already significant compression. Adequate nodal reinforcing however would be obviously essential. If however adequate nodal reinforcing isn't present then tensioned PT rods could help keep the connection together.

That said it would seem the PT tendons that did exist in #11 were essential to holding it together. If we make the reasonable assumption that the PT adjustments that were being made was the straw that broke the bridges back.

Quote (AUCE98)

I worked for VSL as an engineer for two years several years ago, mostly monostrand commercial building design work. My understanding from some former coworkers was that they were in the process of de-stressing one of the threaded bars when it collapsed. So they were attempting to reverse the stresses within the no. 11 diagonal when the collapse occurred.

As mentioned about this likely allowed the connect.on that was already under significant strain to shear. One would expect that adequate reinforcement would have been provided for. But clearly it wasn't.

Quote (Tomfh)

Does anyone know the size and load of those two PT rods in #11?

From Ingenuity's post, "Keep in mind that the 1-3/4" grade 150 ksi bar was stressed to 280 kips (max), about 70% of UTS."

Quote (Tomfh)

Does anyone know the size and load of those two PT rods in #11?

Here's a picture, for what it's worth:

From: https://youtu.be/aeJKqojmHgY?t=154
I believe that yellow tape is either 1.25" or 1.5" wide.

PE2015, in your https://www.youtube.com/watch?v=SBoE34-WZoE

I see strain transducers on most members, a couple at bottom blisters and other sensors along the deck, with red wires connecting to a data-logger.
They were monitoring the structure and logging strain and more during the move.
Appear to be ST350 Strain Transducers http://bditest.com/product/sensors/strain-sensors/...

JAE,
That Oliver McGee character was on TV news, espousing that rubbish. I linked it in the first thread. He is some sort of professor at Texas Tech. He needs stopping.

And from Stuart Grant's https://www.youtube.com/watch?v=SBoE34-WZoE. Another view of the 11-12-deck connection. Unfortunately can't quite see how 11 relates to the deck.

Quote (hokie66)

JAE,
That Oliver McGee character was on TV news, espousing that rubbish. I linked it in the first thread. He is some sort of professor at Texas Tech. He needs stopping.

Oh dear. Anything for limelight, even if you are an idiot with a 'Dr.' in front of your name.

There is a whole bunch of 'amateur' conjecturing and speculation going on in this thread but at least there is virtually instantaneous peer review. Plenty of new sign ups to this forum too (myself included) so who knows what credibility so many of these posts have. But that is the nature of community discussion and in the right contexts good conclusions can emerge.

In contrast Oliver McGee is an idiot, who reporters have no idea that they should ignore.


You would hope they do have excellent forensic engineers examining this. Some silence from them is understandable, however complete silence just allows airtime for idiots.

Again looking for views of the 11-12-deck connection.
From https://youtu.be/qBXnr0FoqA8?t=156


And from https://youtu.be/IM-wppkV9_w?t=99

[Revised: added diagonal dashed line so that red line intersections indicate better position of 11 relative to deck and 12.]

Quote (human909)

That said it would seem the PT tendons that did exist in #11 were essential to holding it together.

Yes that's what I was thinking. Maybe the PT was acting like anti-burst. Not strictly needed for the member, but helped keep everything bandaged up tight.

This discussion has slowed a lot, and with all the guessing we have done, I don't think we know exactly where the failure initiated. Perhaps we never will.

Someone, I forget who, suggested that it would have been easy to conduct a load test in place before the precast bridge was moved. True, just support it a bit off the ground and load it with sand bags or water bladders. Plenty of sand and water in Florida. I suspect it would have failed the load test, and thus the lives would have been saved. We will never know now.

For what it is worth, it is my belief that we do not have the tools available to analyze this structure adequately. The engineers at Figg must have thought differently, but that may have been because their many successes with different bridge types made them overconfident, or perhaps it was just arrogance.

Zero experience with bridges, or concrete, or pre/post tensioning, but also rolling around in my head is the arrangement of the PT strands at the top of member 11. If we believe the not for construction drawings we've seen, one strand goes right through to the top of the blister, past member 10, whereas the other stops short and is embedded in the joint, before member 10.

They were presumably working on the 'longer' PT strand as that lines up with where the construction guys were on top of the bridge before collapse. It doesn't make much sense that releasing compression from the strand would cause failure when the member is acting in compression anyways - technically you should be making the situation better (less compressive stress).

What I am wondering is what else besides applying compression was that 'long' PT strand doing? It looks a lot like any compressive loads in that strand would also help to carry shear between the top ends of members 10 & 11 and the canopy. By releasing the compression in the strand, has more shear load (unsuccessfully) been transferred through the remaining members of the joint?

EDIT. This is the picture to which I refer (from Pt 1 of this thread). If you picture this drawing without the 'long' strand in member 11 (e.g. if compression had been removed from it) the joint as a whole looks awfully weak to me with very poor load paths between all of the members.

@RandomTaskkk:



... shows two PT rods exposed at the joint of 11 and 10, with the lower of the two apparently undergoing adjustment with the jack.

You probably saw the drawing which seems to detail the 2-3 junction.

gwideman - I thought B was one of the strands of member 10? Maybe I am mistaken.

Im a chemical engineer, so to those who know about this subject:

This shows a drawing of the pedestrian bridge when finished from my local (Danish) engineering society paper. Why was the bridge erected without the cable stays? Wouldnt a bridge like this normally be build so that the pylon was build before the span is installed? Was this a part of the "accelerated bridge construction"? Or is the drawing an outdated drawing?

Best regards, Morten

I think hokie66 has nailed it. Every time I see the videos of this collapse I am astonished by the speed of collapse. This thing just broke in two at what seems to be two or three locations within a fraction of a second of each other. Without super high speed filming I'm not sure you will be able to say with any real certainty which element broke first or which was just the subsequent rolling damage. Hopefully some of the testimony of the survivors from the top of the deck doing the work will be able to shed some light.

Someone may be able to show after many days/months of analysis that some element was not properly designed or constructed, but from what I've seen and read here, there were a number of elements which could contribute to the failure and that the analysis tools for such a structure were maybe not fully developed.

The changes to the design (11 ft longer) and subsequent change to the lifting points may be the smoking gun as everyone would be reluctant to go right back to the start design again. Hopefully this will go to show that with newer designs and construction principles you need to test the thing in a safe location before allowing people to drive underneath it or work on top of it.

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

Morten - this has been already covered in earlier comments. No need to repeat it.

Quote (human909)

But reality doesn't seem to fit those drawings.

What was constructed may differ considerably from the proposal... it will be nice if they release the crack photographs and shop drawing records.

Dik

Quote (RandomTaskkk)

It doesn't make much sense that releasing compression from the strand would cause failure when the member is acting in compression anyways - technically you should be making the situation better (less compressive stress).

There's a couple of ways that removing compression load from one PT strand could create a failure-prone condition:

1) Removing load from one strand and not the other would create an internal force imbalance inside the member being destressed. Concrete experts (ie not me) would be better suited to say whether or not it's possible that this force imbalance could possibly create a large enough internal moment that the member could not handle, but on first principles it seems logical to me.

2) In order to reduce tension on a PT strand, tension must first be added- the jack and stool are attached and the tendon is stretched so that the nut on one end can be loosened by hand. So, for a short period, the PT strand being loosened is under significantly more tension, and is applying significantly more load to the member, than the designed installed load. If there's a mistake during the procedure for reducing strand tension, the tendon could fail or the member could be grossly overstressed.

3) Compression load from the PT tendons could be providing confinement to the nodes/joints at the ends of the members, and possibly transferring shear loads across a deeper section than would otherwise be developed in shear. Removal of the tension could reduce or eliminate that 'bonus' joint confinement and make one or both joints at the ends of the destressed member unstable.

jgKRI - 1) Good point, I hadn't considered that.

2) I'd thought about this, but (in my naivety) I find it hard to believe this was what caused failure. Surely you just need a teeny tiny bit more tension to back the nut off by hand. All you need to do is overcome the original pretension by a very small margin in order to free up the nut? .... But I could be wrong!

3) Is exactly what I was trying to suggest in my post, although somewhat less eloquently...

I guess, like most, that the failure occurred with member #11, or the node, or the slab at member #11. I'm not certain, but from the pictures something happened in this area. If you can imagine a portion of the structure, associated with this area 'simply disappearing', the collapse would be similar to what was shown. I have several ideas based on information from this thread, but, have not 'wrapped my ears' around the actual cause of collapse. It's elusive due to lack of information.

This visualisation would be in keeping with the sudden catastrophic collapse. We have very limited information on how the truss was designed, detailed and construction. Hopefully some of this information will be forthcoming. There are numerous things that could have been done to prevent the loss of life, maybe not the collapse.

Hopefully new information will be divulged, and, maybe identify the actual mechanism of collapse.

My $.02US, $.03C.

Dik

Quote (RandomTaskkk)

I'd thought about this, but (in my naivety) I find it hard to believe this was what caused failure. Surely you just need a teeny tiny bit more tension to back the nut off by hand. All you need to do is overcome the original pretension by a very small margin in order to free up the nut? .... But I could be wrong!

You'd have to add enough tension to make up for any creep or force-induced shortening of the member, as a percentage of total preload already applied, this could be a LOT of force. We're talking about 140 tons of preload on one of these tendons, and a process performed manually. Yes the crews performing the work were allegedly experienced and highly skilled, but mistakes can still happen.

I'm not a PT concrete expert either, so I would have to defer to more experienced engineers on how realistic this possibility is or isn't as a potential root cause.

MortenA,

In general you are correct. However, on *this* bridge the pylon and stays were there largely for looks with little, if any, structural purpose. Yes, you read that correctly.

jgKRI....there is some possible evidence of such an imbalance if you look at one of the photos of member 11 where the concrete had spalled along one side.

Quote (Ron)

there is some possible evidence of such an imbalance if you look at one of the photos of member 11 where the concrete had spalled along one side.

Maybe, but not necessarily. Member 11's lower PT rod was pulled down by the falling deck, while the rest of member 11 stayed attached to 12 up at the top of the pedestal. This may have created a zipper effect, pulling off the bottom of member 11.

Quote (human909)

There is a whole bunch of 'amateur' conjecturing and speculation going on in this thread but at least there is virtually instantaneous peer review. Plenty of new sign ups to this forum too (myself included) so who knows what credibility so many of these posts have. But that is the nature of community discussion and in the right contexts good conclusions can emerge.
In contrast Oliver McGee is an idiot, who reporters have no idea that they should ignore.

I'm not sure "amateur" accurately characterizes those participating in this thread, or in Eng-Tips in general.
I think the 400+ comments are all a representation of engineers natural inquisitiveness and DNA compelling pure problem-solving.

As hokie66 suggests, "I don't think we know exactly where the failure initiated. Perhaps we never will." - probably very true until more study/factfinding and analysis is performed by, hopefully, individuals who can successfully get to the bottom of this disaster.

But I think generally that your statement reveals just how great Eng-Tips is as an engineering forum.

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Quote (Ron, what you said reminds me of something the chief engineer at a bridge firm I worked for in the 80"s said "engineers are like prostitutes fighting over a customer".)

Sadly Bridgebuster in the consulting world that seems spot on. Especially with what I see with regards to state DOT marketing.

Quote (Ron)

there is some possible evidence of such an imbalance if you look at one of the photos of member 11 where the concrete had spalled along one side.

I noticed there was a lot of small particle debris, possibly from the concrete spalling from the chord members... I would not normally expect to find such relatively small parts. I've never encountered a failure so catastrophic and sudden.

Dik

Quote (JAE)

I'm not sure "amateur" accurately characterizes those participating in this thread

There's probably a kazillion years of experience on Eng-tips.

Dik

kazillion....let me get my slide rule out to check that...

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Quote (dik)

There's probably a kazillion years of experience on Eng-tips.

I wonder what would happen if eng-tips designed something collaboratively. Replacement bridge design anyone? :)

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

Quote (TehMightyEngineer)

I wonder what would happen if eng-tips designed something collaboratively. Replacement bridge design anyone? :)

Some 400 engineers trying to design something via forum? It would certainly be entertaining to watch. :)

I have a great deal of respect for pretty much everyone I've interacted with on this board.

With that said... there is such a thing as too many chiefs

After watching the video that Meerkat 007 supplied. Around 0:17 seconds we see something protrude out from back side of the interface of member 12, 11, & the deck (bottom chord). My thoughts earlier were this was the end of the PT Bar that possibly broke while being de-stressed. In the roll-in photos we see no openings for the PT Bars in end of the deck. They had to have been dead ended in this area. When looking at the piece that popped out. It came out horizontally, NOT in line with member 11's angle, but in line with the bottom chord and it stays there until the deck (bottom chord) starts to fold downward and we see a puff of dust around this connection. The piece then pulls inward with the collapse.

In the photos gwideman provided it looks as though the PT bars did not break. It appears they did zipper member 11 as gwideman proposed. The top bar stayed top with parts of 12 on the top of the pier and the bottom with deck on the ground. Thus, ripping member 11 down the center. It is concerning that we see in gwideman last photo (from the NTSB)the amount of voids in very close proximity to the high amount of SHEAR in this area. The preliminary plans show only one anchor bolt on each side. The photo show at least 4 Pipe with vertical bars that have not been grouted, plus the duct vents, reducing the area to resist shear. There is a heck of a lot "stuff" in this area. There is a Heck of a lot of SHEAR in this area. Notice the near perfect square that gwideman boxed out in blue, looks like a square piece of concrete is gone. Could it possibly be what we see PUNCH out in the video supplied by MeerKat 007. Could the PT Bar that was being de-tensioned been the force that was keeping this joint from shearing?

Quote (dik)

I noticed there was a lot of small particle debris, possibly from the concrete spalling from the chord members... I would not normally expect to find such relatively small parts. I've never encountered a failure so catastrophic and sudden.

You usually find small particles and debris during extreme failure with concrete, especially if there was high compression involved.

For reference, here's a precast arch structure I designed that was unfortunately damaged during shipping (no injuries except to the drivers pride, thank goodness). It was not prestressed but just look under the fracture in the shadows; lots of concrete debris in various sizes. If this was prestressed I imagine it would look even more destructive.


https://res.cloudinary.com/engineering-com/image/u...

Concrete just doesn't like being thrown at the ground I guess.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
https://www.facebook.com/AmericanConcrete/

For what it’s worth regarding small debris, I recall the following reporting immediately after the accident. I do not have a link. But the owner of the car that was half crushed (the entire rear end) stated that she had heard what sounded like rocks falling on the trunk and turned or was turning her head when the rest of the bridge came down and flattened her car. So maybe half a second to register and react to something like that?

Not sure if that information with the location of her car helps with any of this. Just thought I’d throw it out there.

Quote (hokie66)

JAE,
That Oliver McGee character was on TV news, espousing that rubbish. I linked it in the first thread. He is some sort of professor at Texas Tech. He needs stopping.

If you check his LinkedIn page you'll see that won't be possible. The best part is he appears to me to be commenting on his own engineering word salad article himself using sockpuppet accounts.

Quote (JAE)

kazillion....let me get my slide rule out to check that...

You need a really big sliderule... just to keep track of the decimal points...

Dik

Quote (winelandv)

Some 400 engineers trying to design something via forum? It would certainly be entertaining to watch. :)

When we built my parents cottage/homme, my dad, my brother, and myself all knew how to build things... but, put three people together to do it... we eventually divided up the chores and identified the work we would undertake... can't imagine 400...

Dik

TME... thanks...

Dik

Here is some drone footage of the bridge move. It is on DropBox so there will be a server limit. Link

"... can't imagine 400..."

Famously, it took about 400,000 people to put a man on the Moon before the decade was out. A bit of a Project Management triumph, as much as the other disciplines.

In a post near the end of thread II I mentioned the appearance of something moving spanward from the position of column 12. Further review estabilishes that as a structure in the far background coming into view from behind the column and the crane boom in front of it (dashcam view). Just wanted to correct that. The other points I made there hold up a little better.

Studying the drawings and the readily available pre- and post-incident imagery of the 11/12/bottom chord/pedestal area I note that the remaining portion of 12 seems to have a "chair" shape, the outside surface extends to somewhere above the location of the drain pipe while the spanward face ends higher, around deck level, and there seems to be a roughly rectilinear transition between the two cleavage planes.

The drawings show the PT in the lower chord equally spaced (though of graded sizes) and straddling the "core" area of the deck, where the diagonal members sheer interfaces are. In both the independent span design and the complete bridge design I do not see how the compression force is to be transferred to the PT cables.

Two other things about this design bother me. It's clear that the mast that was to support the false stays was to be supported by a foundation built in place around member 12 of the independent span and the matching vertical member of the north span. The false stays would provide, if adjusted reasonably well, ample guy forces for the mast in the axial direction. But what was to counteract the torque around the base of the mast in the transverse plane? I'm not seeing that in the design either. If it's just cantilevered it seems to me there's a lot of complex moments operating on a complex multi-pour and seemingly narrow column. The proposal package only mentions the construction of the mast briefly, showing it as having two cast-in-place components, sheet B-28. The art showing views along the deck illustrate the "base" part of the mast ("cast pylon") as being only a little wider than the diagonals and the plan view shows no bulging of the deck to allow foot traffic to flow around a wider base. All of while leaves me wondering about the moments in that area in a stiff breeze.

And the design of the drain. Apparently the desire to avoid cosmetic issues or wastewater handling costs at the center pylon led to a design in which all the water collected by the deck was to drain through the central pylon bearing area and then the south abutment. The drain pipe was external to the deck structure mid-span but passed through the end flanges and was partially recessed in a "groove" cast into the bottom of the chord. It seems to me that the groove for that drain pipe would create stress concentrations, my reflexes want to see a much bigger radius there and rebel against the complexity of passing the 8" drain line right through the center of the mast foundation.

"In order to reduce tension on a PT strand, tension must first be added- the jack and stool are attached and the tendon is stretched so that the nut on one end can be loosened by hand. So, for a short period, the PT strand being loosened is under significantly more tension, and is applying significantly more load to the member, than the designed installed load. If there's a mistake during the procedure for reducing strand tension, the tendon could fail or the member could be grossly over stressed."

As I stated in my original post on Part II. Member 11 was originally in tension during transport since the bridge transport trolley was below the first interior panel point when the bridge was being moved. When the bridge was set in place the stress in member 11 was reversed. In my opinion the proper procedure would have been to de-stress the tendons while the trolley was still supporting most of the weight of the bridge. In other words the bridge end was in full contact with the pier but most of the load was still being supported by the trolley. Whether this method was assumed by the designer but not properly communicated to those in the field is not known at this point but it will eventually come to light.
As jgKRI stated above the strands have to be stretched so the wedges can be removed. The fact that there was full dead load on member 11 plus a prestress force that was probably not considered in the design plus an additional force due to the detensioning operation is significant and is probably what caused the failure.
Whether it was a compression failure at the end of 11 or a shear failure for load transfer between 11 & 10 cannot be determined with out construction documents because both the cross sectional area and reinforcing are needed to determine the stresses on each member at the time of failure. With shear strength being significant less than compressive strength for concrete, the shear would be the first thing I checked. It is also not known what the compressive strength of the concrete was the time of the accident. If the concrete was at only 75% of design strength that would be a critical factor.

Stephen Nuchia,
I think you have detected the smell of an architect.

A lot of interesting discussions and seemingly good information going thru these three threads.

I have feeling what may come of this at the end of the day is everyone performed their work to the standard of care. Perhaps at or near the member 11 top node the concrete never achieved strength or possibly had poor consolidation around the PT bar anchorage zone which predicated a punching style failure during the temporary jacking operation to destress the bar coupling this with the apparent lack of redundancy could have lead to sudden and catastrophic failure of the "Truss".

Personal opinion with no basis other than the limited marketing material and photos presented so far.

Edit: based on photo below, I revised my failed node assumption to be at the top not bottom of member 11.

Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering

Quote (Stephen Nuchia (Electrical))

... I note that the remaining portion of 12 seems to have a "chair" shape, the outside surface extends to somewhere above the location of the drain pipe while the spanward face ends higher, around deck level, and there seems to be a roughly rectilinear transition between the two cleavage planes. ...

On collapse, members #12 base was knock out by member #11 and pulled down from pylon together with member #11, by north deck fall.
That can explain the "chair" shape after collapse.

For the time being I'll put aside the question raised earlier about whether this structure would behave like a beam or a truss, and for current discussion assume this acts like a truss. Not a normal truss, however, but a frame with fixed nodes. And this would be a complex frame, with internally applied moments from the external PT.

When modelling, the designer should have accounted for moments to develop at all joints. If they didn't, that's a problem. For a proper review it's necessary to see how the designer modeled the structure in their finite element model (they surely must have done this in order to capture the behavior of this complex little frame). As mentioned earlier in this thread, the axial dead load stress just from compression in member 11 is already high, and was the first thing that caught my eye when I looked at the bridge. Now add to this some undetermined moment load, and flexural stresses could become critical. Then also combine this with the fact that minimal shear reinforcement seems to exist in member 11, and things could become dire (this lack of significant observable shear reinforcement in pictures has also been mentioned before).

And just to note the obvious, the load path to the bearing is thru member 11. It's axial stiffness is much greater than the flexural stiffness of the top chord above it, and as such 11 will attract the load.

Having said all that, the following is a hypothetical collapse sequence based on concerns with design of member 11. To date, I've seen nothing in the limited, grainy, heartbreaking video to contradict this proposed sequence of events.

1) First member to fail - member 11:
Diagonal, under high axial and shear loads, becomes critical with the additon of moments at member ends [This may or may not be connected to the external PT that has been so much talked about. Destressing of the PT rod, however, as some speculate was the operation being performed at the time of collapse, would actually reduce shear capacity of section]. The integrity of the end(s) become compromised, pins develop as a behavior mode, with final result being brittle shear failure at one, or both, ends. Note that from pictures it does not appear that the diagonal buckled, as the center zone of member appears to be relatively intact. Also, the "zippering" along the bottom side of 11 is likely the result of PT bar being ripped out during collapse.

2) Second member to fail - top chord/flange above member 11:
After member 11 fails the structure is "theoretically" still viable, assuming we have frame behavior. However, this is obviously not sound. So next, the top chord near intersection with 11 fails quickly in shear and flexure; top chord is much weaker than bottom so top fails first. It's also possible then that the longitudinal PT force in the top chord, combined with the instantaneous frame bending of structure in this corner, causes 12 to fail in bending at its base in the diaphragm area. And it's even conceivable that member 11 becomes "detached" from the bottom slab and is driven along the top surface of the slab, impacting 12 and causing additional damage to its base. This could explain why the final resting place of 11 & 12 are on top of the pier.

3) Bridge collapse:
After top chord fails the only section remaining is the bottom slab. Location where loads are highest is where the bottom slab intersects the next diagonal 10. Bridge now hinges at this point and falls. At some point during fall the longitudinal PT force in bottom slab pulls this now free bottom chord (section beneath 10 & 11) off its bearings, and that entire end of bridge plunges to ground. Punching out of anchorage blocks on top is likely the result of impact of falling bridge.

>>>
An alternate, albeit unlikely, failure mechanism:
With diagonal member 11 carrying all dead load back to bearings, the zone of shear transfer at the intersection with member 12 and the bottom slab is crucial. The load is entering the bottom slab, and all that wonderful longitudinal PT spread out across the entire cross-section, at a single point--midspan. Adequate shear and confinement reinforcement must be provided to ensure that the horizontal component of load from 11 can be distributed back into the slab. If not, triangular frame of 11-12-top chord could conceivably pop out of end diaphragm in a prying action-like behavior, leading to catastrophic collapse. The video, however, doesn't appear to show this, although that could be merely a trick of the eye. And there's also the fact that 12 appears to have failed in flexure at its base, ABOVE the surface of the deck.

The terminology is important, so please use 'chord', not 'cord'.

I have stopped speculating about where it failed first, but we should not be focusing only on the prestress. As this was concrete, other factors are involved. Shrinkage, creep, etc. We don't know how this was cast, but there must have been construction joints. At these joints, the later cast element is restrained by the earlier elements. The prestress was presumably applied only after the whole thing was cast and hardened, so there would have been some cracking already.

I don't think there will be a call for any more concrete truss bridges in the near future.

Quote (TheGreenLama (Structural))

This can be between 1) and 2) as south structure/frames from member #11 was still working as one.

Quote (LittleInch)

Hopefully some of the testimony of the survivors from the top of the deck doing the work will be able to shed some light.

Firstly god rest the soul who died on the top deck as it fell, and same to all the other victims.

As for the other top deck survivors, the speed that thing went, they won't have has time to sh1t themselves let alone consider the reason they were falling.

I'm also starting to think about the other blameless victims that this event will affect. This must be an extinction level event for MCM & FIGG. Might be tricky time for anyone with those names on their resumes for a while. I truly hope that justice prevails and the genuinely culpable individuals are held to account and more importantly the lessons learnt for future.

gwideman's summary above, I believe, likely captures what happened. As the diagonal member 11 pushes in compression, the bottom slab has to tore away from 11 and 12 first in order for a significant deflection to occur. Since a large portion of the bottom deck is still resting on the pylon, it did not fell down immediately until the deflection is large enough.

A large deflection would not occur if (1)the bottom slab would not tore away from 11 or (2) diagonal 11 buckling in compression.

The top deck between 12 and 11 is not doing really much.

Hokie66,

Hear, hear! All the minutiae about which member failed and how, while ultimately valuable, rather sidesteps the issue: this was a horrible design.

It was designed to be an expensive taxpayer-subsidize landmark. Actually functioning as a pedestrian bridge seems to be a secondary concern. Had they wanted the later it could have been had at probably a quarter of the price and built using a (boring) proven, less risky design. Pride goeth before a fall.

Quote (Archie264)

Hear, hear! All the minutiae about which member failed and how, while ultimately valuable, rather sidesteps the issue: this was a horrible design.

I don't think that is in dispute. But what is the point discussing the obvious. Hence the discussion about more specific aspects. In the absence of great information there is always going to be speculation alongside analysis.

A new statement from NTSB: "The investigative team has confirmed that workers were adjusting tension on the two tensioning rods located in the diagonal member at the north end of the span when the bridge collapsed. They had done this same work earlier at the south end, moved to the north side, and had adjusted one rod. They were working on the second rod when the span failed and collapsed. The roadway was not closed while this work was being performed."

It looks like they have needed to do some chipping around the PT Blister opening. Having a hammer chisel strike the plate washer of the PT strand would account for the strange whipping sound one witness remarked on. It would have somewhat of a warped echo sound.

Bigger:

• http://files.engineering.com/getfile.aspx?folder=92effea6-6a8f-4c07-ba58-ce

Quote (Ron)

This is exactly why an engineering firm should NEVER be a subconsultant to a contractor in design-build.

Design & construct projects are completed successfully every day of the week. Design-only/construct-only projects can and have had the same problems with even larger numbers of fatalities. You think end-clients don't have the same budget and programme pressures and don't project those onto their design-only consultants?

Quote (Archie264)

Hear, hear! All the minutiae about which member failed and how, while ultimately valuable, rather sidesteps the issue: this was a horrible design.

I thought it was extravagant and really unnecessary. It did not have a proper function and the hype with all the useless amenities was a travesty. It was, however, attractive (to each his own). I enjoy an engineering challenge and wait for years before something 'pops up'. I would love to have to undertaken a similar design, a cable stayed unequal flanged beam/truss. As grim as the outcome for the design of this one was, doesn't make it horrible. I think it could have been a design challenge... anyone can design boxes. Doing this is the sort of thing that caused humans to leave their caves.

Dik

epoxybot:

Notice the new style of hardhat...

Dik

Dik - the people in those photos are search and rescue, not construction. Those are standard search and rescue helmets. Note the chinstraps - Emba

I've registered just to put my two cents in. I think that many other comments have already suggested the likely cause so I'm not sure how much more I will add to the conversation. In my opinion the PT bar in 11 was preventing the shearing of the 11,12,deck node by transferring the shear forces to the deck. When the tension is released from this member the shearing is greater than the node can handle, as a result the node has failed first. what we then see is a failure at the 10,11,roof node. As the 11,12,D node shears there would be a moment reaction required at the 10,11,R node which it is unable to support and thus the roof buckles at this node. With the first two nodes failing we then need a moment reaction at the 9,10,D node which it is unable to support and thus the deck buckles(at this point the northern end of the deck is still on the pylon).

Now as to the PT bar being so far extended I believe due to the length of member 11 reducing as the bridge collapsed it was able to be pushed out (part of why the blister moves to it's final resting place) Rather than pinging out if the PT bar had snapped during tensioning/detensioning. I haven't had any experience with PT bars but wouldn't one breaking under tension be energetic enough to result in the PT bar coming clean out? I don't think that the lower PT bar has failed in anyway, there are too many observations that require it to be intact. With the lower PT bar in 11 fixed into the deck a below the 11,12,D node the deck pulling away from members 11, and 12 still on the pylon would have produced the zipper out the bottom face of 11 (as others have noted).

I hope that isn't to ramblely.

-Will

emba... thanks, there's a new style of hardhat coming to the job site, and I thought it was one of them. Catch the link:

https://www.constructionjunkie.com/blog/2017/8/7/s...

Complete with chin straps...

Dik

Far and away what bothers me the most about the failure is the lack of ductility. The near instantaneous collapse due to the brittle failure guaranteed injuries. A more ductile failure would have at least given the victims a chance.

A structure with no redundancies, brittle failure limit states, and located in a high traffic area just doesn’t sit well with me. The ductility can and should be addressed in code changes.

Jerehmy: Agreed... but, this was a structural failure, likely, due to an error in design. Codes require things to be strong enough, and, this for whatever reason failed. Most engineers consider alternative load paths and ductility in design... I don't know how this was designed, and hopefully, this will come out. It may have been the original design had strength and ductility incorporated, and, something went horribly wrong. Intelligent design should not have to be mandated by code.

Dik

For background please review my post in Thread II

In addition to my post in thread II, I believe the following could be relevant :

1) According to the design notes the entire structure with regular rebar was to be completely poured and cured. The section was then to be post tensioned by completing the bottom flange first. If this was carried out in one step then the PT loads would shorten the bottom flange inducing shear along the entire web being higher in both end panels. Was this stress allowed for in the design and did it result in micro cracking which could reduce the reliability of the plain concrete taking shear.

2) PT of either the web or top flange after the bottom flange would introduce stresses to either the web or flange accordingly. After all of the PT was the section overall stress fairly balanced or was there a noticeable bias in deformations between the bottom flange, web and top flange which resulted in an unevenly stressed structure with excessive built in stresses aside from other loads.

3) Had most shrinkage taken place before PT was started, shrinkage in the flanges may have been slightly higher that the web due to being relatively less bulky and the flanges would only be formed on the bottom rather than all 4 sides. Overall creep effects should be fairly equal if PT stresses in the concrete over the section were fairly balanced.

4) Since there were strain gages on at least some members is there strain history available to the NTSB which might assist their investigation.

5) De-stressing the bottom two bars in 11 seems counter intuitive because any additional shear capacity at the deck provided by confinement of the concrete at 12 would be lost. There does not appear to be adequate milt steel reinforcing in the evidence available to transfer the compression from 11 into the deck.

Just a few more factors which confirm my belief that the design concept is more complex that it might appear at first glance.

Ductility in a simple span structure must be available for safety. Regardless of what failed first this was not a ductile failure.

Dik:

Yes, it may in fact had been designed with adequate ductility. We’ll have to wait and see. It just didn’t seem so in the failure. I also understand the sentiment that the codes are already burdensomely large, and that he bigger the codes get the less an engineer gets to engineer. But, I can’t say I’ve ever seen a concrete truss before. I don’t see the harm with code requirements for concrete trusses such as minimum mild steel at joints. New member types should warrant codes requirements.

steveh49....my point was simply that engineers should never be in a subservient position to those making money decisions without technical validation.

"Yes, it may in fact had been designed with adequate ductility. We’ll have to wait and see. It just didn’t seem so in the failure. I also understand the sentiment that the codes"

Use of ultimate strength design was to insure ductile behaviour but it assumes that adequate reinforcing be provided. There does not appear to be more than nominal stirrup reinforcing either in the top blisters, intersection of web members or the bottom flange. The two bars which provided some confinement at the bottom of 11 and 12 were being de-stressed which in my opinion actually reduced the shear capacity. The bottom of member 12 is not even fully connected to the deck as per "gwidemans" earlier posted pictures.

To Dik - just for interest value, on the rescue photo you can see the shirt on the lead person says "FLTF1", that's the "Florida Urban Search and Rescue Task Force 1". Same pic in wider format shows a headlamp on the helmet (https://www.cbsnews.com/pictures/deadly-miami-brid...) Rear rescuer with dog looks like she has a Sierra cup hanging from her belt. -- Emba

I'm with you Dik. We learn from our failures. We pick up the pieces and fail foward. We will all learn from this.

This design was beautiful. I liked the simplicity in the way the they used the canopy as the top CHORD...the sleek deck as the bottom CHORD. Keep in mind the south end had more loading in member 2 than member 11, because it had a smaller angle to the horizontal and the diagonal was longer. Member 2 and its connections survived the impact of the collapse. What happened at the node of 11 & 12?

Did they spend a heck of a lot of money for this???...you bet your sweet silde rule they did.

I guess beauty's in the eye of the beholder, because to my eye it was an ugly phoney kludgey design.

tomfh i did not like the fake stays...even though they were to dampen.

hokie66

Correct after looking at the structure 2 is at a flatter angle than 11 and would have the higher horizontal component, so why the difference.

Joint at 11 is fixed?
An error occurred in de-stressing no. 11?
Was one rod in 11 over tensioned and lifting it off caused it to rupture with the released energy propelling part of it out of the channel?
Was there a particular problem with the cap at the top of 11? or did it punch through due to the collapse of the canopy onto the road?

Don't really know except the failure was not elastic.

You must be looking at the bridge backwards. 2 is the shallow one, 11 is steeper. Yes, the vertical component would be the same for both, but the more shallow one would have the greater force.

hokie66

Gave you an extra star for this one. Also meant that the bearing at 11 is fixed not the joint. 2 appears longer than 11 and therefore 11 would be stiffer than 2 if similar size. I don't have all the sizes but from the only drawing that I have they appear to be similar in depth, if so then 11 could transfer more unbalanced moment into the deck and canopy.

Quote (Jerehmy (Structural)21 Mar 18 23:33 Far and away what bothers me the most about the failure is the lack of ductility. The near instantaneous collapse due to the brittle failure guaranteed injuries. A more ductile failure would have at least given the victims a chance. A structure with no redundancies, brittle failure limit states, and located in a high traffic area just doesn’t sit well with me. The ductility can and should be addressed in code changes.)

Code changes? What part of the AASHTO Bridge Specs are written for concrete trusses? I'm curious how many people on this forum have seen a concrete truss, let alone a concrete truss bridge? I found 3 on bridgehunter.com, the newest built in 1934. Fewer still are those who have designed a concrete truss bridge.

FIGG pushed the design of this bridge to far past the edge of what was possible. Unfortunately, some paid the ultimate price for their arrogance.

For $16.6 million you could have built several pedestrian bridges with proven designs and technology. The USDOT and FDOT share some of the blame for providing funding and going along with such a foolish endeavor. Pride goeth before destruction, and an haughty spirit before a fall.

OSUCivilEng:

Exactly my point. If concrete trusses are going to become more common, they should specifically be referenced in the code.

Also, on your point about pushing boundaries; people said the same thing about truss bridges when the transition from masonry arches to trusses was occuring, suspension bridges bridges when they first came around, steel frames in buildings instead of masonry in the late 1800s, and so on and so on. To demonize FIGG for pushing boundaries when we don’t have all the answers isn’t right. Our profession’s history is full of us pushing boundaries.

Hi All,

Just signed new member. I am an electronics engineer, so other than instrument and cabinet chassis, no structural experience and none with bridges or pre- and post tensioned concrete. I do have an intense interest in engineering disasters as they are virtually always caused by the "holes in the swiss cheese" lining up just so, they typically require a number of failures, and there always seems to be a huge amount of human arrogance and ignorance in play. So while I am not a structures guy, I have about 50 years of common sense engineering that has been hammered into me by various successes, failures, and guys who were at the time older and wiser than me.

Regarding this specific failure, a number of you have noted that that while trusses 11 and 2 both carry equal loads and that truss 2 had even more compression as it was more horizontally oriented than truss 11, it survived where truss 11 failed. You all got my curiosity up, so I went back to all the pictures to see if there was anything obvious, and while all the line drawings show the trusses of equal beam depth (excuse me if my terminology is incorrect as I am not a structures guy), they are most certainly not. In fact, if you see the picture of truss 2, its depth (not sure correct terminology here) is almost DOUBLE that of truss 11!

In addition to this truss having a much lower load per unit area, it will have greater torsional strength against flexing in the horizontal plane. I think the key though is take a look at how the truss is attached to the deck. As the beam depth is almost twice as great, the attachment to the deck has about double the surface area. Therefore, even if the rebar layout is poor, all the loads, including shear, will be roughly half. Last, this doubling of attachment-to-deck surface area means that the loads are transferred to the deck much further into the middle of the deck where the loads can be more effectively transferred to the deck PT cables.

So, while I believe like many of you that this was a terminally flawed design, impossible to analyze at the micro level at the nodes, blisters, and their respective connections to both chords, and that likely would have failed, possibly later killing dozens, or even hundreds of people, there does seem to be a big problem in that two trusses, both of which are the most heavily loaded trusses in the design, are of dramatically different beam sections. And one has to wonder it that last minute "demand" from FDOT, which added 11 feet to the north end, caused a fatally flawed redesign to an already weak structure.

https://res.cloudinary.com/engtips/image/fetch/w_8...
[img ]

Gary

I don't think anyone has posted this yet:
"Experts cite explosive joint failure as cause of Florida bridge collapse"
https://www.newcivilengineer.com/tech-excellence/e...

Their leading speculation is explosive joint failure at base of #11. Note that in the linked article they have the member numbering off by one relative to the MCM-FIGG drawings, so they refer to the failed member as #10.

A number of posters here have noted the awkward path by which compression in number 11 would be transmitted to the tension in the deck's torsion rods/cables.

To the limited extent we can deduce anything from the MCM-FIGG proposal drawing, and the photos posted here, Member #11's lower end pushes against an area of the deck in which forces must work around service channels for the pillar #12, and also travel laterally to the location of the innermost deck tension members. The upper portion of #11's cross section pushes substantially against vertical #12, so those forces create a shear between the base of #12 and the deck, and might meet less resistance.

Summarizing these ideas, compression in the lower end of #11 might look like this figure:



Forces distributed very unevenly over the cross section of a member could result in exceeding the member's resistance to bursting in that high compression region. This is a story which could match the newcivilengineer story, linked above.

Much caution is needed though, since it's clear that #11 and its connection to the deck were redesigned after the MCM-FIGG proposal, not least to add the two PT rods (perhaps to accommodate the transporters relocated inboard), and of course the sizeable PT end plates, that must be somewhere in or under the connection.

[Edited to add a comment about SomewhereOverChina's excellent observation]: I agree that there's a concern to have the end diagonals (#2 and #11) somewhat set back from the end of the deck so that transmission of horizontal forces from these members can be distributed to the laterally offset deck tension rods/cables. On the other hand, each of these members also presents a significant downward load, which needs to go directly down through the supporting pillar, and not create sheer in the relatively thin deck, which it would if positioned further set back from the end of the deck.

Good eye, SomewhereOverChina. Looking back at it, it certainly looks like several of the members are wider towards the south end, with #2 being the largest change from the initial plans. I wouldn't say they added length to the north end; the north pylon was moved and the overall length of the bridge increased. It is likely they mostly scaled the bridge to keep the same proportions.

I wonder if the root cause was making half-considered changes and not re-doing the stress work. It's like they really wanted to keep the diagonal members as thin as possible and then, for some reason, decided they could not, resulting in the drastic change to #2.

I know this is now moot, but I have been wondering about the fake cable stays.
I understand that there can be significant expansion and contraction of cable stays due to temperature changes.
I am thinking about the anchor bolts failing and the deck jumping up about two feet on the bridge up in Canada a while ago.
I would love to hear some comments from the bridge people about the possible issues of expansion and contraction of stays attached to a very rigid structure. Would the stays be stretching in cold weather, sagging in hot weather or both?

Bill
--------------------
"Why not the best?"
Jimmy Carter

Continuing to look at the 11-12-deck area, which we know was redesigned relative to the MCM-FIGG proposal docs, looking at this screen cap from an NTSB video, what are the two items I've labeled with a question mark?


https://youtu.be/aeJKqojmHgY?t=129

They look to be tension rods, and within the footprint of vertical member #12. But it's not clear to me if they are something that extended vertically into #12, or were actually horizontal and bent up during collapse as the end of the deck fell adjacent to the north supporting column. They seem too inboard to be the most inboard of the deck PT rods/cables, at least as shown on the MCM-FIGG drawings, and seen exposed in photos/videos of the end of the deck.

I'm wondering if they might turn out to be part of a feature to distribute forces from member 11 to the deck.

gwideman,

Is it possible that those are tensioners for the future tower that was supposed to support the fake cable stays?

And regarding the same area, does it look like there is no concrete anywhere in that area? None of the tensioners, the plastic conduit, even the rebar. It is almost like that area was purposely voided for some type of service tunnel possibly for the services or other that would have been contained within the cable stay tower. And if that is truly a voided area, then truss 11 and NO SUPPORT to the edge of the deck. It seems a totally open area.

Did they figure when both sides of the bridge were done and the tower constructed that this area would be extremely strong as both truss 11 and its mirror twin on the other side would be in compression against each other? Is it possible the bridge was FEA analyzed as a complete unit? Was the shorter section, with substantially lower forces, supposed to be built first so that truss 11 would have something to work into?

Regarding this link to Civil Engineer:

https://www.newcivilengineer.com/tech-excellence/e...

while I am not a structural engineer, I was not impressed. The author obviously did little or no homework. He did not even have the truss numbers labeled properly, something that has been correct from the first post in this forum, and his "final configuration" analysis shows tension on some of the trusses like they are being supported by real cable stays. Yet if I understand everything I have read correctly, including the original proposal, those stays are almost decorative and may in fact proved more support for the tower as guy wires than as anything to hold up what is a truss bridge (apologies for the true structures guys that make the point that a true truss bridge is always hinge pinned).

This structure seems like a heavy, ugly (I am with Tomfh), costly alternative to a real cable stay bridge, virtually all of which I have seen are quite light, airy, and beautiful looking. Not sure the relative cost of a true cable stay bridge though versus a simple steel passenger bridge.

Gary

Here is the top of the canopy at the north end.



Here's the base of # 12

• http://files.engineering.com/getfile.aspx?folder=92effea6-6a8f-4c07-ba58-ce

@gwideman, Sheet B-8 of the proposal package (linked from thread 1) shows some kind of bolt there but it looks like it's for position-keeping rather than tension.

Quote (SomewhereOverChina)

while I am not a structural engineer, I was not impressed. The author obviously did little or no homework. He did not even have the truss numbers labeled properly, something that has been correct from the first post in this forum, and his "final configuration" analysis shows tension on some of the trusses like they are being supported by real cable stays. Yet if I understand everything I have read correctly, including the original proposal, those stays are almost decorative and may in fact proved more support for the tower as guy wires than as anything to hold up what is a truss bridge (apologies for the true structures guys that make the point that a true truss bridge is always hinge pinned).

Your assessment is correct. While I have not read the link in its entirety, the assumption that this was a cable stayed bridge shows a gross lack of research in the available information. Without the ability to tension the 'cable stays' they will not be under tension in their finished state, except for under transient loads. You can't magically turn a simply supported span into a cable stayed span without adding significant tension to the cable stays.

Quote (Emba)

o Dik - just for interest value, on the rescue photo you can see the shirt on the lead person says "FLTF1", that's the "Florida Urban Search and Rescue Task Force 1". Same pic in wider format shows a headlamp on the helmet (https://www.cbsnews.com/pictures/deadly-miami-brid...) Rear rescuer with dog looks like she has a Sierra cup hanging from her belt. -- Emba

The comment was made because there is a movement within OSHA to replace conventional hardhats with the new style included in the link:

https://www.constructionjunkie.com/blog/2017/8/7/s...

The search and rescue helmets in the photo, likely comply to the new OSHA requirements. These are slowly making their way on to job sites. The new style hardhat is patterned off the helmets that are commonly used for rock climbing. The new OSHA conforming hardhat will likely replace the conventional hardhat; they are safer.

Dik

Quote (apster)

Don't really know except the failure was not elastic.

Even glass behaves elastically... to a point. There was, however, little or no ductility.

Dik

My opinion : At the time of failure the structure where it failed was beyond the elastic range and failed as a brittle material. This is equivalent to reaching the ultimate stress and strain of the material or combination of materials. A concrete cylinder can remain fairly elastic up to the point of failure, steel will remain elastic and continue to stretch i.e. increase strain without increase in load or even with some decrease in load until it ultimately fails.

Quote (gwideman)

I don't think anyone has posted this yet:

There have been a couple, as I recall; Eng-tips is generally on top of it:

Structuraleng89, March 18
Tomfh...great video w the dash cam...i see an explosion at the bottom of the vertical at the support...maybe the strand they were pulling at the top of #11 came out at the bearing node of the truss...the tension diagonal adjacent to #11 appears to punch thru at the bottom as well

gte447f, March 18
...it looks like you can make out something shooting up vertically from the top chord first panel point and in the same frame there appears to be an "explosion" of dust at opposite end of diagonal #11

Dik, March 19
A failure at the end of a member may be different than the panel point 'crushing' or 'exploding'.

Rapt, March 20
At the 3 - 4 frame in the video you see something like crushing in this area with an explosion of material upwards from the top (not as in caused by explosives). The diagonals still seem to be intact at this point. Next frame, the top drops very quickly and you get the rotations in the 10/11 node at the top that have been noted previously as the bottom slab at this location crumbles.

Dik

Quote (dik)

There have been a couple, as I recall; Eng-tips is generally on top of it

I didn't mean to say nobody on the thread had discussed crushing at various locations.

I meant I thought nobody had yet posted a link to this article. Notable because it's from a site that purports to specialize in civil engineering, though as others have noted, a bit careless in this article.

Hmmm, and now, suddenly, I can't access the article -- it wants me to register.

The failure was catastrophic, and there has not been sufficient information to put the cause to rest...

Dik

Quote (Steve Nuchia)

@gwideman, Sheet B-8 of the proposal package (linked from thread 1) shows some kind of bolt there but it looks like it's for position-keeping rather than tension.

Sounds plausible, though I don't think the drawing matches the photo. On B8 "CROSS-SECTION", and "SECTION A-A", I see a couple of vertical rods, but they appear outboard of the inboard-most deck tension rods. Then in A-A and C-C there are a couple of horizontal rods at the canopy level to tie the "Main" and (not present) "Backspan" bridge canopies together. So I don't think the rods in question correspond to that drawing.

I was wondering why diagonal strut #11 has P.T bars, but it hasn’t P.T bars in the preliminary design.
I found a difference between the preliminary design and reality in transporting the main span. In preliminary design, the shoring supports both ends of that span therefore no need P.T bars in #11.
Maybe they narrowed the distance between two transporters to save money for site clearance, but they paid too much for that.

If this Oliver McGee is a licensed Professional Engineer he has likely breached the ASCE and NSPE codes of ethics for his unfounded public statements. Might also be a violation of the Texas Engineering law, but I don't know their stance on public statements. In my state, it would be a violation.

• Safety
• harness

Leubong,

It has already been well established in earlier threads that there was a significant design change whereby the length of the span increased by 11 ft to allow for a future widening of the road. As such the pillar on the N side moved North into what is currently a grass area. This resulted in a change to the planned movement and location of the transporters further into the span and required the additional PT rods in both member no 2 and 11.

They were both supposed to be released once the span was sitting on the piers.

Now why it then took them 5 days to do it and after they re-opened the road is anyones guess and I would think will form part of the investigation into the collapse.

The PT bars in member 2 were apparently (according to the NTSB) released without incident before they started on member 11.

Please take some time to read parts I and II of this incredibly interesting and thought provoking thread, before re-introducing further items which have already been discussed.

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

VSL employee was tied off to horizontal safety line. The safety system became wrapped around the exposed post tension rod. See attached photos. This unfortunate worker seems to have been flung by this while bridge fell. It looks like he dangled from crane because vertical crane line held up the horizontal safety line.

The north end fell fast in part because the center of gravity falls at 1 g, but like a falling smoke stack, the free end goes faster. The marks on the pylon made by the falling deck may help with investigation. The marks begin well below the top of pylon base.

The central web truss does not look right, it looks more like a concrete canopy support system would look in a walkway/plaza on solid ground, not a support system for a 174 foot span. It looks like they were going for clear vistas from sides of walkway, so they put only one web in the middle. It's sad, but FIU/MCM put in writing (the proposal) all the constraints that added together doomed this project. I.E. ABC, a pedestrian overpass that resembles a magistic full size cable-stayed bridge, concrete to be cool in summer, wide ribbon deck...

This bridge will not be completed with this design, you can bet the new design will be highly redundant.

I will keep on open mind until report from NTSB is issued, but I must say now, I'm shocked engineers did not block this project and even more shocked rod adjustment was done while traffic was below.

This bridge failed because deck separated from joint 11-12. Half the steel of 11 went with deck, the other half went with 12. Joint burst and beam zippered, no PT-rods or anchors broke. I agree with other posters the north end depended on support from other section that was not yet in place or the stress path was bad. The "last minute" 11 foot increase in span is scary to me.

PS. There is a public FIU student dropbox with time-lapse of formwork phase. You can clearly see the rods and joints before concrete was poured.

Comparison between bridge collapse movie and theoretical simulation of truss without member #11

Deck fail between member #11 and #12 #10 and #11 is close too.
Courtesy to Zac Doyle for video and to Ivan Markov for online simulator.
Data input : (0, 100)(87, 100)(154, 100)(221, 100)(289, 100)(358, 100)(412, 100)F:(0, 139)(80, 139)(157, 139)(240, 139)(327, 139)R0:(412, 139)(0-1)(1-2)(2-3)(3-4)(4-5)(5-6)(7-8)(8-9)(9-10)(10-11)(11-12)(0-7)(7-1)(1-8)(8-2)(2-9)(9-3)(3-10)(10-4)(4-11)(11-5)(5-12)(12-6)

Quote (jrs87)

There is a public FIU student dropbox with time-lapse of formwork phase.

Care to let us in on the address?

Meerkat007

Can you set simulator to no connection between deck and 12?

Quote (jrs87 (Mechanical)Can you set simulator to no connection between deck and 12?)

For that simulator is same if you remove deck between #11 and #12 #10.
Delete "(11-12)" from data input.
UPDATE: You can remove member #12 entirely together with canopy.

Quote (Jerehmy (Structural)22 Mar 18 03:20 OSUCivilEng: Exactly my point. If concrete trusses are going to become more common, they should specifically be referenced in the code. Also, on your point about pushing boundaries; people said the same thing about truss bridges when the transition from masonry arches to trusses was occuring, suspension bridges bridges when they first came around, steel frames in buildings instead of masonry in the late 1800s, and so on and so on. To demonize FIGG for pushing boundaries when we don’t have all the answers isn’t right. Our profession’s history is full of us pushing boundaries.)

I simply disagree. I think this entire design was irresponsible. Test pilots, astronauts and soldiers at war push boundaries and put their lives at risk. Innocent motorists shouldn't be put at risk with unproven designs built right over their unsuspecting head.

From the MCM-FIGG proposal:

"The superstructure shape for the new signature pedestrian bridge is innovative and one-of-a-kind. We have re-invented the traditional I-beam in a magnificant scale with a special transformation of an open truss down the middle, improving both its functionality and form for a 30’ wide path."

Somebody has been reading too much of their own press clippings.

Reply to gwideman:

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

OSUCivlEng just made some very good points. Jerehmy is right that new is often unfairly ridiculed. But it is never unfair to ridicule failure. Maybe Figg did not push boundary, maybe they went past it. Or the end result was such via chain of events. We all learned in engineering school the right thing. We forget as time goes by... And it's the same old story: engineers not enough power. But physics always prevails.

Anyway, no one wants to demonize anyone in this forum. Let the NTSB investigate.

Meerkat007, the simulation clearly showed the movement of the roller support, akin to the tore out. I am newly registered here, and found this discussion really interesting, with lots of great responses. Thanks a lot.

Starting enumeration of code standards:

1. Redundancy, no single plane of truss
2. Symmetry and no faux in super-structural members
3. No 30 foot wide width for 174 foot span without curve in horizontal plane or true wire stays
4. No ribbon deck so it looks like wire-stay but is box truss (wait it's not even a box truss)

This is not meant to be mean, just a thinking exercise.

PS This accident is very interesting because we know how and why this design came about, it's so obvious. But the cause of sudden collapse is very perplexing. After all it was unloaded when it failed. And there is no doubt about it, some trust of engineering as been lost in America now. There as always been accidents and failures, but this one seems different... Maybe because something so simple as a pedestrian bridge failed in the hands of those with much bigger capabilities.

After this collapse we could say big bridge designer demoted to pedestrian bridges. Oh... sorry.


I wonder too how the thin edge of deck was expected to respond to accidental strike from overhigh cargo.

OSUCivlEng:

How do you prove a design without building it?

FIGG shouldn't be ridiculed for trying to innovate, they SHOULD be ridiculed for not doing their due diligence in ensuring everything went correctly. I don't think they did. If you're going to do an innovative structure, you need to put extra time and effort so that everything goes correctly.

Was there a FIGG engineer on site? If not, WHY? In this book I'm reading on building/bridge history in USA, the design engineer was on-site every single day. He led the project. I feel we've gotten far away from that and I'm not sure why. Especially on a project that is innovative.

I'm also curious into their R&D in using a concrete truss. Did they do any scale models of concrete trusses? Test them to failure?

No single element doomed this bridge. ABC is okay but rules out wire-stay, concrete is okay but rules out thin, central web is okay but rules out no support from wire stays and ABC... Now we need a Venn Diagram and we will see I suppose this bridge will not be in any circle. Engineers are taught that "never-been done before" is a red flag, because maybe it has not been done because it is NOT an improvement.

Jerehmy, are we in the USA anymore? USA used to mean diligence. Not anymore. You make good points, but I think innovative should not be done just to be innovative in and of itself. Not everything needs to be innovative. Be innovative in the Utah bridges, that's okay. Care to share the name of the book?

I know FIU tests elements, but maybe only steel. And then only small parts. This bridge failed as a system that cannot be tested. This bridge was the lab. Right? Too many variables...

Re-posting from the last thread:

Quote (winelandv)

A defense of the design:

To the structurals here, who hasn't, at the behest of an architect, worked their tail off to make something superflous work? Is that not part of our job? Ok, so the "cable stays" really aren't. So?

Any of us who has come into a project halfway through has had the thought, "This, this, and this look really strange and I hope that this, this and this were accounted for." Same situation here - we're all coming in after the fact and slowly learning what the design really is. I, for one, am willing to give Figg some slack here on all the design issues: the un-symmetry, the not-really cable stays, the non-redundancy, the kinda-a-truss but kinda-a-beam-with-holes issues. That firm isn't just 2 people in a garage somewhere. They have designed many, many bridges throughout the United States (including the I-35W replacement in Minneapolis).

Maybe, when it's all said and done, we'll know one way of the other if the final design was safe (and maybe not if everything gets settled and everyone signs an NDA). Until then, all we're doing is throwing thoughts around. Yes, it failed, but there's only a couple hundred other factors that could have ultimately caused the collapse.

Yes, many people here have identified challenges with the design side. SURELY you don't actually think that FIGG didn't try and account for those challenges? To those saying that the design is completely wrong - it did stand for 5 days - surely that's at least proof of concept? Short of gaining access to the design team's notes, hand calcs, and computer models, along with the rebar shop drawings, and the constructor's construct and erection plans, there's a giant pile of unknowns as to the ultimate cause of failure. I'm not saying that there isn't a fatal flaw in the design (as I don't know either way), but let's slow down the blame game - there's enough talking heads on cable handling that. And goodness, the last thing we need is to start revamping AASTHO's spec for bridge design before we have all the facts.

Additional thoughts:
-if you want to see a cable stayed bridge, go look at the conceptual pre-bid documents that TY Linn put together.
-originally, this bridge only had $6,000,000 allocated to it and it was all coming out of the TIGER grant - where did all the other money come from?
-what happened at the army corps that the 11' pier change 1) was necessitated and 2) came so late in the process?
-what was the full sequence of PT stressing on member 11 over the life of the bridge? Was the work being done at collapse to tension or de-tension the rods?
-who in their right mind allows a crew to work on 150ksi PT rods on a non-redundant structure without closing traffic????

Meerkat 007, I would rather not have to remove all those elements, need bending resistance. I know Java model cannot do that as is.

winelandv, six people died, 10 injured, it's like a nightmare. We know Figg is good and asked the same things we are. This was just a pedestrian bridge. This forum is a form of peer review. There is nothing wrong with that. Raw blame game is bad. I wish NTSB all the best with this one.

I have commuted several times under skyways (elevated freeway supported from median of existing freeway) during construction, 110 in Los Angeles and the Metro Manila Skyway. That is darn scary. So we must have trust in our builders and engineers.

As I mentioned earlier, they had all the time in the world to test the bridge while it was on the construction site and before moving it. Since the largest point of the process was to highlight their engineering program, they could have fully load tested the bridge via a couple of semesters of classes, but were no doubt rushed by the pedestrian death and funding concerns and skipped it.

The horror I imagine is the bridge survived this tweak and instead they had a huge party some time, bridge crowded with new graduates and their families, when the overloaded section blew out, pulverizing dozens and crippling hundreds. Any claim that the fact it didn't collapse instantly proves the concept was OK is a bad one.

I also don't understand the repeated references to 'redundant' as if it would fix this. The I-35W bridge had multiple trusses and still collapsed because one side failed. Redundant structures are very difficult to create for spans. An overload failure of one element typically adds that element weight and the load it was carrying to the load of other elements, which should have been equally loaded to the one that failed, causing them to fail as well. It seems like only in special configurations where a defect in an element allows it to fail far below it's design limit for the current load that there is a chance for load sharing to work. And, worse, a really good redundant structure will mask the original failure until such time as the backup also fails.

LittleInch, the NTSB only said the workers were adjusting tension on #2 and #11 on the day of the collapse, not that this was the original, somehow neglected, destressing. On the day the bridge was moved, the project manager said they were going to detension "the two bars on top of the bridge" immediately after removing the SPMTs and some ropes. (twitter)

There's another interview with him in the dropbox linked upthread, where he says they have the whole weekend for the move if they need it; it doesn't sound to me like the destressing was skipped in the rush or forgotten.

3DDave, you are correct, redundant term is overused. It would be more precise to say design and engineer to be best practices available. What really bothers me is all the risks taken, no falsework, no closing traffic during tension adjustment. But then again, redundant engines on airliners is nice. Right? I think what sparked this forum is the fear non-engineering influences are getting stronger and stronger and USA standards are receding.

I have noticed on this forum no one dwells on the cracking. And they should not, amateurs worry about that. So we should give ourselves some credit for that. We went into engineering because we know how things work, we should use that knowledge.

The party scenario you mentioned is truly a nightmare averted.

I wonder if those dropboxes stay open because the NTSB ordered no deleting of data.

Engines on airliners exemplify what I mentioned. Each engine is more than capable of taking the full load needed to keep the plane airborne, but they are helped because aircraft can also shed load at will by changing their rate of climb. Even so, there have been a number of cases where they all failed because of lack of fuel or because identical failed service crippled them all or because they all ingested birds at the same time.

Respectfully gentlemen:
I see two questions here:
1> How did the bridge fail?
2> Why did the bridge fail.

I suggest that the why is the root cause that must be addressed to prevent future "hows".

Why did the bridge fail?
Regarding the calculations of the various forces in the structure:
Calculations depend on values. Some of the values are known, some are reported and some are assumed.
Many of those with a lot of field experience may share anecdotes relating cases where reported values were in error anwhere normally safe assumptions were in error for special cases.
Safe assumptions:
When the bridge was being transported the force exerted on member 11 by the PT rods had an adequate safety factor.
When the bridge was in the final position the force on member 11 due to the weight on the bridge had an adequate safety margin.
A special case where the assumption may be unsafe.
The force on member 11 is the sum of the PT force and the force due to the weight of the bridge.
Now add the unknown force of the de-tensioning procedure with an already reduced safety factor.
Another suspect assumption:
Over tensioning a PT rod is not dangerous.
I assume that this was an experienced crew. They were sent to work alone without direct supervision.
Normally a structure is supported by some type of false work until post tensioning is completed.
A failure of a PT rod or anchor may be inconvenient, it may be dangerous to the workers and it may be expensive.
However the false work will prevent a catastrophic collapse.
I think that it is safe to assume that this was an experienced crew. They have done this many times.
They have probably over stressed PT rods a number of times without any issues.
They may even have stressed rods and/or anchors to failure a number of times without a collapse.
We may never know truthfully what the instructions and warnings were given to the crew before the work began.
It is doubtful that the crew was aware of the reduced safety margin.
It appears that one worker falls from his safety harness. If true, that says something about this workers profesionalism.
The man who will take shortcuts with important personal protective equipment may also be the man who will overtension a PT rod without a second thought.
I hope that this is not the case.
I believe that the root cause of the wye may be a tiny grain of sand or other foreign matter jamming the threads of the PT nut and the following cause was over tensioning the rod in an effort to free the jammed nut.
Yes the "How" is important, but identifying the "Why" is the best way to avoid future "Hows".
I hope that someone has checked the PT nut to see if it is free to turn or if it is jammed. After the failure this will not be conclusive but if accurate information makes its way into the investigation may provide a strong indication of a possible root cause.
All the design calculations may come to naught if the field crew imposes a tension 250% of design tension on a structure.
Bill
--------------------
"Why not the best?"
Jimmy Carter

Not sure why you're arguing against redundancy. Non-redundant structures are avoided a majority of the time, and, when they can't be avoided, the safety factors are much higher. As for the elements of a redundant structure, they are always designed with some reserve capacity with load redistribution in mind should one element fail. Inherently, Load Resistance and Factor Design makes provision for that by increasing raw loads with load factors and decreasing capacity with phi factors. The more critical the failure mode, the lower the phi factor. If i recall correctly, the issue with the I-35W bridge in MN, was that dead load had been added over time and the inspection frequency was very low. Had the bridge been inspected more often, they likely would have noted which joints were becoming more and more overloaded.

As for redundancy fixing the issue with the FIU bridge, it would have prevented catastrophic failure. The structural concept in general could not be non redundant because they chose to go with a single, giant, post-tensioned, simply supported I-beam. The critical feature of their design was going with the open-web, truss-like structure. I could only imagine focusing moment, shear, and axial forces as well as the post-tensioned forces at nodal points would be very hard to design and detail for and equally difficult to construct properly. I would think that a solid web would have been easier to design though you would lose the aesthetics of the open web.

Any have a clue what role had PT anchor of member #11 into #12 from structural design point of view ?

Quote (dik (Structural))

Looks like someone is clamming up, From Fox News:

“The city attorney is advising us not to speak about anything to do with the bridge,” said Sandra Antonio, a spokeswoman for the city of Sweetwater,

...National Transportation Safety Board had forbidden engineers or contractors from talking about the project pending its investigation.

A FIGG spokeswoman, Cheryl Stopnick, said it would be “inappropriate to discuss specific detail about bridge design” during the investigation.

A spokeswoman for FIU did not immediately respond Tuesday to questions about the pylon relocation in the bridge design.

Happy as a clam...

Dik

This issue was noted in other accident discussions. All parties take a vow of silence until the NTSB delivers its final report on an accident. So secret is the process that for some portions of the inquest the partners gather in a secure section of the NTSB building that is equipped with a unique computer system that allows no communication outside the room. Partners at those sessions take notes on color-coded paper that is collected before they leave the room.

NTSB Form

This is not a 'conspiracy', but a means to determine the cause of the accident as well as to prevent future accidents. If all parties to an accident stand in the public square, behind their attorneys, point fingers, and fail to disclose what they know, the true cause of an accident may never be determined.

This accident has developed intense interest, but it will take some time before the accident investigation is finished.

jrs87 - Condit, C.W. 1983. American Building: Materials and Techniques from the First Colonial Settlements to the Present. University of Chicago Press. ISBN 978-0-226-11450-7

The I-35W gussets had been known to be buckling for some time and were just being observed. I think the accidents reports indicated they weren't specifically analyzed by the original team who considered the frame members to be the load path which is why no one noticed they were undersized.

Regardless of later developments in use, the I-35W trusses were seemingly redundant and still allowed a catastrophic failure.

Winelandy:

I gave you a star for your post, but, it comes with a caveat. Maybe half a star, and, someone can provide the missing half. This posting goes to others in the thread, just to bring our feet back to earth.

We have a reasonable likelihood of having identified the collapse mechanism, or, something similar to it. We could be wrong. We are basing our conjecture on some poor photos, some poor videos, and a preliminary construction 'document' (Photos are generally consistent with the preliminary drawings).

The final as constructed documents may differ substantially from the preliminary ones. The preliminary document could have provided design simulation material, simply for marketing. To do anything, but speculate, we need final construction documents and shop drawings. I think our current endeavour is close, but, there could be 'big holes' in it. Myself, and, others likely don't have enough information of offer a real cause and may have some reservations/concerns about the Eng-tips proposed failure.

Regarding codifying everything, there are a few thoughts and maybe more. Overall, this is a *BAD* idea.

Codifying goes a long way to indemnifying an engineer... all is done to prescribed standards, and, it is more difficult to fault an engineer. The Nuremberg defense comes to mind.

Codifying also curtails the profession by tending to restrict innovation. With things not codified, you can reach beyond a bit, using your engineering skills.

Another issue with codifying is that there are so many problems in engineering that don't fall into a 'cubby hole' and there is no prescribed manner of addressing them other than engineering skill. All you have to remember is that E=mc^2 and you cannot push on a rope (you can, but, you reduce the effective prestress)... and go from there.

There is talk about insisting that construction be ductile. As engineers, we know how to build redundancy into our Work so there are multiple load paths to failure, or enhanced safety factors, if ductility is limited. A ductile structure is a little easier to accommodate. It started, I think, with Ronan Point.

We don't know, that the original structure may have had some ductility built in, but, this failed.

The failure appears/was a brittle failure from some mechanism that may have eluded the original designer. There was no redistribution. Just so much is unknown. We have to sit calmly back, and, let things unfold;, else, we put ourselves in the same position as 'the Professor'. Additional information should be forthcoming and we will be able to provide more enlightened responses.

Dik

Quote (bimr)

This is not a 'conspiracy',

Never thought it was... just reminds me of the old saying, "When everyone is thinking alike, no one's thinking."

Dik

Quote (jrs87)

I wish NTSB all the best with this one.

It's not luck they need... hopefully, they can come up with an explanation of the cause of the failure. If not, they can speculate on the possible cause (and identify it as a reasonable, possible cause), but, the report should not have to stipulate on an exact cause. They should strive for that, but, not be 'driven' by it.

Dik

Quote (3DDave (Aerospace) Regardless of later developments in use, the I-35W trusses were seemingly redundant and still allowed a catastrophic failure.)

Quote (The NTSB concluded that the probable cause of the collapse was the inadequate capacity of the U10 gusset plates, which resulted from an error by the original bridge design firm, who failed to perform all of the necessary calculations to properly design the main truss gusset plates.)

I don't get your point. Redundancy role isn't to cover design errors.

Dik,
The known facts are:
1.Bridge design as per AASHTO Guide Design Specification for Pedestrian Bridges, and AASHTO 2015 (Drawing B-2)
2.Overall SF factor (DLx1.25+PLx1.75)/(DL +PL)=1.35
So, applying blindly the code it's not the best idea. I do not think that the authors of the "design guide" envision a pedestrian bridge with such ratio of DL to PL.
Any bridge suppose to have a minimum SF of 1.7 or above for bending, and 2 for shear.
And under-reinforced transition from blister to the deck was not ductile - it was relying on the shear strength of uncracked concrete

Meerkat 007,

This bridge seems to have failed due to design deficiency and many are claiming that redundancy would have prevented the failure.

There is no difference to the people who are killed between a design failure to account for expected loads and a design which fails because it is loaded more than the expected loads. The only thing that is important is if the failure mode is such that people have some warning that significant failure is imminent. Redundancy isn't guaranteed to do that.

OTOH overcapacity and designing a specific failure sequence would be helpful, but I have rarely seen it done. For example, I don't see many cases where there is a levee that has a separate, higher levee structure placed behind it. Instead there are designs such as the space shuttle booster O-ring that allows the secondary o-ring to cover for failure of the primary o-ring, resulting in catastrophic loss of the entire space shuttle.

Meerkat007, your comparison between bridge collapse movie and theoretical simulation of truss without member #11 looks similar. It tells that the diagonal #11 crumbles at the base first, otherwise, it would have pushed out #12 a great distance based on the geometry as shown below, before the base slab tore away from #11 and #12.

Please forgive me if I missed this in skimming the previous threads but:
-I noticed the PT sheath that I assume is from member 11 is nearly intact. I would think it would be destroyed as it ripped out.
-Maybe the old eyes missed it but if the PT strand was within the ties of member 11, I would expect to see the broken tie ends projecting where the PT rod ripped them apart. Was the PT rod a late addition outside the ties when it was discovered that the transporter needed moved inward and member 11 became a tension member (temporarily)?

Relevant webinar today:

The Role of the Design Professional During Construction

Quote (dik (Structural))

It's not luck they need... hopefully, they can come up with an explanation of the cause of the failure. If not, they can speculate on the possible cause (and identify it as a reasonable, possible cause), but, the report should not have to stipulate on an exact cause. They should strive for that, but, not be 'driven' by it.

Dik

Probable cause will be stated.

Example: Board Meeting : Collapse of I-35W Highway Bridge, Minneapolis, Minnesota, August 1, 2007

RetiredSE - see below...





You can see more details here.

Quote (wiktor)

The known facts are:
1.Bridge design as per AASHTO Guide Design Specification for Pedestrian Bridges, and AASHTO 2015 (Drawing B-2)
2.Overall SF factor (DLx1.25+PLx1.75)/(DL +PL)=1.35
So, applying blindly the code it's not the best idea. I do not think that the authors of the "design guide" envision a pedestrian bridge with such ratio of DL to PL.
Any bridge suppose to have a minimum SF of 1.7 or above for bending, and 2 for shear.
And under-reinforced transition from blister to the deck was not ductile - it was relying on the shear strength of uncracked concrete

That's where I have a problem...

minimum design standards as you have noted... could have been more than AASHTO required, or, may not have been designed in conformance with AASHTO... It should have been, but, just don't know this fundamental information...

Safety factor predicated on AASHTO design... see note above.

DL to PL ratio *should* make a bridge more safe because the variable quantity is diminished... not, that the standard includes for this loading differential... this weight disparity may be reflected in any geotech report, but, not likely in a bridge standard.

...appears to be under-reinforced, else, it shouldn't have collapsed catastrophically. I cannot yet get to the 'real' cause of the collapse although I think the thread is going in the right direction.

Dik

Quote (bimr)

Probable cause will be stated.

That's great...

Dik

Quote (winelandv)

who in their right mind allows a crew to work on 150ksi PT rods on a non-redundant structure without closing traffic????

More common than you think.

Ever since the 1950's when Finsterwalder pioneered free-cantilever (non-redundant) post-tensioned bridge construction, which are constructed over traffic-occupied freeways and highways all over the world to this day.

For match-cast segments consider that the only thing passing through the precast joints are PT (and UNbonded until grouted) and some epoxy, without a lick of mild steel reinforcing.

Correct me if I'm wrong but with the pylon moved 11 feet further to the north, this part of the bridge was 186 feet (57.2m) long instead of 175 feet.

FDOT reasonably distances itself from this project because Florida International University received Local Authority Program (LAP) Certification to oversee the project. Link

FIU installed Smart Bridge Technology on a new flyover from eastbound State Road 836 to northbound State Road 826 in Miami back in 2013, why they didn't do so here is frustrating. Link

The City of Sweetwater really has no roll in this project. They transferred title of the land to FIU at an inflated price of $1.3 or $1.6 million and agreed to participate in a shuttle transportation program, requiring they provide a "refurbished" 10+ passenger bus. When the bridge was moved from one side of 107th St. to the other, they agreed to assume the costs of completing the remaining improvements to the existing "historic" pedestrian bridge access. They did it on the cheap. Their approval of the ADA work is laughable.



According to the FDOT LAP guidelines FIU had the responsibility to have the FDOT sign off on the Right-of-Way BEFORE requesting Bids for the project. The delay & redesign fall squarely on their shoulders. TY Lin seems to have been providing Planning Services since 2014 for the global "University City" plan. Should they have been prompting FIU to the ROW requirement? Or did FIU just bite off more than it could chew.

Watching the videos in the DropBox, it seems they paused at one point to adjust the chains securing the bridge on the north end. I think this was needed in order to raise the bridge enough to clear the pylon. Finally, the bridge just sat in one place until a daylight hour convenient to ~dignitaries~.

Quote (3DDave (Aerospace))

The I-35W gussets had been known to be buckling for some time and were just being observed. I think the accidents reports indicated they weren't specifically analyzed by the original team who considered the frame members to be the load path which is why no one noticed they were undersized.

To be clear, the Collapse of I-35W report also indicated other factors:

"The National Transportation Safety Board determines that the probable cause of the collapse of the I-35W bridge in Minneapolis, Minnesota, was the inadequate load capacity, due to a design error by Sverdrup & Parcel and Associates, Inc., of the gusset plates at the U10 nodes, which failed under a combination of (1) substantial increases in the weight of the bridge, which resulted from previous bridge modifications, and (2) the traffic and concentrated construction loads on the bridge on the day of the collapse. Contributing to the design error was the failure of Sverdrup & Parcel's quality control procedures to ensure that the appropriate main truss gusset plate calculations were performed for the I-35W bridge and the inadequate design review by Federal and State transportation officials. Contributing to the accident was the generally accepted practice among Federal and State transportation officials of giving inadequate attention to gusset plates during inspections for conditions of distortion, such as bowing, and of excluding gusset plates in load rating analyses."

Thanks Meerkat. PT ducts are tougher than I thought and the widely spaced ties are not as ductile as I imagined. I envisioned member 11 looking like a centipede where the PT ripped out.

Quote (waross)

Over tensioning a PT rod is not dangerous.

Yes it is. How many have you personally stressed to state this? I have personally stressed (and sometime intentionally failed) many PT tendons - some longer than 1,000 ft (yes, not a typo - one-thousand feet long). Over stressing PT is a big deal. However, it is somewhat moot in this discussion, because the PT bar did not fail in tension, so therefore it was NOT overstressed.

Quote (waross)

I assume that this was an experienced crew. They were sent to work alone without direct supervision.

For more than 15 years the Post-Tensioning Insitute (PTI) has conducted training and certification for field personnel pertaining to PT. I would expect that at least one of the field crew on the canopy undertaking the stressing works that day would be PTI certified. Additionally, the parent company of the subcontractor who did the PT work has possibly one of most comprehensive and industry-leading training and safety program in the US.

Quote (waross)

Normally a structure is supported by some type of false work until post tensioning is completed.

Often, but certainly not always, highly depends on the type of works. For temporary PT works, often PT is undertaken under full-traffic. For more complex (higher risk?) PT, there may be a traffic shutdown, but falsework, generally speaking, is a big time and cost item. I am not saying it was 'not justified' for this structure, it may have been. A traffic shutdown would have been PRUDENT, on a Saturday evening!

Quote (waross)

We may never know truthfully what the instructions and warnings were given to the crew before the work began.
It is doubtful that the crew was aware of the reduced safety margin.

I think it will be revealed precisely what instructions were given by FIGG after the 2-hour trailer meeting after cracking was noticed and necessitated the stressing works. There are also two surviving members so the stressing crew who I am sure will do the honorable thing and report the facts of what they did on that canopy - it is the very least they will do for the sake of their now deceased work colleague and 5 others.

Quote (waross)

It appears that one worker falls from his safety harness. If true, that says something about this workers profesionalism.

Total speculation.

Quote (waross)

The man who will take shortcuts with important personal protective equipment may also be the man who will overtension a PT rod without a second thought.

You really have it in for the field guys, don't you! I have spent half of my 30 year career in the office and half in the field (with dirt on my hands and under my fingernails) - and for all the $hit field guys get from office 'pimps' it is not justified in the majority of cases.

Quote (waross)

I believe that the root cause of the wye may be a tiny grain of sand or other foreign matter jamming the threads of the PT nut and the following cause was over tensioning the rod in an effort to free the jammed nut.

Highly unlikely. Consider than member #11 (and it PT bars) is more than 30 feet in length. So the elongation of the PT bar stressed to 70% of yieldEDIT ultimate is (70%*390 k = 273 kips) about 1-1/4". To achieve 'lift off' of the nut to the bearing plate all you need to 'daylight' a gap - let assume 1/8" (very conservative). So the bar has now elongated 1/8" above its original 1-1/4" so it elongates a total of 1-3/8" - an increment of 10% - and therefore a 10% increase in corresponding force (stress) too. 110% of 273 kips is 300 kips - way below ultimate tensile of 390 kips.

Additionally, the very design of the threads to PT bar is to handle the field conditions. It is a very robust thread - a grain of sand or cement paste splatter has little effect. Additionally, it is common for the manufacturer to supply a 'thread cleaner' nuts that you 'run' over the bar projection to clean away any deleterious materials BEFORE you install a stressing setup.

Quote (Ingenuity)

Over stressing PT is a big deal. However, it is somewhat moot in this discussion, because the PT bar did not fail in tension, so therefore it was NOT overstressed.

Is relative strength of the components an active design consideration when designing PT members with relatively thin sections and a low number of tendons?

In other words- for the web members of this concrete truss, they are relatively slender and with a low quantity of PT tendons. We know the level of strain applied to the PT tendons was insufficient to load them to UTS, because they did not fail. But was the level of strain applied to the PT tendons high enough to fail the concrete member in compression? Does that extra 10% applied during the destressing procedure ever run the risk of cracking or otherwise damaging the PT member?

Off topic, but I'd love to see the PT procedure for a 1,000+ foot length tendon.

Quote (epoxybot)

FDOT reasonably distances itself from this project because Florida International University received Local Authority Program (LAP) Certification to oversee the project

Because they 'have big pockets', they may not be able to distance themselves far enough... we'll have to wait and find out... hopefully, their presence on site, during any part of this work, is nil or next to... If they were frequently on site, or attending meetings, they may have a problem.

Dik

Quote (jgKRI)

Off topic, but I'd love to see the PT procedure for a 1,000+ foot length tendon.

It was closer to 1,100 feet long, 31 x 0.5" dia 7-wire multistrand tendons (all 31 pulled one-time), 8 tendons total. Approx 8 feet of elongation. I may have a photo I can dig up...

thanks Ingenuity...finally someone with actual experience in PT design and construction....

bimr - I have read the entire I-35W report and still have in mind the photos of the warped gussets. I didn't feel that pasting the full contents was required to make the point that a bridge that had the same redundancies as suggested as a fix for this one still failed because a fundamental design element was never addressed.

Quote (jgKRI)

Is relative strength of the components an active design consideration when designing PT members with relatively thin sections and a low number of tendons?

In other words- for the web members of this concrete truss, they are relatively slender and with a low quantity of PT tendons. We know the level of strain applied to the PT tendons was insufficient to load them to UTS, because they did not fail. But was the level of strain applied to the PT tendons high enough to fail the concrete member in compression? Does that extra 10% applied during the destressing procedure ever run the risk of cracking or otherwise damaging the PT member?

Yes, the engineer would (should?) have considered all aspects of component capacity - all the PT components and the structural member with its load histories/cases and including reductions for area occupied by the tendon ducts (not yet grouted etc).

For member #11 with 2 each x 1-3/4" PT bars on a 24" x 21" concrete cross section, with PT bars stressed to about 70% of UTS - that is about 540 kips over about 500 in² of concrete - so about 1,000 psi of axial compressive stress. Self weight support reaction of 950 kips and a member #11 angle of 36^o equates to a compressive force of about 1600 kips (3,100 psi axial compressive stress) - add the self weight force to the stress induced by the PT to member #11 and you get a net of about 2,140 kips (or 4,100 psi axial compressive stress). IF (big IF) there was a 10% overstress to PT bars that would only increase the force (and stress) by 54 kips (100 psi) to the cross section, for a total of about 2,200 kips (4,400 psi). For 8,000+ psi concrete, adequately reinforced with mild steel reinforcing, (including significant confinement reinforcement) and the level of stress to the member should be okay...now the joint of #11 is another matter!

[quote dik][That's where I have a problem...

minimum design standards as you have noted... could have been more than AASHTO required, or, may not have been designed in conformance with AASHTO... It should have been, but, just don't know this fundamental information...

Safety factor predicated on AASHTO design... see note above.]

Yes, we do not know what the bridge was designed for - but we know for sure that it collapsed, and based on this fact, we know that there was some major error somewhere leading to the catastrophe. I have seen already another design driven by the appearance, without any understanding of the V-pier design.
And I have seen indiscriminate application of the code, without understanding of the meaning of these factors, and resulting safety of the design.
No code, or design guide are perfect - and there are many variables which should be taken into consideration, like the percentage of the particular loads in the overall design forces. And perhaps the codes should specify the limits, to which particular formulas applies. ("when DL/PL ratio is >2, use DLx1.6" - just example)
Almost anybody, with some basic computer skills, could operate Midas, or Bridge Designer, or Robot program, to name just few. One could model a bridge, and size it without even knowing how the structure works.
And I really hope, that that's not the case, and the cracking moment and shear for the span was checked, and compared with the design loads and minimum sensible safety factor achieved.
And I also hope, that somebody designing another heavy pedestrian bridge is reading this, and will alter a design to achieve minimum 1.7 SF.

>>>One could model a bridge, and size it without even knowing how the structure works.<<<

Given our current overly-complex codes and our reliance on software to navigate those codes I'm afraid I agree with you. I've also come to believe we may have built better in the days of the slide rule. Just my opinion.

wiktor, regarding this discussion of load combinations, the ratio of DL/PL concerns are exactly why bridges also have to satisfy the Strength IV combination, using DLx1.5

Maybe not the 1.7 you're shooting for, but a higher SF than has been discussed to this point.

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

Received this eMail at home; I've been discussing this with a couple of engineers on DesignToEurocodes...

I’m not a member of the ENG-TTPS [sic] forum, but I thought you might be interested in this:
https://facilities.fiu.edu/projects/BT_904/FIU-Ped...

Other internet finds:

RFP:
https://www.flrules.org/gateway/notice_Files.asp?I...

FDOT Pedestrian Bridge Info
http://www.fdot.gov/structures/structuresmanual/20...

The information may already be posted, but, I didn't notice it.

Dik

Quote (Archie264)

we may have built better in the days of the slide rule. Just my opinion.

Still have mine... just in case...

Dik

For those who don't have one: http://www.mathsinstruments.me.uk/page3.html

The discussion will continue in Part IV. This thread is closed.

Post here:

thread815-436924: Miami Pedestrian Bridge, Part IV