Miami Pedestrian Bridge, Part I 65

[JohnRBaker]

Multiple Fatalities After Pedestrian Bridge Collapses Near Florida International University

https://www.nbcmiami.com/news/local...orida-International-University-476984523.html

As investigators continue to search the site of a deadly collapse involving a 950-ton pedestrian bridge near Florida International University in Miami Thursday, officials say the death toll has risen.

Early Friday morning, the Miami-Dade Police Department confirmed that six people have died as a result of the collapse....

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
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[structuralengr89]

Are we sure it was the first diagonal in compression that failed? What about the second diagonal which is running the opposite way and is in tension? How do you handle the connection at the bottom between the tension member and the bottom chord of the post tensioned slab? Is there a tendon or two that run thru a sleeve and then anchor into tendons running thru the diagonal?

It looks from the video like the bottom slab/chord buckled in flexure between the first and second diagonal.

 

[EngineerEIT]

Here is a link of the proposal from the design build team. I have a hard time imagining that the project would progress to construction without a design that considered the simple span condition before the the back span was placed and the two were tied together. I don't design bridges but aren't precast girders designed for simple span DL and continuous span LL (assuming 2+ spans)?

https://facilities.fiu.edu/projects/BT_904/MCM_FIGG_Proposal_for_FIU_Pedestrian_Bridge_9-30-2015.pdf

 

[Archie264]

Well it's certainly a gorgeous design but I guess the devil was in the details, unfortunately. Complexity begets failure, to the sorrow of us all.

 

[haynewp]

By those drawings, stressing a bar could cause eccentricity on the diagonal.

 

[structuralengr89]

EngineerEIT...Great Find!!!!

On Sheet 115/173..the first compression strut is labeled #11...It looks like there is no P.T. strand for that member...but there has been talk of that member Tensioned in the field???

Member #10 shows 280k and is in tension...I see the connection detail at the top...but what about the bottom where it ties into the tendons running thru the deck?
Member #9 (in compression)and doesn't indicate any PT Tendons...Was there mild reinforcing in the compression member???

 

[racookpe1978]

Funding was largely federal, from the previous administration:

Collapsed FIU Bridge Was Funded by Federal Grant Program Criticized for Shoddy, Politicized Review Process
The TIGER grant program has come under fire for putting politics ahead of technical concerns.

By Christian Britschgi
March 15, 2018

TIGER was created as an economic stimulus measure under President Barack Obama and morphed into a permanent program. It has awarded $5.6 billion in nine rounds of grants since 2009. Members of Florida's congressional delegation publicly lauded the TIGER award to FIU.

"Thanks to this TIGER funding, FIU students will be able to walk from their student housing to class through a pedestrian bridge across Southwest Eighth Street," Rep. Debbie Wasserman-Schultz (D-Fla.) said in 2013. "More jobs will be created in our community thanks to this grant, and I look forward to celebrating the project's success with everyone in South Florida."

Rep. Mario Diaz-Balart (R–Fla.) made similar comments on Saturday. "FIU has come a long way since the TIGER grant that funded this pedestrian bridge was awarded in 2013," he said. "This project represents a true collaboration among so many different partners at local, state, and federal levels, and in both the public and private sectors."

 

[SAIL3]

I would assume that the bridge under construction and transportation had intermediate supports and a corresponding pretension of the strands....when this section of the bridge was in place and the intermediate supports were removed then it experienced for the first time the forces and deflections caused by spanning the full length of the first span resulting in more compression in the top chord and thus a lessening in the tension present in the strands and thus the attempt to re-tension them...for a simple single span the top chord would always be in compression so why is the added tensioning required at this stage in construction...could it be possible that the temporary compression in the top chord due to the single span during construction plus the added design tension in the strands based on the completed bridge have contributed to the collapse...again, this is all speculation on my part, at the moment....

 

[Ingenuity]

I don't understand the detensioning of the top deck PT.

I think the tendons being de-tensioned are TEMPORARY PT bars in the top of the top flange- the wheeled trailer support was centered about the first interior node point, so maybe that 'overhang' necessitated the temporary PT.

And this would explain the stressing operations occuring in the collapse photo - the photo above I posted may be the crew de-stressing the top bars (NOT the compression diagonal, as I first stated) - there is a large stressing stool in front of the ram - that is often used to access the nuts from threaded PT bar and therefore enable destressing.

 

[kmart30]

For a short time I did inspections on elevated PT slabs and Ive seen them snap from over stressing during tensioning. It can be very violent and destructive to the concrete. Maybe a cause? I believe they said they were tensioning it when it collapsed right? One report said someone heard a loud "cracking whip" sound a few hours before and I know these things are loud when they snap. If its been up since Saturday what else would of caused this? Vibrations from the traffic? There must of been some type of external force that caused this right? I dont design bridges but if the design wasn't adequate enough to resist at least its own weight do you think they would of noticed cracking, deflections, ect. once they removed the supports?

 

[Ron]

I saw a traffic cam video of the collapse. They were tensioning tendons in the first diagonal when there was a sudden collapse, apparently buckling the top chord plate. Under a simple span condition, that would be a compression strut. Under the cable stayed condition, there might be load reversal (depending on the support scheme)....thus a need for tensioning. Any possibility they tensioned this member out of sequence? Tensioning of this member should probably have waited for the cable stays.

We're all guessing at the reasons....could be all or none, but certainly interesting though incredibly tragic. Unfortunate for all involved and their families for their loss. This will gone on for years to come.

 

[AggieYank]

From looking at drawing package erection procedure (page 125):
Link
* They were supposed to tension the strands in the longitudinal direction prior to setting main span and removing shoring.
* It was planned to set the main span, and 2nd span, and only then install the guys (which were 16" pipe).

Presumably they would have checked the engineering of the "main span as simple span", as its shown in their drawings and also in their construction schedule.

Its not clear to me whether they were re-tensioning the long strands or not. If they were, then this was either a sequencing screw-up (extremely unlikely in my opinion), or, when they removed the shoring they must have had structural problems and were trying to fix it by re-tensioning strands etc.

So based on the above, its possible that the construction sequence was followed, but the performance of the single span action was lacking, whether it was an engineering screw up or a materials screw up.

 

[slickdeals]

Is it possible that there were multiple tendons in web member and may have needed to be stressed in a balanced manner. Maybe that sequencing wasn't followed and all tendons on one side got stressed creating a large moment which could have buckled the truss diagonal?

 

[Brad805]

We have one load case for the temporary construction where the supports were provided by the lifting trailers. After erected I can see the need to increase the jack forces due to the change in support conditions. When you look at those complicated joints I would not be shocked if they had some sort of bursting failure. It seems unlikely they could jack all the strands at the same time at some of those joints, and you could easily end up with a short term zone where you had a problem.

Looking thru the drawings I am confused when we needed to make our bridges into an art project? At one time the chicken merely needed to get to the other side of the road, but now it seems he needs to do so very elegantly. We could put in twice as many simple structures for what silly things like this cost.

 

[epoxybot]

I don't know why the won't release the actual video that shows the collapse. You can see a crew of 3 or 4 working on the canopy at the point where they tensioning equipment was located. When the bridge collapses, one of the workers it temporarily suspended in mid air. I wonder if he got hooked on the strand.

 

[minerlax4]

On Sheet 115/173..the first compression strut is labeled #11...It looks like there is no P.T. strand for that member...but there has been talk of that member Tensioned in the field???

The photos Ingenuity posted clearly show a tendon protruding from member #11. I'm assuming that was a change in the design sometime after that document in the pdf was created on 9/30/15.

Marco Rubio tweeted this last night: “The cables that suspend the #Miami bridge had loosened & the engineering firm ordered that they be tightened. They were being tightened when it collapsed today.” So it seems like it was either over-tensioned and the concrete crushed or a cable snapped and the jolt lead to a failure. The eccentric loading theory from tensioning sequence seems plausible too, particularly since based on the details in that pdf document, they'd have to go to the underside of the bridge to get to the other tendon(s) in the diagonal.

It's very obvious in hindsight they should have closed traffic for that since it's a heavily loaded compression diagonal, but they were touting their construction methods as causing minimal traffic impact, so I'm sure there was pressure to avoid any closures. This will probably be added to the list of failures covered in ethics courses or least to safety protocol for post-tensioning.

 

[JohnRBaker]

This just confirms something that Professor Henry Petroski (the author of 'To Engineer is Human: The Role of Failure in Successful Design' as well as other books on the subject) said once (I'm paraphrasing), "That eventually, particularly when designing bridges, all new types will be successful until the first failure is encountered, at which point we will learn what the minimum acceptable design criteria was, the last one built that DIDN'T fail."

His basic premise was that bridge design in particular was susceptible to a unique phenomenon and that is that since the vast majority of them are financed using public funds, that the pressure to continue to optimize the design is enormous, trimming away at safety factors and the costs of materials used, in order to win the bidding process, and that this will continue until we learn what constituted going just a bit too far. At that point, the consensus will be that the last successful project will be the standard by which all future ones will be be modeled. That is until a radical new design is introduced and the process will start over. In this case, it might not be the design of the bridge itself which was the victim of this phenomenon, but rather the method and processes used to construct it.

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

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

 

[Archie264]

John, while that's quite an insightful statement by Professor Petroski, I can't imagine that this particular showcase bridge in any way optimized public funds. Indeed, while skimming through the proposal the thing that jumped off the pages at me (as it was on, seemingly, every page) was how "environmentally friendly" the end product would be. When that issue is that much of a priority on a project designed to do one very difficult thing: defy gravity, well, in my opinion we've lost our way.

I believe what was optimized was political pressure; specifically, the pressure not to close down a road. It brings to mind a topic that was discussed on the structural forum, preventing a high-incidence / low-consequence failure (in this case preventing traffic impediments) leading to a low-incidence / high-consequence failure. And here we are.

 

[hokie66]

Thanks to EngineerEIT for posting the proposal documents, which shed a lot of light on what was done. Not the construction drawings, but enough.

The way I read it, this was a concrete truss bridge with stays added for vibration control, or to look like a cable stayed bridge.

The proposal committed to an independent peer review. Wonder if that was done.

Concrete truss/frame, bad idea. Unbonded PT, bad idea. Very little bonded reinforcement, bad idea.

 

[Archie264]

Wow, stays primarily for cosmetic effects? I did see that part of the proposal was to be able to have seasonal lighting projected onto them but it never occurred to me that that might be their primary purpose.

 

[structuralengr89]

minerlax4"The photos Ingenuity posted clearly show a tendon protruding from member #11. I'm assuming that was a change in the design sometime after that document in the pdf was created on 9/30/15."


Are you sure about that? Could it be the tendon in the photo is actually Member #10...a tension member? Member #11 is under the greatest compressive force already...why would you add additional compressive force...if we assume this is a purely axial compressive force in a truss?...The only reason I would want to further compress this member, is if I thought the member had bending forces in it..and I needed additional moment capacity of member.

And the drawing shows that they were to apply the jacking force from the bottom of the truss...but the photo shows the stressing ram from the top...so the design is different than shown on the proposal (but that is not surprising).