fiu bridge collapse–what happened? [Updated]

March 16, 2018 § 17 Comments

collapse getty

Getty Images

One consequence of having taught structures for years using failures as examples is that whenever something big comes down my twitter feed and email erupt.  Yesterday’s news that a pedestrian bridge at Florida International University collapsed brought a bunch of links and questions, so with the usual caveats (I don’t know anything about its design or construction process, not a licensed engineer, investigations will need to be done thoroughly, etc.), here are some thoughts…

bridge-model

Florida International University

Only a few media outlets reported at first that the bridge was, in fact, under construction when it collapsed–the fatalities appear to have been people in cars underneath it, and workers on it.  What was there was only part of a cable-stayed bridge, and that’s the first clue.  The picture above is from an FIU press release when the design was first announced, and you can see that–when finished–the structure would be a fairly traditional cable-stayed one, with a compression mast and tension cables connected to raking diagonals below.  These have been popular since WWII, when cable-stayed designs were used to quickly replace bridges in central Europe that had been destroyed by bombing raids–Tampa’s Sunshine Skyway bridge and the Millau Viaduct in France are two archetypal cable-stayed designs.

These work by translating the load of the bridge deck into diagonal tension members.  This has a couple of consequences.  First, the ‘pull’ required in each of these cables isn’t just vertical–it’s also horizontal, meaning that the bridge deck has to be designed to withstand huge compression loads along its length.  Second, unlike suspension bridges, there’s no good way to build a cable-stayed span piecemeal.  Large sections of the deck have to be brought in at once so that each of the cables has something to pull back against.

construction fiu

Florida International University

And that is exactly what occurred over the weekend, when one whole span of the FIU bridge was brought to the site in one piece and raised into place.  Seeing this, a couple of things make sense.  First, the shape of the deck itself was probably designed for two conditions: one when the cables were all in place and working with the tower and the deck, and second, when the deck was in place before the tower and cables could be erected to hold it up.  That explains the cross section of the bridge, which includes a heavy concrete roof.  This, I suspect, was designed to work as one of two flanges in a rough I-beam shape.  The floor of the bridge deck would have worked as the other flange, and the raking diagonals that would eventually have continued the lines of the cables were supplemented by more vertical members to create a truss-like web.  In other words, the deck appears to have been designed as a giant beam that could self-support until the tower and cables were in place.

This is also a common feature of cable-stayed design–decks that can support themselves before cables are connected.  Why put in the cables at all?  The decks can be designed to just barely hold themselves up, but often with enormous deflections that would make the bridge unserviceable, and margins of safety that are less than would be required for a fully occupied bridge.

millau

Millau is a great example of this–in this famous shot you can see the deck sagging before the cables were tightened.  They’re self-supporting, but you wouldn’t want to drive on such a bridge…yet.

There are a couple of clues in the images above that suggest possible reasons for the FIU collapse.  First, you can see that the collapse seems to have happened at joints in the web truss–in other words, the two end triangles seem to be intact, albeit rotated, while the other panels of the truss are smashed.  This is evidence for a failure in bending.  In some recent collapses, we’ve seen evidence that the cross section twisted or bent in ways that reduced the bridge’s section modulus (the 2004 I-70 collapse in Colorado is the paradigm of this).  But here it looks like the top flange fell directly on center and didn’t ‘twist’ out of the way.  Failure in bending, as SCI-TECH alums will recall, results when one of the flanges actually fails–either in tension along the bottom edge or in compression along the top edge.

If you look at the image of the deck being placed, you can see that the end of the bottom ‘flange’ has a line of small gray cylinders sticking out of it.  These are ducts for post-tensioning cables, ‘super-reinforcing’ that, once tightened, would take the huge tensile force in a bridge deck acting, temporarily, as a beam across its span.  These may have been tightened before the deck was put in place, or the bridge may have been waiting for the tower and backspan to be installed, so that cables could be run through the entire length of the bridge and tightened at once, holding all of the pieces together.  If that’s the case, then the deck would have been particularly vulnerable to failure along its bottom, tensile flange.  Another possibility is that the top flange could have failed in compression.  From the images of the collapse, there appears to have been buckling there, but it’s hard to tell whether this occurred before or after the deck impacted the ground.

Collapses like this are always shocking, but invariably contain some lessons within them.  Very often they highlight not only structural principles, but also problems of actually constructing such large spans, and this may well be an important example of how the two phases of bridge structures–under construction and in service–present very different static issues.

Update, 17 Mar 2018:  Two updates in this morning’s news:  1) There are reports that an engineer saw cracks in the span soon after it was put into place.  Depending on where these cracks were (haven’t been able to find more details), this would be consistent with greater-than-anticipated deflection in the lower (tensile) deck.  2) More interesting are reports that the deck was being ‘stress-tested’ when the collapse occurred, and that ‘cables were being tightened.’  Here, I think there’s some confusion–much of what I’ve read assumes that the ‘cables’ mentioned were the stays attached to the tower, but it’s clear that the tower hadn’t been erected yet.  More likely they were the post-tensioning cables mentioned above.  At least one report mentions a loud ‘pop’ a few minutes prior to the collapse, which would be consistent with a cable (or its anchorage) breaking while being tensioned.

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§ 17 Responses to fiu bridge collapse–what happened? [Updated]

  • Stephen Buonopane says:

    As a structural engineer, I want to commend you on your fabulous description. I agree that the initial press coverage was confusing. It took a bit of digging to verify that the two span bridge was under construction even though I thought I was seeing the continuity steel visible at one end of the wreckage. And even more digging to find out it was designed as a cable stayed bridge.

    It will be interesting to see which party (designer, contractor, etc.) proposed the idea of accelerated construction which led to the single span having to support itself as a beam until the rest of the structure could be but.

    Ciao,
    Steve

    Liked by 1 person

  • Laurance Page says:

    It was not a cable stayed bridge. The stays were steel pipe intended to control harmonics, not to structurally support the spans.

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  • Phil Garris says:

    I am sure cracking had to be noticed in the columns and in the span before the collapse. The column by the pond looked very small to hold such a load without support from tensile forces which I am sure was intended with the completion of span over the water. In the world of construction money is provided on percent of completion so if you install the main span you may get more money and gain some time on the critical path, I hope greed was not the case. FIGG Bridge previously had another collapse on the Jordan Bridge, dropping a 90-ton segment.

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  • Kate Thuesen says:

    Tom, thanks for this! I’ve been looking all over for an explanation of what happened, but anything official is understandably delayed as the investigation begins. I appreciate your thoughtful approach, and I’m glad you have shared some potential reasons for the failure. My curiosity has been piqued even more, and I learned a lot from your post. Your discovery of the cable-stayed bridge and tower was helpful. Will you do a follow up when a more ‘official’ cause is announced?

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  • Richard Schneider says:

    Thanks, best explanation anyone’s given to date. The officials and the media seem to be struggling for this kind of background. There’s a good moment-of dashcam video on Miami Herald’s page (http://www.miamiherald.com/news/local/community/miami-dade/article205704039.html). It shows a good-sized crane apparently positioned over the first suspension anchor point. Could it have been lifting at that point? I don’t know why, but I can’t figure out another reason for it being positioned there. Given that, what happens if the attachment fails under load? Wouldn’t the whole span twang like a plucked guitar string? Presumably if the structure needed a damper system, it couldn’t tolerate a bounce or ripple. So without the suspension system in place to dampen this twang … what happens?

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  • Nobody important says:

    I’m not a structural engineer, but from the info and photos I can see online my best guess is that the span may have been damaged whilst positioning across the road. Was it being supported in the right places? With adequate load spreaders?
    On the day of the failure there appeared to be workmen on top of the bridge and I believe they were ‘stressing’ the PT cables. Not sure if this was standard procedure or because they had a ‘problem’. But they may not have seen the tension coming up in the cables (due to the overall structural failure in the bridge) but just kept stressing the cable in a vain hope it would solve everything. Of course the tension curve would have straightened out and let them put a huge about of tension on the cable until ‘bang’ the cable goes and the shockwave takes out that part of the bridge section then the whole bridge goes like a domino.
    Clue. Look at the photos. You will see the top flange section where they were using a hydraulic puller. The cable is sticking out a long way from the flange. e.g. the cable failed and went flying out of the flange.

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  • John Herbster says:

    If you haven’t see it yet, a Wikipedia article has been started: https://en.wikipedia.org/wiki/Florida_International_University_pedestrian_bridge_collapse
    Note it’s reference to a driver and a construction worker hearing loud “crack” at 8:56 a.m. before the collapse. Thanks for your discussions. -JohnH

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  • John Herbster says:

    “Late last year, Nakin Suksawang, a professor of civil and environmental engineering and a researcher with the Lehman Center for Transportation Research at FIU’s College of Engineering & Computing, along with two graduate and two undergraduate students, installed 96 stainless steel embedded sensors in critical locations over six segments of the bridge prior to the concrete being poured.” https://news.fiu.edu/2013/10/miamis-smart-bridge/67283

    “BDI was contracted to conduct monitoring while the bridge was moved into place.” http://bditest.com/2018/03/follow-up-pedestrian-bridge-collapse-at-fiu/

    I would like to know if the output from those sensors was being monitored and/or recorded *after* the bridge had been moved into place across the roadway?

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  • Joe Maffei says:

    Phil, thank you for providing the link to the technical proposal. It is a compelling proposal with a lot of detail on the design. … and fodder for us Structural Engineers who want to start early speculating.

    The proposal PDF shows that it is designed as a spanning truss with somewhat “fake” cable-stay pipes for stiffness only. If it is an engineering mistake, rather than something done wrong in construction, the suggestion of a flexural failure in either the bottom or top chord is possible. However, failure of a web member could also be a cause; the web members look small compared to the deck and canopy that are the chords. The web members are shown on Page 115 of the PDF. Members 3 and 10 have to each take nearly 1/2 of the span weight in tension, having 4 prestressed rods each.

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    • Phil says:

      Do note that the technical proposal differs slightly from the bridge as finally delivered – I believe the crew were tensioning rods in the final (11th?) bridge segment which are absent in the proposal.

      The speculation on r/engineering is that the design was changed to include tensioners in all the bridge segments in order to accommodate the change in stress on the bridge during installation – the original proposal placed the mobile supports under the ends of the bridge, but due to local obstacles in the field, that had to be changed to support the bridge towards the middle on one side (under the bottom of the first “V” IIRC), altering the forces on the bridge.

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  • JCalvertN says:

    This was a mad design for a bridge. There were so many design faults that they must have racing each other to collapse first.

    Firstly, from a wider viewpoint, the chosen form was wrong. A real cable stayed bridge (instead of this fake one) it would have been better. Cable stayed is an inherently robust and resilient form for a bridge. A truss on the other hand is not.
    The problem with trusses is they will collapse after a single point of failure – i.e. they have zero redundancy. This was noted after the failure of the Minneapolis I-35 bridge by the NTSB who commented that such a bridge form would not be adopted today due to its susceptibility to single points of failure. Were the designers of the FIU bridge unaware of the findings of the Minneapolis disaster?

    But even the I-35 bridge had TWO planes of bracing (on either side). The FIU bridge only had ONE (along the centreline) This was sheer madness! This means that at least half the weight of the bridge was being carried on a single diagonal bracing member – and if it failed the whole bridge failed.

    Secondly, (also from a wider viewpoint) the choice of concrete as the material for a truss bridge, was unfortunate. Concrete is almost never used for truss bridges.

    Thirdly (now zooming in to the level of individual members) we see from NTSB videos and photos that the critical diagonal bracing member had very little rebar in it.
    But it did contain two pre-stressing tendons. This is a member that is already subjected to enormous compressive forces. Why did the designer think it needed to have even more compressive force added by pre-stressing?
    There seems to be a misconception which goes like this, “Concrete is strong in compression, but weak in tension, and this is why it is reinforced with steel bars.” It would be more correct to say, “Concrete is weak in compression, and negligible in tension.” Steel is 10x stronger than concrete. Steel is also ductile whereas concrete is brittle. (Ductile = good; brittle = bad) This is why compression members like columns and bridge piers are densely packed with thick bars and wrapped with lots of links and stirrups. By my crude estimate, this one should have had a row of 32mm (1.25 inch) bars at 100mm (4 inch) centres .

    Fourthly (now focussing on joint design) All too often bridge engineers relish the tasks of conceptual design and optimising the various members, but the tend to shy away from the joint design. It’s much more difficult and rather unglamorous. In this case of the FIU bridge, there is a very concentrated force coming down the diagonal strut at an angle that is flatter than 45 degrees. Where does it go next? It must be converted into vertical and horizontal components – and, due the flat angle, the horizontal component is huge. The vertical component is transferred to the supporting pier – and that is quite easily achieved by placing the main bearing directly under the joint. But the huge horizontal compressive component must be ‘turned around’ and converted into tension in the wide lower deck. This would have required a lot of rebar and a great deal of care and attention to detail.

    When we study the wreckage of the collapsed bridge we see that the last diagonal brace and associated vertical member have completely separated from the deck. They are left sitting on the top of the supporting pier while the rest of the deck has fallen down to the ground. This strongly suggests to me that they must burst away from the rest of the bridge – because there was insufficient tensile strength in the lower deck to counteract the immense horizontal compression component from the diagonal bracing.

    Liked by 1 person

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