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Was The Florida Pedestrian Bridge Collapse Triggered By Post-Tensioning? (enr.com)

Posted by EditorDavid on from the striking-a-chord dept.

A new lawsuit claims post-tensioning triggered the collapse of the pedestrian bridge at Florida International University, killing five motorists and one worker. Engineering News Record reports According to the lawsuit, the March 15, 2018 collapse occurred while a crew was post-tensioning bars in a diagonal member at the north end of the concrete truss that was the bridge's main element. The post-tensioning compressed the diagonal so that it overstressed a joint in the top chord, the lawsuit claims, triggering hinge failure at a connection in the lower chord and resulting in the catastrophic failure of the rest of the 174-ft-long structure. Post-tensioning that modifies the stresses in a structure is inherently risky and should be performed "in the absence of traffic," the lawsuit claims. [The lawsuit] draws heavily on video of the collapse, a voice message about cracks in the structure that were deemed superficial at that time by the engineer of record and design drawings in the design-build joint venture's proposal.
Slashdot reader McGruber writes: Interestingly, just two days after the collapse, an Anonymous Coward posted that post-tensioning likely led to the collapse of the bridge... A March 21, 2018 NTSB News Release said "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."
The Miami Herald reports that "how and where precisely the bridge broke apart likely won't be known for months, until the National Transportation Safety Board issues an official finding." While summarizing the leading theories, they're also calling it "the sort of baffling accident that makes structural engineers break out in sweats."

24 of 138 comments (clear)

  1. Plastic stress strain curve by Cassini2 · · Score: 4, Interesting

    In the elastic region, increasing strain increases stress / tension. When the member enters the plastic region, steel under tension starts to neck. In this region, increasing strain can result in decreased stress. Eventually, the member fails and you have lots of strain and no stress / tension.

    When tensioning, the question every structural engineer must ask is: Am I in the elastic region? For sure?

    Structural engineers tend to use ridiculously small assumptions for material strength to guarantee being in the elastic region. However, one good crack or subsurface fracture, and fracture can occur. High performance work tends to use fea to predict areas of stress concentration, and then eddy current, magnetic and x-ray inspections to prevent these failures. This is not common in structural applications.

    1. Re:Plastic stress strain curve by burtosis · · Score: 5, Interesting

      As a mechanical engineer, I'd just like to point out that you should place emphasis on it is crack or subsurface fracture in the tensioning member and not the concrete. The news media was making waves about some visible cracking in the concrete which would not necessarily be a concern here as the whole idea is concrete is quite strong in compression. A crack that was stable under compression, not allowing movement, would simply be compressed together and still retain structural integrity.

    2. Re:Plastic stress strain curve by aaarrrgggh · · Score: 2

      Depends on the cause of the crack...

    3. Re:Plastic stress strain curve by Woldscum · · Score: 5, Informative

      AVE on YouTube called it on March 16. Very informative visual demo.

      " I ran a test to see why the post tension rod was sticking out of the rubble. There was a problem with cracking on the pylon side of the bridge."
      https://youtu.be/KtiTm2dKLgU

    4. Re:Plastic stress strain curve by nukenerd · · Score: 2

      The news media was making waves about some visible cracking in the concrete which would not necessarily be a concern here as the whole idea is concrete is quite strong in compression. A crack that was stable under compression, not allowing movement, would simply be compressed together and still retain structural integrity.

      Except when you consider buckling. As far as I can make out from the accounts, videos and artist's impressions, what they were pretensioning was a concrete strut under compression. You need to keep the resultant compressive force in a concrete or masonery strut within the centre third in order to avoid tension at the outside edge and hence potential bucking, which would of course be preceeded by cracks there. The pre-tensioning may well have been meant to keep the resultant compressive force within the middle third, but if they over or under-tensioned one of the rods it may only have made matters worse by taking the resultant force outside of it.

      OTOH, that strut is visible in some of the post-failure pictures and looks fairly intact and lying on top of the deck, athough it is hard to tell. So it might simply have punched its way out through the top deck due to lack of sufficient steel rebar in the area connecting it stress-wise it to the next downward sloping strut at that point (which would have been in tension). That rebar would have been local and cast into the concrete, not tensioned, but if it was insufficient, or if the concrete around it was not cured enough (or just crap), then fiddling with the post-tension rods near to it might have caused that area to disintegrate, allowing our strut to punch through.

      These are all things the investigation will need to consider.

    5. Re:Plastic stress strain curve by burtosis · · Score: 4, Informative

      Not directly. It depends on the shapeof the crack. If it was orthogonal to the bridge, structural strength would be just fine. It's analogous to setting one brick on another with a weight on it and still supporting the load. Now if it was a clean radius, say by two cracks in a V, so the V chunk pops out below a center tension bar, it is free to bend like a hinge. Now if it was cracked because it was dropped, and the center member damaged, this could be a failure of the tensioning member. But if they looked at it and it looked ok, I doubt the crack was a problem itself.

  2. Post-tensioning by Megahard · · Score: 4, Informative

    For those of us who are not structural engineers, here's an good easy-to-read article (pdf) that explains it.

    --
    I eat only the real part of complex carbohydrates.
    1. Re: Post-tensioning by sl149q · · Score: 2

      https://www.youtube.com/watch?...

    2. Re:Post-tensioning by Anonymous Coward · · Score: 4, Informative

      The short explanation is that concrete can handle very high compression, but fails quickly under tension. For post-tensioning, there are steel rods going through the concrete between anchors at the ends of the concrete part. When these rods are tensioned, they compress the concrete, so that any loads on the concrete at most lower the compressive force on the concrete, but don't cause it to go into tension. When a rod is overtensioned, it breaks and removes that compressive bias on the concrete. This weakens the concrete immensely and it breaks.

  3. AvE by DrewFish · · Score: 3, Informative

    AvE (crusty canadian enginerd on youtube) had a couple of interesting videos on this:
    https://www.youtube.com/watch?...
    https://www.youtube.com/watch?...

    1. Re:AvE by DCFusor · · Score: 3, Insightful

      It was obvious to me he got it right, actually, and I AM an engineer. And it was most likely caused by some construction worker tightening something that was supposed to be tight already and no be messed with in the field in the absence of an engineer. When it yielded instead of getting tighter...boom. Now, why someone thought it needed that is another question. Concrete gets harder and drier for *years* after initial set. Wanna bet someone moved it too soon and too wet?

      --
      Why guess when you can know? Measure!
  4. Re:$15,000? by Sarten-X · · Score: 2

    Or in other words, grab $15K before the NTSB report comes out, as a hedge bet against the report saying it was unforeseeable and nobody can be sued for it.

    --
    You do not have a moral or legal right to do absolutely anything you want.
  5. How is this a "baffling" mystery to solve? by geekmux · · Score: 4, Insightful

    "...they're also calling it "the sort of baffling accident that makes structural engineers break out in sweats."

    Uh, how exactly is this some kind of "baffling" mystery here? Instead of summarizing theories, let's review the facts:

    "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 were working on the second rod when the span failed and collapsed."

    Seems pretty damn clear to me as to the cause of the collapse. Let's review the fuck-ups that lead to disaster and lives lost:

    "...a[n ignored] voice message about cracks in the structure that were deemed superficial...Post-tensioning that modifies the stresses in a structure is inherently risky and should be performed "in the absence of traffic,"...The roadway was not closed while this work was being performed."

    Seems pretty damn clear who fucked up and who should be held accountable here. Of course, this also happened in the United States, which means insurance companies are going to drag out pointless "investigations" for the next 2-3 years in order to keep millions in their coffers for as long as immorally possible.

    1. Re:How is this a "baffling" mystery to solve? by burtosis · · Score: 2

      The cracks were likely inconsequential because the concrete was to be used in compression and not tension.

    2. Re:How is this a "baffling" mystery to solve? by thegarbz · · Score: 2

      Seems pretty damn clear who fucked up and who should be held accountable here.

      Then please share for those of us less enlightened. I mean in your quote you already mentioned at least two separate people, but since you've done the investigation I'm sure you can also tell us about:
      - How you've reviewed the engineering drawings and design to ensure that it was designed correctly.
      - How you've reviewed construction to ensure that construction was as per design.
      - How you've reviewed the composition of materials.
      - How you've reviewed that pre-tensioning was completed correctly.
      - How you've reviewed that there was no damage during movement.
      You're clearly a structural engineer with detailed knowledge of the shape and effect of the crack on the bridge too I see, so likely you've reviewed the fact that the previous engineers were wrong when they looked at the cracks, otherwise you wouldn't have quoted that part about an "ignored" voice mail that wasn't so much ignored as it was discussed in committee for an hour. But you knew that already right?

      While we're at it, why not tell us about the process. Apparently you know the exact failure so what was it? Was the tensioning done incorrectly? Was it equipment which failed? Was the hydraulic system calibrated properly? I mean it's amazing that you know all this, last I heard people weren't even sure if they were tensioning or detensioning.

      But please enlighten us on all the above since you know the cause of it. If there's anything there you don't know then how are you certain that you actually found the cause and who's at fault? By the way you should become a super incident investigator since you've solved in mere days what normally takes months to identify. Kudos to you!

    3. Re:How is this a "baffling" mystery to solve? by thegarbz · · Score: 2

      NOT having the sense to clear traffic while performing an "inherently risky" operation is the most obvious fuck-up

      And is also not a root cause which is why investigations actually take months despite your claim to the contrary. The point of investigations is not to come up with what happened, but rather the reason of why it happened. So... since you know it already please share with us the detailed assessment that lead to them not closing the bridge including the risk assessment that was performed. After all I will humour you and assume you understand that post tensioning comes in a great variety of different forms, some are inherently risky such as post tensioning a structure to make it stable, and others less so, such as adjusting post tensioners for a pre-tensioned bridge. You did know this bridge was pre-tensioned right?

      And my "ignored" comment was referring to a voice mail that was left by an engineer two days prior to the collapse to report structural cracking, but was not picked up until the day after the collapse

      Except it wasn't ignored. That was just some initial quality journalism. The only part of the ignored voice mail is that the actual crack was identified and a proper engineering assessment done to it making the subsequent discovery of a voice mail irrelevant, and as many have pointed out not only did the engineering assessment (before collapse) not consider the cracks a concern, but depending on the shape and position of a crack in a tensioned concrete bridge they are often completely irrelevant and quite common.

      Had more than one organization reviewed the damage surveyed prior to the collapse, tragedy may have been avoided.

      Tell me again what you think the crack had to do with a post tensioner failing? Better still given that structural member 11 was a critical supporting member with no redundancy tell me why you think a perfect bridge with absolutely not a single crack in it would survive a critical tensioned member suddenly ceasing to exist.

      Don't get caught up in the crack hysteria. No doubt in a few months after the investigation is finished you'll see it was completely irrelevant, risk assessed, and quite normal for a tensioned concrete structure.

      Regarding ultimately finding fault, that will likely never be found due to the insane amount of variables you have already cited.

      Well it won't if you don't risk assess properly. But people actually do, and it's not insurance companies as you so claim. The root causes i.e "fault" will be identified. There may be more than one, there certainly will be several mitigation failures as well as an incident such as this often relies on a perfect storm of several failures in order to cause the problems it does, but ultimately there barely any structural failures in the west where root causes were unable to be identified. I'm sure lawyers are salivating.... well evidently they already are.

      the cause of the collapse is a bit more obvious when they were in the middle of tweaking structural integrity on an already damaged structure when it failed

      As said, immediate causes are irrelevant. Blaming tensioning on the collapse is like blaming someone who bought a car for the fact that they were involved in an accident. If your cause ends at a perfectly normal and completely routine activity, then you haven't found the cause, just a useless intermediary and you need to keep looking.

  6. Highlights the problem with our legal system by Solandri · · Score: 4, Interesting

    This actually highlights the problem with our legal system more than it does what caused the collapse. Lawyers have filed a lawsuit (i.e. are certain who is blame) while the investigation has barely started and is still collecting evidence, and is probably a year away from reaching a conclusion.

    If you want to argue that the lawyers aren't certain, they just want be first to get their speculative lawsuit in, then that's yet another problem with our legal system. That the penalty for filing a frivolous lawsuit is so lacking that lawyers can file speculative lawsuits with impunity without a shred of evidence to back up their claim, gambling that such evidence might turn up in the future. Thereby forcing countless innocent defendants to waste money preparing a defense against lawsuits which never should have been filed in the first place.

  7. Re:So basically operator error? by aaarrrgggh · · Score: 3, Interesting

    Post tensioning is a bit of an art, especially on a non-redundant structure. I would think for a bridge with traffic running below they would be slowly tensioning the system in increments following the load path.

    Maybe they set the "final" tension all at once?

  8. How does the State award contracts? by Anonymous Coward · · Score: 2, Interesting

    https://www.vdare.com/posts/th...

    There is a lot or pressure to give contracts to minority owned firms. If you ever enter into one of these government bidding competitions, the number of questions devoted to the diversity of your business is greater than questions related to your ability to fulfill the contract.

  9. Re:So basically operator error? by Anonymous Coward · · Score: 2, Interesting

    Possibly. However, from my understanding, the 2 PT bars in truss #2 at the other end of the bridge had been DEtensioned to some degree along with the upper PT bar in truss #11. Leaving the lower #11 PT bar remaining to be adjusted. May seem counter-intutive that DEtensioning both PT bars in #11 could have led to collapse. However, if there was already cracking in truss #11 and/or in lower chord (deck) and/or at base of the pier (vertical #12), the PT bars could have been what was helping to hold it together. Another theory to consider.

    While some have questioned the choice of a large concrete truss-like structure for a pedestrian bridge, it's notable that truss #2 (which was enlarged from the preliminary drawings) on the other side of the bridge along with nearby support members remained relatively intact after collapse. Why was #2 enlarged, but not #11 is a question some are asking.

    Also, it's important to note the bridge was shifted over 11 feet from its original position, as per Florida Department of Transportation to allow for a future lane, placing the pier on the north side closer to the canal necessitating last minute changes in late 2016. Could the soil there have compromised the pier on that side. Another aspect to consider.

    In short, it appears the tensioning adjustment trigger the collapse, but likely not the underlying reason for failure.

  10. Re:$15,000? by Richard_at_work · · Score: 3, Informative

    NTSB reports cannot be submitted as evidence into any court case, civil or criminal, in the US, by either side. They cannot be used to support a prosecution or action, and they cannot be used to defend against a prosecution or action.

  11. Re:So basically operator error? by Highdude702 · · Score: 4, Informative

    My thoughts were, Why the FUCK wasn't the road closed during all post-tensioning. That would have been the smart thing to do.

  12. Re:$15,000? by Sarten-X · · Score: 2

    Eh, we're both half right.

    I'll admit I confused the issue in my earlier post, but a quick bit of research straightened me out. Apparently, NTSB factual reports are admissible, but final reports are not. The factual reports that say who followed proper procedures would still be pretty damning to a lawsuit.

    --
    You do not have a moral or legal right to do absolutely anything you want.
  13. Re:$15,000? by rtb61 · · Score: 2

    Obviously you have not idea how good structural engineering design works. Failure should never result in collapse but in excess deflection and cracks, at design load, at that time you should stop using the structure and then analyse the failure for possible repair, but generally due to the alteration of the materials especially steel, when it deforms https://en.wikipedia.org/wiki/... it gets stronger due to altered crystalline structure but becomes more brittle and is likely to fail completely. So they saw cracks in the bridge and did nothing, that is criminal negligence and those responsible should be prosecuted for it. The bridge worked somewhat close to design, it failed partially, the failure was visible in the form of cracks, at that time action should have been taken, not longer use the bridge, or if bridge failure was with light load use, immediately tear down the bridge. The engineers were partially in fault, poor design leading to failure, those who own and control the bridge, cracks become visible and they took no action, were criminally negligent (those cracks are actually a design element, the first signs of failure and should be cause for immediate serious investigation).

    Cracks in those kinds of structural members spanning a gap, are cause for immediate action, the owners are screwed, the engineers are in a bun fight with the builders, materials suppliers, as they go through they structure to analyse every possible element in the point of failure but they were at most fifty percent but all the way down to ten percent or even nothing. Failure to take actions drops it right onto the owners and operators of the bridge, based upon them becoming aware of the cracks well before it's actually collapse, the second the cracks become visible action should have been taken. America 'D' grade infrastructure (that's second world grade, not first world grade be clear on that) due to insane greed but hey you got the F35 Flying Pig that Israel pretends to fly over Iran at the US government's request in vain attempt to shore up F35 Flying Pig sales, ohhh, so lame ;DDD.

    --
    Chaos - everything, everywhere, everywhen