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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."

9 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 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

    3. 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 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. 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.

  4. 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.

  5. 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.