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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."

7 of 138 comments (clear)

  1. 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.

  2. 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?...

  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 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.

  5. 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.

  6. 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.