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Was The Florida Pedestrian Bridge Collapse Triggered By Post-Tensioning? (enr.com)
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."
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."
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.
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.
If you know how this process works, and watch the video of the bridge falling apart like a stack of cards, it is kind of obvious.
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?
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.
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.