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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."