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Submission + - Lawsuit Claims Post-Tensioning Triggered Florida Pedestrian Bridge Collapse (enr.com)

Submitted by McGruber
McGruber writes: Engineering News Record is reporting that a lawsuit claims post-tensioning triggered the failure that brought down the pedestrian bridge at Florida International University, killing five motorists and one worker.

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. (https://www.enr.com/articles/44210-lawsuit-claims-post-tensioning-triggered-bridge-failure)

Interestingly, just two days after the collapse, an Anonymous Coward posted (https://slashdot.org/comments.pl?sid=11867553&cid=56274677) that post-tensioning likely led to the collapse of the bridge. AC explained " It's theorized by some that truss member #11 at the junction of the pier was initial failure point. In preliminary drawings, #11 is shown with no post-tensioning bars, but the actual construction shows it with two. While those bars in #11 may have been necessary due to the move, since the ends of the bridge were cantilevered (which is different than shown in the preliminary drawings), they likely weren't needed after placement; not needed to be post-tensioned, since #11 would be in high compression." A March 21, 2018 NTSB News Release (https://www.ntsb.gov/news/press-releases/Pages/NR20180321.aspx) 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."

Submission + - Is Tricking a Robot Hacking? (ssrn.com)

Submitted by Anonymous Coward
An anonymous reader writes: A new paper by security researchers and legal experts asks whether fooling a driverless car into seeing a stop sign as a speed sign, for instance, is the same as hacking into it. Can you get into trouble under anti-hacking laws for tricking machine learning? Do you have inadequate security if your product is too easy to trick? Interesting read.

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