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Florida International University ("FIU") needed a foot bridge to cross a canal and busy street. An FIU committee selected a design without redundant structural support because they were wanting a dramatic landmark (the bridge looked like it was a cable stayed design, but it wasn't -- the faux cables were almost entirely aesthetic). The original specs had called for structural redundancy so that the failure of one structural member would not cause a collapse -- the committee ignored this requirement in favor of visual appeal.
The engineering firm selected for the bridge (FIGG) made an error in calculation for a critical member at the end of the span. The engineering firm providing peer review of the design (Louis Berger) has refused to turn over to OSHA, certain documentation regarding what it was supposed to evaluate and what it actually did. When the bridge section which had been built on the side of the road was moved into place, experienced workers became extremely worried about cracks that started appearing and made their worries known to those up the chain. A FIGG engineer examined the cracks but determined they "did not present a safety concern even though its engineers did not know what caused them — and despite clear evidence that they were growing daily." Apparently, the guidelines are that cracks deeper than a half inch are to be taken seriously and these were much deeper and growing daily -- one photo shows a crack 4" deep.
The final bridge would have two sections -- the long section over the roadway and a short section over the canal. The canal section was to be built in place and tied into the long section. Had the canal section been built first, the risk of collapse for the section over the roadway would have been reduced because it would have shored up the longer road section.
The bridge collapsed killing six and permanently disabling another.
Article regarding the OSHA report: https://www.miamiherald.com/news/local/community/miami-dade/article231428938.html
Article regarding independent engineering review with some good explanations which I, as a non-engineer, found informative: https://www.miamiherald.com/news/local/community/miami-dade/article212571434.html
Time Lapse Video of Bridge Collapse (released by FIU): https://www.youtube.com/watch?v=vrBOF2jugFM
Original Soylent item: https://soylentnews.org/article.pl?sid=18/03/19/1746219
Link to the OSHA report itself: https://www.osha.gov/doc/engineering/pdf/2019_r_03.pdf
(Score: 5, Informative) by Anonymous Coward on Thursday June 13 2019, @07:06AM
The idea was a hybrid of a truss and cable stay. The truss was done in concrete and was expected to handle most of the load, including being self-supporting during installation. The cable stay portion was needed to handle resonance caused by the motion of humans and wind. At the time of the failure the cable stays were not in place, but it wasn't particularly windy and there were only 3 humans on the bridge. The materials have been tested; they were as specified.
Modern engineering doesn't leave much of a safety margin and often uses inappropriate materials. Since the time of ancient Rome, we've known that concrete sucks for tension and shear forces. Romans used nice solid arches, and later people used steel trusses. Nobody sane uses concrete trusses. Well, somebody thought a thin concrete truss would be pretty and cheap, so they tried it. That was a mistake that should be obvious if you aren't blinded by a determination to make it for cost and aesthetic reasons.
Maybe do concrete suspension cables next, eh?
A big problem comes at the joints that make up the truss. They are fixed joints, like the sutures of the human skull. They really are joints, with the concrete poured separately. Here you're asking thin pieces of concrete to resist bending and shearing, and you have a joint right at that stress point. That is nuts. Of course it will crack.
For the transverse direction, nothing was done to deal with that. In the other directions, they hoped to solve the problems with post-tensioned metal bars. On the day of the failure, these bars were tightened to excess in a futile attempt to stop the rapidly growing cracks. In some places the bars snapped. In some places the concrete got crushed, with blow-out events that involve chunks blasting out sideways due to the pressure.
CAD was used of course, including structural analysis. The designer modeled it with two different kinds of software. An independent reviewer again modeled it, with a third distinct piece of software, and raised some concerns that were mostly dismissed.