Two weeks ago, FIU’s bridge abruptly collapsed. Here’s what we know so far

[Elderberry]

miamiherald.com by By Andres Viglucci And Jenny Staletovich 3/30/2018

Did it crack when it was lifted into place? Was it a snapping steel support that did it in? Or was it a fatally flawed design from the start? And what was the doomed construction crew member doing when the bridge buckled under him?

In the two weeks since the deadly collapse of a pedestrian bridge under construction at Florida International University, engineers the world over have obsessively dissected photos, scrutinized video clips and picked over preliminary plans for the singular structure in search of the mysterious and elusive breaking point.

Never mind 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: On engineering online discussion boards, around office water coolers and computer monitors, divining the cause of failure for a structure by one of the country’s best-known bridge designers has become the professional cause du jour.

The upshot so far: several compelling hypotheses and lots of sharp assessments, some loopy detours and a sharpening focus on a pair of support trusses that appeared to shear and shatter just as the unfinished concrete bridge suddenly crashed to the roadway bed below, but nothing close to a conclusive explanation.

More: http://www.miamiherald.com/news/local/community/miami-dade/west-miami-dade/article207358659.html

[thackney]

Isn't that the other way around? During transport it would need additional tensioning because it was being stretched and the additional tension needed to be higher than the stretch force. Once in place it was being compressed and the tension could be relaxed (we're talking about support #11). Support #10 is exactly the opposite. It looks to me they were adding tension to the wrong support, #11.

@DB

Your question from a previous FIU bridge thread is answered here.

...The preliminary plans for the bridge called for the tension rods to be adjusted twice: They would be tightened before the bridge was lifted into position from below by special transporters. Because concrete is weak when it’s pulled on, that tightening would provide needed support to the bridge span ends, so that the structure would retain its integrity under the immense stress of the lift. Once the span was settled into place, the diagonal beams would become compressed by the weight of the bridge, providing the needed structural stability. The rods would then be loosened.

In a video taken just after the bridge was installed, an MCM employee tells an interviewer that the next step would be to de-tension, or loosen, the support elements at either end....

[Frank Cannon]

Gravity Brings Down Bridge. News at 11.

[DB]

@DB

Your question from a previous FIU bridge thread is answered here.

Thank you. I don't see how loosening it could have caused the failure - if they were working on the correct tension rod... I'm sticking with my guess they further tightened the rod that was supposed to be loosened and crushed the concrete support member.

[thackney]

Thank you. I don't see how loosening it could have caused the failure - if they were working on the correct tension rod... I'm sticking with my guess they further tightened the rod that was supposed to be loosened and crushed the concrete support member.

As I understand it, the only way to loosen is to first tighten a little to lift the tension of the retaining nut, before it is then backed out.  If that was done too far, if the controls jammed, if, if, if...