NTSB analysis of last year's pedestrian bridge collapse in Florida

"Dusty Ventures" (dustyventures)
12/11/2019 at 15:15 • Filed to: None

DISCUSSION (11)

ttyymmnn > Dusty Ventures
12/11/2019 at 15:43

Damn. What a screw up. All those cracks and they didn’t close the road. I’d like to hear from the engineer who designed that bridge. Sounds like it was doomed from the start. “A single failure of one part would cause the bridge to collapse. Sounds like it was designed by Boeing.....

Ash78, voting early and often > Dusty Ventures
12/11/2019 at 15:44

They didn’t even close the road while tensioning the truss after cracks were seen repeatedly over days. Just wow.

“ CAN’T TRUSS IT” —Carlton Douglas Ridenhour

ShrimpHappens, née WJalopy > Dusty Ventures
12/11/2019 at 15:46

They’ve done a great job of explaining what , but it leaves me wanting to know why .

I’m a civil engineer, but I work in transportation roadway design, so this sort of thing is way out of my wheelhouse. But I do know that in basic truss design, members are considered to be pinned at the joints (and in old bridges, they actually are). In this truss, the connections are fully fixed, so it doesn’t act the same. Was this fully accounted for in the design?

Excessive shear at the failure joint was mentioned in the video. It looks like at the joint that failed, the shape of the concrete was causing the axial load of the member to translate to weird things the connection couldn’t handle.

Edit: I’ll add that I remember from my structural steel and concrete design classes that design of members (beams, columns, joists, walls, slabs) is fairly straightforward, while the connections are really tough. Seems to hold true for this bridge.

VincentMalamute-Kim > Dusty Ventures
12/11/2019 at 15:48

So AvE’s theory that it was post tensioning cable failure is wrong? It seems like they installed it without tensioning (other than the ones at the ends) and then were in the process of tensioning them.

TorqueToYield > Dusty Ventures
12/11/2019 at 15:53

That was really interesting.

I wonder why they went with concrete trusses instead of steel, actually I don’t wonder , I’m sure it was cost and time . Seems like concrete isn’t the best material for a truss., but I’m a simple Mech E not Civil.

But hey, lets have decorative cable stays

Highlander-Datsuns are Forever > Dusty Ventures
12/11/2019 at 16:28

You know when I see shit like this it really pisses me off. The moment they saw cracking at the connection point that structure should never have left the ground. I understand there was some weird things going on as to who designed it and how the design was completed as well. There was a complete failure of the contractor and construction manager to understand the severity of the situation . What did they think they could just tighten up some bolts and it would take up the 3" crack?

Levitas > VincentMalamute-Kim
12/11/2019 at 18:20

I think AvE’s theory is mostly correct. The NTSB video says the rods were tensioned before moving the span, then released after it was put on the pylons. But they noticed the cracking to continue to propagate, so they decided to increase tension on the PT rod in #11 to reduce the cracking. AvE posited the increase in tension caused a failure in the rod causing the failure of the span.

Granted, the NTSB video doesn’t seem to have any discussion on potential changes to the movable supports that AvE discussed.

VincentMalamute-Kim > Levitas
12/11/2019 at 18:39

Thanks. Been a while since watching AvE’s video and didn’t want to rewatch.

I assume the NTSB written report would have a more detailed explanation of the physics? Their video has minimal explanation - at least for laymen. Unlike AvE.

wafflesnfalafel > Dusty Ventures
12/11/2019 at 21:52

wow - all the warning in the world. The disaster is entirely a human failure...

VincentMalamute-Kim > Levitas
12/12/2019 at 11:19

NTSB actually appears to be blaming the three stage concrete pour. I think the report said instructions for a rough surface at the truss member/walkway junctions weren’t given . The area was smoothed before the truss pour.

The forces carried by the trusses couldn’ t be transmitted and spread out over the deck. The forces instead sheared the trusses which then caused the post tension rods to fail.

RacinBob > Dusty Ventures
12/12/2019 at 13:44

Kind of a salty guy but he knows his stuff

Another video explaining the construction