Miami pedestrian bridge collapses, 'several dead,' multiple vehicles trapped beneath

Like everyone here, I can’t speak to the actual cause of the failure, but the design firm (Figg) is a long established engineering group that does nothing but bridge design. I would be very surprised to hear that the cause was a design flaw. My money would be on the construction process, and whether each aspect met design specs.

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It’s a bit strange to me that a good number of contractors and construction companies have problems with safety. I routinely see flagrant OSHA violations around Austin when i’m driving by construction sites. The company i work for prioritizes safety above all else, and everyone’s yearly bonus hinges upon our safety record. I have a hard time understanding how other companies get away with cutting corners.

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Well you’re in the glorious independent state of Texas, where individuals need not fear overbearing gubbermint bureaucracy controlling you with wimpy feminine things things like safety!


And I wasn’t searching for that one, I was gonna post a link to an article about the other less recent chemical plant explosion that more people know about, but, well, it’s Texas!

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Until recently… same here. My previous employer was incredibly focused on safety, empowering all to speak up when some unsafe condition or practice was observed, and even empowering us to stop unsafe operations however important. My new employer (and especially the government ‘entity’ that we do work for)? Well, they talk safety, but if a fix requires $$$, expect to wait and wait and wait until (and it’s happened) someone gets hurt… then something gets done.

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just heard this in some place and thought it should go here…

https://play.google.com/store/music/album?id=Bqybs5hlajwnkd6zz6ofihjexge&tid=song-Tcd53t25wupn63ix2uh2g24spne

item #2

Whether or not this may be what happened at all, some worthwhile glimpses to ponder for us laymen

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All that paper adds up…

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It might not be engineering or construction, but operational in how the stress tests were done. They’ll need more fingers.


Associated Press Posted: Mar 16, 2018 6:16 AM ET

I don’t know if this was cable tensioning:

They put some weights to hold down the popped up section and reopened it to traffic.

Looks like these were improperly sized bolts that failed under tension. The bridge I was referring to carries subway trains and has a bike route attached to it. Of course, none of those things were present when the bridge was standing without tensioned cables.

If you ever found yourself wondering why indemnity clauses exist in contracts, this, right here, be a prime example of the reason.

A good discussion about the status of the bridge at the time of collapse - it was not complete, and was still under construction. The author has identified “post-tensioning” hardware which is a method by which reinforcing steel is put under tension for its final state. The tensioning of this reinforcement may or may not have been underway.

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I have this…bridges, not tunnels. (I don’t love driving in tunnels, because lanes tend to be narrow and pavement slick, but it doesn’t turn my knuckles white.)

As for bridges, I’ll usually choose a driving route to avoid them if I can, though sometimes it is difficult (I drive in and out of San Francisco pretty often, for example). Once I had to drive across the Humber Bridge on a day that was so windy that they closed it a few minutes after I’d crossed. That was delightful.

Honestly, as phobias go I think this one is pretty rational.

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Given that a pedestrian bridge currently going up here in Spokane is projected to cost $12m, it doesn’t seem like a particularly whopping number.

(There is, of course, a councilman questioning why a pedestrian bridge costs almost as much if not more than a major automotive bridge, but it’s not like a lengthy bridge is any less complex because it’s not carrying cars, and isn’t there always That Guy?)

Yes, good read, thanks for the link.
From the link:

If you look at the image of the deck being placed, you can see that the end of the bottom ‘flange’ has a line of small gray cylinders sticking out of it. These are ducts for post-tensioning cables, ‘super-reinforcing’ that, once tightened, would take the huge tensile force in a bridge deck acting, temporarily, as a beam across its span. These may have been tightened before the deck was put in place, or the bridge may have been waiting for the tower and backspan to be installed, so that cables could be run through the entire length of the bridge and tightened at once, holding all of the pieces together. If that’s the case, then the deck would have been particularly vulnerable to failure along its bottom, tensile flange. Another possibility is that the top flange could have failed in compression. From the images of the collapse, there appears to have been buckling there, but it’s hard to tell whether this occurred before or after the deck impacted the ground.

This isn’t more than speculation from a gut feeling, but I’d say that the tensioning cables were tightened before the bridge segment was put in place. Possibly the idea was to have the lower flange act like a beam that would carry the whole segment until the pylon and cables were assembled. Thing is, this is in effect a structure in prestressed concrete - the tensile force in the tensioning cables has got to go somewhere. It also means that an element that would have to bear tensile forces in the finished bridge would have to bear tensile and compressive forces temporarily. Looking at the cross-section of the flange in the picture, the ducts for the tensioning cables take up quite an area (or rather volume) of the flange. Now the anchor points of the cables would be the critical points of the construction; but if the ratio of massive concrete (which has to bear the pressure) to cable duct is, let’s say competetive, any imperfection in the flange’s concrete is potentially a point of desastrous failure.
So maybe too much tension in the cables and either tension failure in a cable or compression failure of the lower flange.
Or maybe not enough tension in the cables and compression failure in the upper flange.
Or maybe something else entirely.

In any way, if you put a half-finished structure over a highway that is in use, you should be damn sure that this works.

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Well, that sure doesn’t make your life easier.

At least I don’t live across the Humber from the airport anymore. The Richmond and Golden Gate bridges are a piece of cake by comparison.

My impression of public works engineering has always been that up to the 1950s the engineers were a conservative lot who overengineered everything. In the post-Reagan era public expenditures (outside of the military) have been so heavily scrutinized that trying to meet the specs and not an iota more seems to be the rule. I think that this is an issue not only for immediate functionality (as in the FIU bridge) but also for long-term durability and maintenance.

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Dash cam of the span collapsing. It breaks on the left side right where there is a crane. Crane appears to be supporting something just over the junction of the diagonal members.

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Horrifying. Instant failure. I’ve only ever seen prestressed structures fail that fast (not live, videos). And somewhat frustrating as you still can’t really see what exactly happened. It seems like the lower flange failed first, but I’m far from sure.
What was the crane doing; lifting up/lowering down? Hooking on/unhooking?
Why wasn’t whatever the crane was supposed to do not done while the highway was closed to traffic?

I think if they were adjusting tension on the post-tensioning tendons as I’ve read reported, it would seem that they were working on the steel bars in the diagonal members that met at the upper deck/flange where the crane was operating. If they were adjusting tendons in the upper or lower deck/flange I’d expect them to be at either end.

If something failed relative to one of the diagonals, the span would loose its truss configuration and the upper and lower deck would likely snap or fold like cardboard. Thats what the collapse looks like, breaking right at the place where the second diagonal met the lower deck/flange.

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