This is the best NSFW explanation of the Florida bridge collapse


#1

Originally published at: https://boingboing.net/2018/04/06/this-is-the-best-nsfw-explanat.html


#2

I love AvE’s tool teardowns, as well as his colorful variations on the old Canadian saying “Keep your stick on the ice!”

For the bridge collapse, however, I’m waiting for the official report.


#3

But the way he talks is so annoying. Nerdy-cute. I’ll bet ten bucks he knows a few filks, and may even write them.


#4

Although I do have a slide rule for LARPing purposes.

It’s not going to get better than that today, signing off.


#5

Shout out to @ctg for linking to this vijaho in a boingboing comment 21 days ago.


#6

the diagonals look ok but am having a bad day
the uprights should be bigger I think


#7

As a long time AvE fan, I feel obliged to share a tip: do not actually put your dick in a vice. Although applications where this procedure might be useful are certainly conceivable, I have found that not to be the case for most DIY and maker purposes I have been dealing with.


#8

I already tried, it didn’t work - kept slipping out. I think it needed to be pre-tensioned.


#9

I want him to explain what’s up with that pencil he is holding.
He won’t let go of it.
It’s useless for penciling so he just holds it.
He doesn’t use it as a pointer.
It has bits missing from it.


#10

Here’s a part 2 of sorts with more detailed (educated armchair) analysis, eh.


#11

If anyone is interested in the detail of where the bridge designers went wrong then we’ve been discussing it
here http://www.sciforums.com/threads/florida-international-bridge-collapse.160673/
and here https://cr4.globalspec.com/thread/118962?frmtrk=cr4sd&Pg=2


#12

The gaps has something to do with using it as a battery charger. You could probably find the viseo about it somewhere in his playlist.


#13

About the pencil: he was using the graphite ‘lead’ as a variable resister in another video. Got pretty hot if I recall.


#14

This is pretty convincing. I didn’t realise there was supposed to be a central column. Most bridge failures happen before the things are finished. The box-girder bridge failures are a classical example. When the box-girder is complete, you have a strong square tube. When it isn’t finished you have a set of smaller beams, with stress raising at every unfinished join. And there’s probably a crane sitting on it too.

Here’s how it’s done…

This one is 125 ft span instead of 175 ft, but it is only made of stone, and it was finished in 619 (yep, just coming up to its 1400th birthday). Looks nice too.


#15

I learned about this “pre-tensioning” of concrete in a physics class (a long time ago), and the professor noted that it had been used on all the sections of Miami’s metrorail, which are long concrete sections like the bridge.

I wonder why they put it into place incomplete. And didn’t one of the engineers actually report a crack in the bridge earlier in the day before it collapsed?


#16

He did not do so bad with it, at least did not over-reach.

His focus on what he called “false work” - lets just call it temporary support - is not wrong, but suggests gross obvious mismanagement. First off, there wasn’t any, period. Pinning the collapse on this suggests that the bridge could not be set without temporary support, could not support itself without temporary support, and somebody made the grossly obvious error of not using any temporary support.

I don’t think thats what happened.

I suspect they were following a strict installation plan, and if temporary support was required it would have been there. Would it have been prudent? Yes. Saved lives? probably. But required by the calculations - apparently not.

What that means is that the failure was more nuanced, and its not obvious from observing the collapse or the aftermath if it was an installation error, material deficiency, or design error. You just can’t tell.

What you can see is where it failed, and how it fell. I suggest finding the dash cam video which actually captures the entire span at the time of the collapse. It did not happen quite as described in the video, but he probably did not see it. What you do see is there is a lane closed and a crane in that lane holding a piece of equipment above the bridge - this is likely the device used to tension the post-tensioning steel. From this positions its probably working on one of the steel tendons that follows the diagonals between the deck and roof of the bridge. What you see happen is bottom deck cracks right below the second diagonal coming out of this spot on the roof. Once that cracks both halves of the bridge rotate around the other ends, and fall to the street. The longer part hits the street (and cars) and then sequential cracks along each joint along its length with most of the deck falling to the road, and the roof on top to it. The other side rotates to the street and amazingly more or less remains in one piece.

So what happened? I could write a book. Here is the short tell. The bridge was in the form of a truss with a top member and bottom member and diagonals connecting them in triangular form. The top and bottom are much too small to span as far as the whole truss, and not nearly strong enough. But the diagonals tying them together enable them to span much further than they do alone. So what happens if you break one of those diagonals? Well the whole house of cards will fall down.

So what I think happened is the work that was happening with the steel tendon in the diagonal went wrong - the steel tie broke. It was tightened too much, or the wrong tension value was used, or that piece of steel had a defect, who knows right now - but it broke. Now suddenly the strength of the truss configuration was gone, and all the force of the weight came to bear on the bottom deck alone - and it cracked right there, then the whole thing came down.

The questions around this are too many and too deep to figure out here what was the actual mistake or failure. I don’t think it was an obvious mistake like it should have had temporary bracing. That would have been a precaution, sure, but not likely the source of the failure.

My best guess is there was some confusion about the dual spanning modes for the part of the bridge. It was installed as a simple span - truss spanning across the road. But it was apparently supposed to be a cable supported bridge. So that would mean it would have to be designed for both modes. And the tension requirements on the steel tendons may have been different for both those conditions. Something may have been adjusted wrong for this stage of the work, or the temporary spanning condition may have put the steel closer to its ultimate strength in a way that made it susceptible to a defect. No way to know now. We won’t find out until a detailed investigation is done.


#17

This bridge is post tensioned, and cracking in concrete isn’t necessarily and indication of a structural failure.


#18

I and chief engineer @TheirFeldspars have done some extensive research, and we have determined the reason for the failure:


#19

I assume you’re joking. It wasn’t finished yet.


#20

You’re darned tootin’ it wasn’t finished yet. I keep reading that the pylon and cables were supposed to be essentially decorative. That makes no sense. You don’t build the deck of a suspension bridge and then later add the suspension cables, because it would fall down and go boom. Kind of like this bridge did.