Bridge Collapse question: How did it all come down at once?

I can understand varying degrees of disrepair. That bridge did not receive a clean bill of health last time it was looked at. At least that is what I garnered from the news this morning.

But does anyone know how the whole thing came down at once? I can see a section of it coming down, but the whole damn thing?

Am I being whooshed by basic engineering?

It looked to me like there were two spans - a long one over the river, and a shorter one. If the main span failed at the far end, the weight of the bridge would have pulled the rest of it across and disconnected it from the other side, using the “short-span” support as a pivot.

In any structure, when one component fails, load is transferred onto the others - it looks to me (and I’m not an expert, nor am I intimate with the specific design of this bridge) as though an initial failure in one place may have resulted in a cascade effect where the redistributed load took the adjacent components beyond their limits, causing them to fail too, resulting in even more load on even fewer components, and so on, until the whole thing comes crashing down.

In the thread over on MPSIMS (sorry no link) someone posted a structural report carried out on the bridge that said it was a “non-redundant structure”, i.e. if one bit breaks, the whole lot breaks.

A common problem is that a load is being shared across multiple supports. If one or more of those supports fails, the remaining supports have to deal with a sudden increase in load. This can produce a cascading sequence of failures.

Sudden catastrophic failure is the danger of non-redundant bridges. The bridge had two longitudinal main truss members. If one truss breaks, the span collapses. If there are other spans that were continuous with the collapsed span, those go down too. If just one main member of a truss breaks, the whole truss is gone. If I had to bet, they’ll find one gusset plate in the connections of one of the trusses had fatigue cracks and the crack opened up suddenly, two members became unconnected and down she went.

Is it safe to say that the temperature here played a part in this mess? When it’s 90 degrees today and 10 below zero six months from now, that’s a lot of expanding and contracting for all that asphalt, cement and metal. Fatigue would surely be an issue, would it not?

The bridge was designed to span the river without supports in the water. So, basically you had one long span and when one end gave way it pulled the other end out and the whole thing just slapped down.

I posted a link to a security camera video in the MPSIMS thread. Security cam video

It clearly shows what other posters are saying: Part of the bridge failed, then the other part, unable to hold itself up without the first part there, came down as well.

This video of the collapse shows the that road surface remained remarkably level on the way down. Unfortunately, the interesting structural bit is off camera, but lack of twist suggests a simultaneous break across the entire support structure; pier failure, perhaps?

I’d be truly amazed if it was a pier failure but can’t say it’s totally impossible. It certainly looks like the end nearest the camera went down first and it does seem quite level. But I think the time between failure of one truss and the consequential failure of the other could be small enough that the twisting time would be quite short and make the collapse appear level.

Dunno - that video shows a big bit falling flat, then another bit falling and tipping - to me, it looks like maybe some components in tension might have rapidly failed in sequence across the top on the end nearest to the camera, but that the rest of the structure held it up until the failure had propagated all the way across, then it all fell as a big slab, tearing away the far end as it went.

Me too, but ISTM that the sequential failure of two trusses could only look like that if both structures were very weak. Perhaps the initial failure took place further to the right than is obvious, and there’s some twisting over there we can’t see.

Looks to me like the end nearest the camera failed and as it began to fall, pulled the other end of the span off of it’s expansion joints, hence the nearly flat fall. The spans on either side of the fallen center then collapsed because of the loss of the counter-balancing force.

That’s what I want to know. It doesn’t make sense that a huge structure could just fall down due to the loss of support. Were there any squibs? Grainy footage to analyze? Quick, someone look for pools of molten steel…we’ll get to the bottom of this!

Is that supposed to be a whoosh?

Post-failure photos show that both piers remain standing, although one seems to have tilted a bit.
I noticed the twisting of the truss when looking at the first photos to come out. One end was twisted off its pier (it seemed to me). At the other end, the lower members are bent down into the water.

Yes, post #15 is making fun of 911 WTC skeptics’ arguments.

I got hold of the 2001 U-Minn Fatigue Evaluation Study for this bridge. Some choice clips:

“The bridge’s deck truss has not experienced fatigue cracking, but it has many poor fatigue details on the main truss and floor truss system. The research helped determine that the fatigue cracking of the deck truss is not likely, which means the bridge should not have any problems with fatigue cracking in the forseeable future.”

“The approach spans have exhibited several fatigue problems; primarily due to out-of-plane distortion of the girders. Although fatigue cracking has not occured in the deck truss, it has many poor fatigue details on the main truss and floor truss systems.”

“Inspection of the bridge revealed Category D details on the main truss members and Category E members on the floor truss. No fatigue cracks were found by visual inspection of these members.”

The document has 82 pages and is more that I can digest in one sitting. They obviously had enough concern to fund this study, but it strikes me as odd that they didn’t use more than visual inspection on the fatigue sensitive details. Their recommendations seemed quite timid, only recommending the 24 month frequency for most of the members and 6 months for the ends of the “fin” attachments reinforcing the splice welds. “Since these can be inspected easily from the catwalk, they should be inspected every 6 months.”

Not really. Thermal expansion is almost perfectly linear, as well as expected, and therefore designed for. Since the amount of stress induced in the steel would be a small percentage of the load, fatigue should not be an issue. However, with proper design and NDE (nondestructive evaluation) throughout the life of the bridge, proper preventive maintenance could have avoided any fatigue cracking problems.

Concrete (both bituminous asphalt and portland cement) doesn’t really have a fatigue life, since both can only support loads in compression. 90 F is a very reasonable temperature for steel, and it can withstand much higher (think engine block). I would be more worried about the 10 below zero F, because depending on the type of steel, it could be below the DBTT (ductile to brittle transition temperature), and the steel could act brittle instead of ductile, which is the regime in which it would have been designed.