Divers Look for Bridge Collapse Victims

Lazy engineers cutting corners, and this is what happens. Hopefully this will be a wakeup call.

I think the very fact that it collapsed points to evidence of there being a structural problem. As of last night there were several conflicting reports from structural engineers, however, none of them had actually done work on this bridge, specifically, nor have seen the wreckage as it was just too soon. But the media needs to fill the airwaves, even if they don't know the actual how's and why's. It'll be months before they figure it out, and at no time are you going to hear a politician say "We knew it was structurally deficient but decided to see what happens."

The next couple of years will be a good time to be in the bridge inspection business no doubt.

@Glinda -- any bridge has a number of problems at any one time, almost all of them irrelevant to its structural soundness. Some are caused by direct loads, some by vibration, some are cosmetic or roadbed issues. And sometimes there are judgment issues, as to whether or not some particular crack or loose rivets or whatever are "progressing" into a problem. I believe that "structurally deficient" doesn't by any means mean that it's going to fall down, generally it means that it's lifespan will be reduced if it isn't strengthened or the loads limited. So it will have problems with it in 25 years rather than 50.

I would note that the entire region has been heat-soaked (for Minnesota) in 90-degree-plus weather for several weeks and that could have had some impact on things -- certainly not melting it or anything, but it could have expanded much of the structure in a way that lead to damage.

However, its collapse was somewhat anomalous -- designs like this one are made to fail in sections, the center span failed first but oddly it brought down multiple spans on either side of it, which points to either something odd, or a major design problem.

We DO have a governor here who does not like to spend money on transportation in any way, but in general that has lead to our using up our infrastructure-capital by (as discussed above) shortening the lifespans of our physical assets and deferring maintenance in any possible way.

blaming "lazy engineers" seems a little premature, especially at a time when so many departments and agencies are underfunded. sure, it could be negligence on the part of engineers, but it could be so many other things.

designs like this one are made to fail in sections

I correct my statement above, it now appears that the individual arches were tied together via the roadbed, which would lighten the structure but would then make the failure of one arch propagate to the others, which is what happened here. It seems to me this is a questionable design approach.

Re the incompetence of engineers, I would point out (as an engineer) that engineering is an exercise in constrained optimization -- the engineer is working within the constraints of the purpose of the structure, cost, physical positioning, and countless other things. Cost is always a factor, and it was so in the days of the Romans. Many Roman stone bridges still stand, not because they were especially competent (although they were very good), but because they used very high safety factors in their designs. Clients nowadays (especially governments) require much more cost-effectiveness in their designs which leads to tighter margins and generally lower overload safety factors. So don't blame the engineers if you aren't willing to spend the money to build in higher redundancies.

@Jim
There was an interview on the radio during my drive home (ironically, I guess) with Saeed Mirza who is a professor of civil engineering at McGill University - his whole thing is about infrastructure maintenance. To make his point, he was comparing the regular checks on a $20-30,000 car which you might expect to last 10 years, compared to a structure like a bridge or building.

over ten years the accumulated stress cracks in the steel caused the bridge to lengthen a foot.

Interesting. The only way that cracking could have lengthened it by a foot would be if the stressed member had enough cracks opening enough to add that much length, which I would think would have to have lead to a dropping of the top of the span, which should have been noticeable in an inspection. But it meshes with my thought above that heat-based expansion (especially of added to that lengthening) could have caused serious problems. Good data, though.

this style bridge had a large unsupported span in the middle to allow river barge traffic

But all the bridges along there have the same issue, if it was well designed it shouldn't be a problem -- but of course who knows what the design load factor was for that truss. Apparently that was the span that failed first. That part of the span always looked a little flat to me, for the distance it was spanning.

governor pawlenty has a stubborn concpet of no new taxes he has pushed down the state legisltures throat which has led to many areas of state responsibility being under funded and road and brdiges was one of those.

Amen, per my statement above. He's just burning up our infrastructure because somebody else will have to pay for it later!

Good picture of the bridge pre-collapse, showing the relatively flat arch of the main span: here.

And, good little article in the Times about the issues with bridge inspecting.

They're quick to add public taxes for private ballparks, however.