This Post is Structurally Deficient

Looks like it's time for another engineering post. Today's discussion is about structurally deficient bridges. Inspired by all the finger pointing due to I-35 falling into the Mississippi River out in Minneapolis.

Let's get the stats out of the way first. Right now, Tennessee has 1,202 bridges rated structurally deficient. That's out of 19,519. So 6% of our bridges are structurally deficient.

So what does that mean? It means that the bridge has a problem of some kind. The kind of problem that should be watched. Structurally deficient is a classification that means we watch that bridge a little more closely than the others just to make sure the problem doesn't grow. It's a way of prioritizing bridge needs. It does NOT necessarily mean that the bridge has to be immediately fixed. The ones that need to be immediately fixed are easy to see. They're the ones that you aren't allowed to drive across. And trust me, these things are watched very closely. I know the guys that do it.

So what if it was structurally deficient??? That does't necessarily mean it needs to be replaced. I'll go so far as to say Minnesota no doubt had the money to replace that bridge if they'd wanted. But they were probably spending it on bridges that had been closed for safety reasons. That's what the structurally deficient watch list is for. Prioritizing. There was just apparently something bigger wrong than anyone thought. Maybe somebody screwed up. Maybe they didn't.

Sometimes bad things happens and it's not any ones fault. Bridges are complicated. The only way to know for sure what kind of shape it's in is to take it apart. I think the problem with that method is obvious. You have to kill the patient in order to find out how healthy he is. So you have to settle for what you can find out by looking and doing things that don't hurt the patient.

I did some research on bridge failures back in 2000 when I wrote my thesis, and maybe I'll go into those a little more if I decide it's not too boring, but that's for another post. My rant today is about all this finger pointing. Save it for later. None of you have any idea yet what caused that bridge to fall down. Trust me, we'll find out. As soon as the survivors and the bodies are out of the way the National Transportation Safety Board (NTSB) will be there. And they'll be crawling up the nether regions of every single person involved in building that bridge, inspecting that bridge, or even thinking about that bridge. They'll tell us why it failed. In detail and in triplicate. The NTSB doesn't hesitate to point fingers. Once they have evidence.

If you only take one thing away from this post, here's what it should be. Sometimes it's just impossible to tell when a bridge will collapse. It's the bane of every bridge engineer, and I've written before about how it makes me worry. I've worked with a few guys who designed bridges that failed. It isn't something that's easy to live with. So quit blaming people until you know who to blame.

And now I'm going to reverse myself a little for those of you who still feel the need to blame someone for this tragedy. Let's talk for a minute about closing dangerous bridges. I think the Demonbreun Street bridge proves that TDOT is willing to close a bridge when it needs to be done, despite the consequences.

It's also a good case study for the pressures on bridge engineers. Everyone and their dog complained when that bridge was closed. There were cries of everything from "But how will people find my business?" to "I want the head of whoever let it get this bad!". Now, by show of hands, who thinks the whining and recriminations foster doing the right thing?

It's simple really. There are so many pressures that you just don't close a bridge until you're really absolutely sure that it's dangerous. Especially not a huge one that a hundred thousand commuters use every day. Something has got to be very wrong in order to close the bridge, and the folks in Minnesota probably just didn't see anything that looked very bad. Obviously they missed something.

If you can't take the rest of the speech to heart and still feel the need to blame someone, then point some of that blame on everyone who has a car. And remember this the next time you complain about too much road construction going on around town.

** I admit this is a little bit of rant. Lack of infrastructure funding is a pet problem of mine, and it's glad to see some non-engineers talking about it. But I hate the way things are already being oversimplified and politicized. For once, I agree with Chris Wage.

here for details on the lack of infrastructure funding, and here for a few facts and figures about bridges in Tennessee.


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