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Minnesota Bridge Collapse
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[QUOTE]Originally posted by Kiernan: [QB] Since I'm a registered professional civil engineer in two states, I'll try to answer Train Lady's question. Lots of things can cause a bridge to collapse. We've all seen film of the Tacoma Narrows Bridge--sometimes called Galloping Gertie--as it failed in wind. No weight at all on the bridge. Certainly an earthquake could make a bridge come down, as happened in the East Bay during the Loma Prieta earthquake, which also took down the bell tower at my high school. What happened in Minnesota was probably the failure of some specific component on the bridge. That bridge was designed in the B.C. years--Before Computers. It's quite simple now to make a computer model of a bridge and allow the computer to simulate component failure. And then modify the design. The Minnesota bridge was a steel arch. Arches are common bridge structures, the Roman aquaduct bridges were built with arches. The Roman bridges, though, used stone or masonry for their arches, an ideal material. Steel arches are generally made from pieces that are connected together, usually with rivets or high-strength steel bolts. These bolts or rivets can fail, particularly when the road maintenance crews use salt in the winter. For these reasons, bridges have to be inspected carefully. Federal law requires inspections every other year. Many years ago, my first civil engineering job was as a bridge inspector. Inspectors now take a two week training class and regular update classes. I don't do that kind of work any longer, but my office mate does and he's off to Seattle next month for his refresher class. I guess it would take weeks to inspect a bridge like the one that collapsed. Scoll up in this thread and look at the picture that Mike Smith posted of the St. Louis bridge. The white stuff that hangs down is call "efflouresence." I think I got the spelling right. It's a sign of concrete deterioration, and huge chuncks of the concrete have fallen off exposing the reinforcing steel, which is hanging down. The steel carries the tension load in the reinforced concrete, but to carry the load it needs to be bound to the concrete. Which it is not in that picture. If that bridge was to fail, would it be "weight" or bad maintenance? When I was riding the Acela from D.C. up to Stamford, CT, a few weeks ago, I saw lots of bridge like that, and I wondered how are we going to pay for that. Generally, bridges are quite safe. Transportation management, though, needs to listen to the inspectors and act on the recommendations. Sometimes the way a bridge acts can be deceptive. I remember being stopped in traffic on the Interstate 10 bridge in Baton Rouge and feeling the bridge bounce under the deflection caused by the eighteen-wheelers. Bridges deflect and it's perfectly normal. It is creepy, though, to look out the window and see the water when you're crossing the river. [/QB][/QUOTE]
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