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Alas, part two is finally done! Going to have to do a third, though, as there's just so much to talk about with so little space to do it in effectively. I suppose that's why there are full volumes of books on this stuff. You may find it here.

I thought I'd share the actual cause of the I-35W bridge collapse in Minneapolis for those of you who are interested and couldn't get any information from trying to deal with generic media sources. The official forensic modeling results were released not too far back, and the conclusion is that the bridge failed due to poor construction and poor engineering examination. Basically, truss bridges are a variation of the following standard design:
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The long bars between the diagonals are called chords, and the connections occur on things called gusset plates. Though a number of factors came into play, the gist of the problem was that the gusset plates were in sad shape due to human error: a number were bent, and 16 of them were installed as 1/2" plates instead of the designed 1" plates. As a result, the truss connections were in shoddy repair, and stripped the bridge of its ability to handle large loads safely. Thus, when a bulk of construction equipment was added to the deck during contract work, it caused enough of a load increase to send the plates into failure, ultimately snapping the connections between the members and causing the bridge to basically fall apart at its seams.

Another factor, too, was that the members were poorly designed for thermal expansion. Believe it or not, temperature fluctuations play a huge role in how structures behave. Let's say that you have a 10" steel beam that's sitting overnight at 45 degrees Farenheit, and that it warms up the next day to 95 degrees; the beam, assuming it only changes 50 degrees (and we all know hot metal gets compared to the ambient external temperature), it will expand 0.0033 inches. Certainly not visible to the naked eye. Yet, while this may not seem like much, all that expansion is converted into an applied load if the beam is locked into place, and can quickly become of massive proportions. If the beam is given a mere 0.001" breathing room, it cuts the thermal stress inside it by about 50% (9.43 KSI to 6.53 KSI.) Keeping in mind that this steel fails at 36 KSI, you don't need to be math-minded to see how big a difference that is.

Well, in simplified terms, a number of chords in the bridge were installed poorly, which allowed for their thermal-breathing mechanisms to freeze over. Keeping in mind that these members are tens of feet and not a mere 10 inches, the thermal stresses were already tearing the bridge apart internally. The NTSB said in its report, "[This bridge] was in danger of collapsing the day it was completed." Combined with the gusset plates, it's a wonder it stayed up as long as it did, as there were two design failures acting concurrently on the same points.

And yes, these problems were all noted during previous bridge inspections. We knew this thing was coming apart but we did nothing about it - a truly scary thought.

In case anybody was wondering what I do with my life, I thought I'd share a tidbit. I was playing around with AutoCAD earlier, and decided I'd throw up an arbitrary beam diagram for practice with a few of the commands. In addition, to help me study, I thought I'd break down and analyse the situation to see if my completely random and arbitrary design holds. :)

Well then, without further adieu...