What a difference a week makes! Lots of information has come out in the NTSB board meeting last Thursday and Friday. Also I attended a presentation given by Prof. Roberto Ballarini tonight at the University of Minnesota.
Four thoughts now -
- It's clear the U-10 gusset plates failed due to the load. The gusset plates should have been thicker. As it was, the safety factor was only 1. (That's the ratio of the load that can be supported to the load that is actually being supported.) It should have been 2. The bridge had been sitting on the verge of failure for years.
- I asked Prof. Ballarini a question at the end of his talk. I mentioned the reports that the bridge had been wobbling in the days leading up to the collapse. I asked if such wobbling could have been an indication of imminent collapse. He thought that was a good question. From his response, it sounds like there could indeed have been wobbling in the days before the collapse, related to the loss of elasticity in the gusset plates.
As I wrote August 11, 2007, "When a bridge starts wobbling, best not to ignore it." Get people off wobblers. - Someone has lied about whether or not there were sparks flying between the work crew and MNDOT about mixing concrete on the bridge, either the people involved or the short grapevine. The stories are at odds.
- Concrete was being mixed on the bridge and had been for days. Heavy ingredients, sand and gravel for making concrete, were in piles on the road above the U-10 gusset plates. This concentrated load was the straw that broke the camel's back, or perhaps the lead pipe.
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