Bridge Collaspe
This is what I've read so far in explanation of the collapse.
MNDOT did an experimental stress test of the bridge in 2001. They drove heavy trucks of known axle weight over the bridge during normal traffic, and then using carefully placed sensors were able to determine the structural stress loads on the various parts of the bridge. The road surface is supposed to experience no structural stress, it's just supposed to lay there. In fact it experienced great stress: it was the road surface that was holding the bridge together! The reason they had to use this experimental method was because all the supporting trusses had been painted with a heavy green paint for aesthetic reasons, and so they weren't able to adequately visually inspect for those fractures.
Apparently what happened is that when the maintenance crews were resurfacing the roadway they found a great deal of poor material used in the initial construction so that rather than just resurfacing the top four inches they had to dig down deep and sometime entirely through the road bed. They hadn't been informed that it was the roadbed, anchored at each bank, that was actually holding the bridge together. In effect they cut that supporting structure in half. That allowed the surface roadway to slide. The stress fractures hidden by the green paint meant that the basic support trusses were too weak to hold together and separated; everything pulled off the supporting piers and the whole bridge collapsed.
Pretty interesting. It's still only speculation but it's the best analysis I've so far found.
Incidentally, the hot weather we've had recently would have increased the latitudinal stress on the roadway due to the expansion of the concrete, increasing the possibility that it would just split in half. As well as that there was overloading due to all the cement trucks and construction materials all gathered in one spot. --It was also noted that there was a lot of rebar sticking out with no concrete adhereing. That would be consistent with weakly mixed cement.
MNDOT did an experimental stress test of the bridge in 2001. They drove heavy trucks of known axle weight over the bridge during normal traffic, and then using carefully placed sensors were able to determine the structural stress loads on the various parts of the bridge. The road surface is supposed to experience no structural stress, it's just supposed to lay there. In fact it experienced great stress: it was the road surface that was holding the bridge together! The reason they had to use this experimental method was because all the supporting trusses had been painted with a heavy green paint for aesthetic reasons, and so they weren't able to adequately visually inspect for those fractures.
Apparently what happened is that when the maintenance crews were resurfacing the roadway they found a great deal of poor material used in the initial construction so that rather than just resurfacing the top four inches they had to dig down deep and sometime entirely through the road bed. They hadn't been informed that it was the roadbed, anchored at each bank, that was actually holding the bridge together. In effect they cut that supporting structure in half. That allowed the surface roadway to slide. The stress fractures hidden by the green paint meant that the basic support trusses were too weak to hold together and separated; everything pulled off the supporting piers and the whole bridge collapsed.
Pretty interesting. It's still only speculation but it's the best analysis I've so far found.
Incidentally, the hot weather we've had recently would have increased the latitudinal stress on the roadway due to the expansion of the concrete, increasing the possibility that it would just split in half. As well as that there was overloading due to all the cement trucks and construction materials all gathered in one spot. --It was also noted that there was a lot of rebar sticking out with no concrete adhereing. That would be consistent with weakly mixed cement.
posted by mouse at 2:31 PM
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