Posted on 08/04/2007 11:49:45 AM PDT by kathsua
I've been looking at some of the video showing the collapse of the I-35W bridge over the Mississippi River in Minneapolis and it looks like the collapse could have resulted from a major mistake by the company working on the bridge's surface. The bridge carries eight lanes of traffic, four each direction. To maintain an even load on the bridge supports during construction the contractor should have either worked on the two inside lanes of both sides of the bridge or the two outside lanes. Instead the contractor worked on the inside two lanes of one side and the outside two lanes on the other.
The result was the support on the side in which the outside lanes were carrying traffic had to support a greater load than the side on which the inside lanes carried traffic. I'm oversimplifing, but essentially the support on this particular side would have been carrying the load of the traffic directly above it as well as a portion of the traffic on the other side. The support on the other side would only have been carrying a portion of the load on its side.
If you've watched videos from the scene you know there is a school bus and semi truck next to each other in those outside lanes. There is another semi just behind them. These heavy vehicles went across the supports just before the collapse. The supports on this side of the river shifted toward this side with the roadway shifting toward the other side as if the weight of the heavy vehicles may have been too much for it to carry in a weakened state
My background is in math and physics rather than engineering so I'm not familiar the precise distribution of weight and how the supports carried it, but the basic math would indicate more stress on one side of the bridge than the other.
A computer analysis of the remains of the bridge and its design will be necessary to determine the precise amount of stress on the different pieces of the bridge and their physical condition. This analysis will show exactly which part(s) failed. Fortunately investigators have a computer program using Finite Element Analysis developed at the University of Minnesota.
If the bridge was only 17 years old I doubt it was caused by structural failure.
Just spreading concrete/asphalt around is pretty stressful I imagine. Even a thin layer weighs quite a bit when resurfacing
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It was 40 years old. Built in 1967.
It was built in 1967, making it 40 years old, and it was a non-redundant design, meaning the failure of any structural element could cause catastrophic failure of the whole structure.
I heard it was built in ‘67...I heard it on NBC, so I’m not married to it. The uneven distribution of weight was the direction I was leaning. I also wondered if perhaps any de-icing agents might have gotten to the sub-structure.
That’s why armies don’t march in step across bridges.
I thought the mythbusters debunked that?
It is something that I have been wondering about, but of course, we do not know yet.
Also note that the milling off of the existing surface strata would exacerbate the imbalance issue.
They did actually get the bridge to resonate (with a relatively small piece of equipment) but couldn't get it to shake like an earthquake.
If they did, they are idiots.
Thanks for the correction.
They were trying to duplicate an experiment by Tesla, and it shocked the crap out of them when they felt that tiny weight shaking the bridge from about 100 ft. away.
The military has known for years that troops marching in step can create enough vertical force to destroy a bridge. It is standard practice for soldiers to break step at every bridge crossing.
The LSM needs to make up its mind... Last I heard, GWB is into ranching, not construction.
Resonance caused by pedestrians:
http://en.wikipedia.org/wiki/Millennium_Bridge_%28London%29#History
Resonance apparently resulting from wind and poor design: “Galloping Gertie” Tacoma Narrows Bridge, 1940:
http://www.youtube.com/watch?v=HxTZ446tbzE
Construction?.....nah, from all I’ve heard on TV, George Bush did it.........
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