What effect does stress have on a steel arch deck truss bridge?
Wednesday, August 25th, 2010 at
6:58 am
I wonder if the re-routing of eight lanes of traffic onto four lanes would cause a stress factor of which the four lanes were not designed to with-stand. Thus causing, as in the case of the Minnesota I-35W , a collapse of the structure.
Filed under: General
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It depends on the structure added.
If you reroute traffic to double capacity then how are you going to hold all that traffic? If you increase the road bed then you increase the ‘dead’ load; which is the load that has to be constantly held and thus increase the load on the bridge. If you put in another road deck on top then you are doubling the weight and inviting collapse. If nothing else the foundations wouldn’t be able to support the load and so would sink.
If you are simply making tighter lanes and pushing more cars across then you have increased the ‘live’ load. In the case of bridges the live load is usually only a fraction of the ‘dead’ load so that may not be such a big problem. However, the added stress and shaking caused by the increased traffic could be a major problem.
The Minnesota I-35W Bridge didn’t collapse because the loading was changed but because the bridge was allowed to wear out. The bridge, like many in the US was old and had too many cracks in it. The scary part is that there are a lot of bridges in worse shape that are major parts of the US infrastructure. One of them could collapse at any time if they are not repaired; but the cost to do that is too much.
You have to remember that bridges have to hold their own weight as well as the weight of the road bed. Then they have to support themselves in the wind and under the stress of traffic.
One of the most famous engineering disasters is what happened to the Tacoma Narrows Bridge. The wind reached the bridge’s resonance frequency which is its vibration frequency so bridge started to flux like a vibrating string. Galloping Gurdy as it was called literally shook itself to death. It could support the weight of the cars just fine, it was still new and didn’t have any cracks in it, and it was strong enough to support itself. It was NOT designed for the high winds in the canyon.
The moral of the tale here is that you can never account for all the stresses that will be encountered by a building or structure in real life so you have to include a safety factor; a fudge factor. The bridge should have had years left in its design life, witnessed by the fact that the east bound lanes were still open. Maintenance was the issue not design or an increase in stress, but a simple lack of proper maintenance.