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Name: Syllie
Status: other
Grade: other
Country: South Africa
Date: May 2, 2011


Question:
Why are bridges still strong, even after many cars are still passing over it?



Replies:
As long as the integrity of the structural members remain uncompromised, the bridge will stand. Problems arise when snow, ice, other weather cycles, and salt accelerate corrosion of the support members to the point where the support members and the bridge fails. Vehicles that are heavier than the weight the bridge is designed to carry can also cause a bridge to deteriorate or to fail.

Bridges are also built to withstand more load than the peak load. This factor is referred to as the safety factor. For example, a bridge that carries 100 cars at any one point of time may be built to carry 200 cars at any one point of time even though that many cars wouldn't fit on the bridge.

You can find a copy of the Oregon bridge engineering manual online at the following URL: http://www.oregon.gov/ODOT/HWY/BRIDGE/standards_manuals.shtml#Bridge_Design_ Drafting_Manual

Sincere regards, Mike Stewart


Hi,

Well, let me reverse your question... why would you think that cars driving over a properly engineered bridge should make it slowly get weaker?

Bridges are engineered so that under maximum loads, none of the structural members will be subjected to stresses that exceed their so called Yield Points. The Yield Point of a material is that amount of stress that will permanently cause it to deform. When stresses are below the material's yield point, the material behaves "elastically"; that is, it springs back harmlessly time and time again after the stress is removed.

As you probably can guess, structural members in a bridge are designed so under load, they never even come close to reaching their Yield Point.

As an example, fasten a ruler to a table so most of it hangs out over the edge. Now press down at the overhanging end, so it deflects downwards one or two millimeters. You can continue to press and release forever, but the ruler will never break because it never gets close to its Yield Point. The same situation holds true with bridges.

Regards, Bob Wilson



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