Strength of In Service Bridges
Country: South Africa
Date: May 2, 2011
Why are bridges still strong, even after many cars are still passing over it?
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
Well, let me reverse your question... why would you think that cars
driving over a properly engineered bridge should make it slowly get
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
As you probably can guess, structural members in a bridge are
designed so under load, they never even come close to reaching their
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.
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Update: June 2012