CE 3102
Fall 2008
Thought Problem 15
From a number of laboratory tests on beams of A285 steel, the number of loading cycles
to fatigue failure appear to be well-modeled by a lognormal distribution with an
estimated mean of 430,000 cycles and a standard deviation of 215,000 cycles. All tests
were run at 36,400 psi.
It is anticipated that beams in a bridge, for which you are the engineer of record, will be
subjected to a loading equivalent to 300,000 cycles at approximately 36,400 psi during
the life of the bridge. Estimate the probability that fatigue failure will occur before the
bridge is retired.

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