FATIGUE Concrete vs. Steel

^^ W J ^ ^ ^J ^^ U '^ ^l FATIGUE Concrete vs. Steel by W. Jack Wilkes, P.E. Senior Vice President/Manager Figg and Muller Engineers, Inc. Austin, Texas C atastrophic failures are the ultimate nightmare of structural engineers. The greatest number of bridge failures occur during construction, and the second largest cause of bridge failure is from flood water either from scour or from overtopping. However, the failures that seem to attract the most media attention are those that occur to aging bridges that have been in service for many years, par- ticularly if there is loss of life involved. In many cases, the offending culprit, an overweight truck on a load posted bridge, can be identified because it is trapped in the wreckage. Unfortunately, it is often only a lightly loaded vehicle that is on the bridge when it collapses. It may be unfair to point out that, in- variably, the collapsed bridge or span is a steel beam or steel truss bridge; in years past, most long span structures in the United States were constructed with steel. Therefore, it is rare that the collapsed structure is a concrete bridge. When one searches for a reason for the collapse of older bridges, the usual cause is a fatigue failure of the steel members. Since all concrete structures require steel elements, strands or bars, to take the im- posed tensile stresses in the structure, there needs to be an explanation of why concrete structures are tougher, more durable and more tolerant of frequent overloads. 76

FATIGUE Concrete vs. Steel

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