Fatigue

What It Means

Fatigue is the progressive and localized damage that occurs in steel members subjected to repeated cyclic loading, such as the stress cycles produced by truck traffic. Unlike static failure, which requires loads exceeding the ultimate strength of a member, fatigue cracking can develop at stress levels well below yield if the number of cycles is sufficient and the detail geometry creates stress concentrations. Fatigue is governed by the AASHTO fatigue detail categories A (best, smooth base metal) through E' (worst, certain welded details with high stress concentrations), each with a distinct S-N curve relating stress range to cycles-to-failure. A Category E detail, for example, has a constant-amplitude fatigue threshold of only 2.6 ksi, meaning repeated stress cycles above that level will eventually produce cracking regardless of how many millions of cycles it takes. Fatigue-sensitive details in older bridges include cover plate ends on girder flanges, transverse stiffener-to-flange welds, web gaps at cross-frame connections, and pin-and-hanger assemblies. The Mianus River Bridge collapse in Connecticut in 1983 killed three people after fatigue-driven corrosion jammed a pin-and-hanger assembly and caused a suspended span to fall. Fatigue is accelerated by increasing truck weights and volumes, many 1960s-era interstate bridges were designed for HS-20 loading with much lower truck volumes than they now carry. Fatigue evaluation is required for steel bridges with high truck traffic under the AASHTO Manual for Bridge Evaluation, and fatigue-critical members are subject to hands-on inspection similar to fracture critical members. Remediation options include retrofitting with cover plates, bolted splice plates, ultrasonic impact treatment, or hole-drilling to arrest crack propagation.

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