Identifying Effective and Ineffective Retrofits for Distortion Fatigue Cracking in Steel Bridges Using Field Instrumentation

It is estimated that nearly 90% of all fatigue cracking is the result of out-of-plane distortion or other unanticipated secondary stresses at fatigue-sensitive details. Neither design specifications nor evaluation specifications provide any guidance on how to evaluate the in-service potential for fatigue cracking at these details. Often, as a result, the effectiveness of various retrofit procedures is questionable and ill fated. There are many examples where implemented retrofit procedures did not work and fatigue cracking reinitiated or continued. Implementation of one or two prototype retrofits is an attractive alternative to ensuring effective retrofits are developed where many details have to be retrofitted. Field instrumentation and testing is an effective means to determine the effectiveness and behavior of a given retrofit strategy. Behavior that may not be anticipated can be identified prior to installing retrofits on a large scale, thereby preventing future problems. This paper provides guidance on how to instrument these details and examines one example where initial retrofit strategies did not work, as demonstrated by their performance through field instrumentation.