Distortion-Induced Fatigue in Multi-Girder Steel Bridges

This report summarizes the findings of a project with the following goals: to implement a field instrumentation and monitoring program for a typical multi-girder steel bridge on skew supports that may be susceptible to web-gap distortion; to assess the frequency and magnitude of the distortional fatigue stresses at the web-stiffener connections; and to evaluate the impact of these stresses on fatigue life. Measurements from 12 independent strain gauges were continuously monitored and recorded for more than three months on Minnesota Department of Transportation (Mn/DOT) bridge #27734. Truck loading tests also were conducted. Predicted web-gap fatigue life based on the long-term monitoring data from Mn/DOT bridge #27734 ranges from 45 to 75 years. Comparison of web-gap stresses with primary design stresses reveals that web-gap distortional stresses are comparatively high. The report also highlights a detailed finite element study to better understand the web-gap stress mechanism and to compare experimental results with theoretical predictions. Study results have important implications for investigators of distortion-induced web-gap fatigue. They indicate that the actual stress at the so-called hotspot may be as much as twice the stress measured at the strain gauge. The report includes a method for estimating girder deflections and web-gap stress.