Fatigue life prediction for distortion-induced cracking of steel bridges

Fatigue is the main reason for the limited service life of welded plate girders built before the mid-1970s. Although, current bridge design specifications provide estimates of fatigue life for load-induced cases, fatigue cracking due to out-of-plane distortion is not adequately addressed, despite its severity. This paper presents a three-dimensional (3D) finite element analysis (FEA) of two units of the I-345 Bridge using the commercially available software LUSAS. The modeling procedure is used to determine the magnitude of the distortion-induced stresses, the fatigue life, and the effectiveness of the retrofit strategies. The results in terms of contour plots for the as-built and repaired connections show a high stress concentration at crack initiation sites, in agreement with the bridge inspection reports. The used retrofits methods, when properly carried out, provide a full restoration of the service life of the connections. A relationship is proposed to predict the fatigue life for distortion-induced cracking.