Finite Element Study of Distortion-Induced Fatigue in Welded Steel Bridges

Out-of-plane distortion-induced fatigue cracking is caused by relative rotation and displacement between longitudinal girders and transverse members framing into these girders. Procedures for determination of secondary stresses are not specified in the design or rating process. Appropriate finite element method procedures to analyze distortion-induced fatigue behavior are presented. A multigirder bridge developed web gap cracks near the girder bottom flange in a positive moment region. The affected diaphragm–girder connections were repaired by installing additional reinforcing splice plates to the web and attaching connection stiffeners to the flanges. Because no structural modifications were made to similar details in the bridge that had not developed fatigue cracks, concerns remain that these details may also be subjected to high-magnitude fatigue stresses that may lead to future cracking. By using finite element submodeling techniques, potential crack initiation sites in the bridge were identified and the corresponding distortion-induced stresses were determined. The most stressed detail reached yielding with an out-of-plane displacement of only a few thousandths of an inch. On the basis of the analytical results, a linear stress–displacement correlation was established for prediction of the secondary stresses. Repair analysis indicated that web gap stresses can be significantly reduced if a rigid stiffener-to-flange attachment is used. Thus, a bolted repair is recommended for the positive moment region connections, and a welded repair is recommended for the transition and negative moment region connections.