FATIGUE EVALUATION OF WELDED BRIDGE DETAILS PRONE TO DISTORTIONAL CRACKING BY LOCAL APPROACHES

摘要

Distortion-induced fatigue cracking in various welded details is an often encountered problem in steel and composite bridges. This type of fatigue damage is generally caused by some kind of unintended- or otherwise overlooked - interaction between the bridge members. Distortion-induced fatigue cracks are mainly generated by out-of-plane distortion of the web in regions where web stiffe-ners are dissociated from the web and a part of the latter is hence unstiffened (referred to within the text as "web gap regions ") which in many cases results in very high local stresses. This kind of local effects can rarely be captured by "simple " global models: a refined and detailed model is thus required to represent the local effects for the calculation of fatigue design stress. Consequently, for fatigue analysis, local approaches such as the structural hot spot stress or effective notch stress usually need to be used. In this paper, the applicability of the nominal stress method to the problem of distortional cracking in a welded bridge detail is studied and the results compared to those obtained from refined calculations with the structural hot spot stress and the effective notch stress approach. The results of the study show that welded details prone to distortional cracking should be evaluated by local fatigue assessment methods. The conventional nominal stress method fails to capture the high stress gradients caused by bending. When considering the modelling efforts and computation time, it is recommended to use the structural hot spot stress method. Analysis with the effective notch stress method requires multi-level sub-modelling which results in a substantially larger modelling efforts and computation time.