Development of fatigue cracking has led to severe structural deficiency of many Kansas Department of Transportation (KDOT) welded steel bridges. In most circumstances, cracks were found to have developed at the connections of transverse structural members and longitudinal girders due to out-of-plane distortion. Procedures for determination of secondary stresses are not addressed by the current bridge design or rating specifications, since the details subjected to distortion-induced fatigue are often confined to highly localized regions and their corresponding stress fields are difficult to predict using conventional design or analysis approaches.;This dissertation presents the application of finite element methods for evaluation of out-of-plane fatigue behavior and recommendation of appropriate retrofit. Through proper modeling of the interaction between longitudinal girders and transverse structural members, causes of distortion-induced cracking were determined and different repair options were assessed. The central focus of the research is the case studies of five KDOT bridges with typical superstructure types and cracking scenarios. The Arkansas River Bridge developed fatigue cracks at the coped floor-beam flange to connection plate fillet welds, while the Westgate Bridge, the Winfield Bridge, the Hump Yard Bridge, and the Tuttle Creek Bridge developed fatigue cracks at the unstiffened web gaps where the transverse connection plates were not rigidly attached to girder flanges. To link the global structural behavior under truck loading to the local stress concentration of crack prone details, a two-level finite element modeling approach was employed during the investigations. Stick frame to finite element modeling was used for the analysis of the Westgate Bridge, and direct finite element coarse-to-fine submodeling was used for the analyses of the other four bridges. Both modeling procedures successfully determined the stress magnitude and distribution caused by out-of-plane distortion, and at the same time minimized the computation effort as well. (Abstract shortened by UMI.).
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