Analysis of Girder Differential Deflection and Web Gap Stress for Rapid Assessment of Distortional Fatigue in Multi-Girder Steel Bridges

Distortion-induced fatigue cracking in unstiffened web gaps is common in steel bridges. Previous research by the Minnesota Department of Transportation (Mn/DOT) developed methods to predict the peak web gap stress and maximum differential deflection based upon field data and finite element analyses from two skew supported steel bridges with staggered bent-plate and cross-brace diaphragms, respectively. This project aimed to test the applicability of the proposed methods to a varied spectrum of bridges in the Mn/DOT inventory. An entire bridge model (macro-model) and a model encompassing a portion of the bridge surrounding the diaphragm (micro-model) were calibrated for two instrumented bridges. Dual-level analyses using the macro-and micro-models were performed to account for the uncertainties of boundary conditions. Parameter studies were conducted on the prototypical variations of the bridge models to define the sensitivity of diaphragm stress responses to typical diaphragm and bridge details. Based on these studies, the coefficient in the web gap stress formula was calibrated and a linear prediction of the coefficient was proposed for bridges with different span lengths. Additionally, the prediction of differential deflection was calibrated to include the influence of cross-brace diaphragms, truck loading configurations and additional sidewalk railings. A simple approximation was also proposed for the influence of web gap lateral deflection on web gap stress. Gary Peterson from the Mn/DOT Office of Bridges and Structures for their help on the progress of this project. Great appreciation is also expressed to the previous researchers on this series of research, Evan Berglund, Benjamin Severtson and Frederick Beukema for their valuable assistance to this project. Without their previous work and findings, this project could not have been completed efficiently. In particular, special thanks must be given to the advisor, Arturo E. Schultz, whose expertise and guidance were essential to the success of this research. Finally, the author would like to thank her dear friends and family, their support and advice are highly appreciated. Many multi-girder steel bridges built before the 1980s are experiencing distortion-induced fatigue cracking at diaphragm-girder connections. The distortion-induced stresses in the unstiffened web gap area that create the fatigue problems result from secondary forces developed at the ends of the diaphragms as a result of relative deflections between the girders. Cracking due to distortional fatigue seldom results in catastrophic failure of multi-girder steel highway bridges, but its occurrence is frequent enough in the bridge inventory of the Minnesota Department of Transportation (Mn/DOT) to warrant …