Overload Flexural Distribution Behavior of Composite Steel Girder Bridges

Using equations proposed by the AASHTO LRFD specifications, live-load distribution factors in bridge superstructures were calculated based on the linear-elastic behavior of the system. In this study, the applicability of these equations in predicting girder distribution behavior in the presence of high material nonlinearity and oversized loading scenarios was investigated. Two representative composite steel girder bridge superstructures, which are in service in the state of Michigan and had been previously subjected to a live-load testing program, were selected for this study. Sixteen cases were analyzed to study the effect of boundary conditions, loading position, and load configuration on the girder distribution behavior of the selected bridges as they approach their ultimate capacities. Comparing the results obtained from nonlinear finite-element analysis with those proposed by the AASHTO LRFD specifications demonstrated that the code-specified values for the distribution factors are overly conservative. Much of the variation in distribution behavior occurs when plasticity propagates in steel girders, whereas the initiation of material nonlinearity in the concrete deck (cracking) has a negligible effect on the evolution of distribution behavior. The results of this investigation also highlight the importance of accurate modeling and analysis in the load rating practices of existing in-service bridge superstructures. DOI: 10.1061/(ASCE)BE.1943-5592.0000671. (C) 2014 American Society of Civil Engineers.