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Load Testing a Double-Composite Steel Box Girder Bridge

This paper describes the load testing and results of a full-scale double-composite steel box bridge girder tested in a cooperative research study by the University of South Florida, the URS Corporation, and the Florida Department of Transportation to develop design rules for double-composite steel bridges. The section modeled is a 53-ft-long, 4-ft-deep trapezoidal double-composite steel box girder that replicates the negative moment region of a continuous steel box girder bridge. The specimen was tested to evaluate fatigue, service, and ultimate design provisions. After 5.6 million cycles of fatigue loading, there was a 17% loss in stiffness. The service tests showed that 1% reinforcement for the top slab was adequate. The specimen failed because of crushing of the bottom concrete slab and buckling of the bottom steel flange in the final service test. Finite element analysis was used to simulate the failure and showed that the bottom flange buckles at relatively low loads, but because of composite action with concrete at shear stud locations it can still effectively carry additional compressive load until the bottom flange yields from plastic buckling. The concrete bottom slab carries all additional load until it crushes. Supplementary provisions are proposed for designing double-composite members. They limit the maximum compressive stress in the bottom slab to 0.6 fc′ and set a requirement for the location of the neutral axis to ensure ductility. Because of the strain limit on the concrete bottom slab, it may not be possible to achieve net section plastic capacity.