Design and Evaluation of Steel Bridges with Double Composite Action

This report presents findings from a cooperative USF/URS/FDOT research study undertaken to develop design rules for double composite steel bridges. In the study, a 48 ft long, 16 ft wide, 4 ft. 10⅛ in. deep trapezoidal HPS 70W box section designed to AASHTO’s LRFD 2004 specifications was fabricated and tested. The section has an 8 in. thick top slab and a 7 in. thick bottom slab and represented the entire negative moment region of a full-size, continuous bridge. The specimen was tested to evaluate fatigue, service and ultimate provisions of the AASHTO code. Instrumentation was provided to monitor load, strain, slip deflection and crack widths at critical locations. Results showed that after 5.6 million cycles of fatigue loading there was a 17% loss in stiffness but no slip. The service tests showed that 1% reinforcement for the top slab is adequate. The specimen failed due to crushing of the bottom slab caused by buckling of the thin (3/8 in.) bottom 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 due to composite action with concrete at shear stud locations, it can still effectively carry additional compressive load until the bottom flange yields due to plastic buckling. Subsequently the concrete bottom slab carries all additional load until it crushes. Supplementary provisions are proposed for designing double composite members. These limit the maximum compressive stress in the bottom slab to 0.6f\u27c and set a requirement for the location of the neutral axis to ensure ductility. Due to the strain limit on the concrete bottom slab, it may not be possible to achieve net section plastic capacity. An illustrative numerical application of these rules is included as a MATHCAD file