Top Lateral Bracing of Steel U-Shaped Girders

Steel box girder systems are being used more frequently for curved bridges because of their torsional stiffness and aesthetic appearance. These systems typically consist of U-shaped girders placed side-by-side with a composite concrete deck acting as the top flange. A critical design stage for these girders occurs during casting of the bridge deck, when the non-composite steel section must support the entire construction load, including the wet concrete. During this period, the top flanges are in compression and are susceptible to lateral-torsional buckling. Lateral bracing, typically in the form of a horizontal truss system, is installed to prevent the flanges from buckling and to increase the torsional stiffness of the girders. There is currently no existing codified design method for the lateral bracing of U-shaped girders. This research report describes the analytical and experimental tests conducted on steel U-shaped girders with top-flange lateral bracing. The scope of the study focused on X-type top lateral truss systems. A series of pilot tests were also conducted to evaluate the performance of girders with metal deck panels used as a top lateral bracing system. Results include girder buckling loads, buckled shapes, brace forces, and girder torsional stiffnesses. Deficiencies in current code specifications are also discussed.