Buckling of U-Shaped Girders with Top-Flange Lateral Bracing

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. Because the bracing is not utilized once the concrete deck has cured, minimizing the amount of bracing will lead to more efficient designs.

In order to develop a design procedure, the behavior of U-shaped girders with top-flange lateral bracing was studied. A series of experimental tests were selected based on elastic finite element analyses. Variable parameters included brace stiffness, geometry, initial pretension force, and connection detail. Results included girder buckling loads, buckled shapes, and brace forces.