TOP FLANGE BRACING SYSTEMS FOR BOX GIRDERS

A critical design stage for steel girders occurs during casting of the concrete bridge deck, when the non-composite steel section must support the wet concrete and the entire construction load. Although a composite box girder has a high torsional stiffness in the completed bridge, during construction the open section is relatively flexible in torsion. A horizontal truss system is usually installed at the top flange level to increase the torsional stiffness. Finite element results presented in this paper show that large forces can develop in the horizontal truss system due to vertical bending of the box girder. Many current design methods and computer programs do not consider the truss forces induced from bending. Expressions to estimate the forces in the lateral truss system due to bending are presented.