Behavior of Steel Box Girders with Top Flange Bracing

A critical design stage for steel girders occurs during casting of the concrete bridge deck, when the noncomposite 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 and form a quasi-closed section. This paper presents results from a finite-element study on the bending behavior of trapezoidal box girder systems during construction. The results show that large forces can develop in the horizontal truss system due to vertical bending of the box girder. For truss systems with a single diagonal, the forces induced from bending result in large lateral bending stresses in the top flanges of the box. Many current design methods and computer programs do not consider the truss forces and top flange lateral bending stresses induced from bending. Expressions to estimate the bending forces in the lateral truss system as well as the lateral bending stresses in the top flange are developed. A numerical example illustrates the use of the expressions.