Brace Forces in Steel Box Girders with Single Diagonal Lateral Bracing Systems

Abstract

Trapezoidal steel box girders (also known as tub girders) may be at their critical stage during construction because the noncomposite steel section must support both the wet concrete and the entire construction load. A lateral bracing system is usually installed at the top flange level to form a quasi-closed box, thereby increasing the torsional stiffness during construction. Typical lateral bracing includes a single-diagonal type (SD type) and a crossed-diagonal type (XD type). The brace forces in box girders with XD type lateral bracing systems can now be estimated with good accuracy. In some horizontally curved box girders with SD-type systems, substantial axial forces, possibly up to 35% of the total force, are developed in struts due to induced torsion and distortion, which are neglected in previous studies. In this study, analytical equations were formulated to compute the brace forces developed in bracing members by taking into account bending and torsional actions of tub girders with SD-type lateral bracing systems. Brace forces in both diagonals and struts can be estimated by these new equations more accurately than with the previous procedures. Member forces due to bending and torsion computed using the new equations were compared with those evaluated by three-dimensional finite element analyses, and excellent correlation was found.