Distortional Loads and Brace Forces in Steel Box Girders

Abstract

Trapezoidal steel box girders are often used in curved bridges due to the large torsional stiffness that results from the closed cross section. However, the torsional loading on the box girders can cause the cross section to distort from its original shape, which results in distortional stresses in the plates of the box section. The distortional stresses are controlled by internal cross frames that restrain the cross section from distortion. Current design specifications do not account for forces in the bracing members due to box girder distortion. This paper presents an analytical study on the distortional behavior of box girders with a trapezoidal cross-sectional shape. Typical torsional loads on curved box girders are discussed, and the distortional components of these applied torsional loads are studied. The distortional components from different torsional loads on trapezoidal box girders are identified and used to derive expressions for the brace forces in the internal cross frames for quasi-closed box girders. The results from the approximate equations for brace forces due to cross-sectional distortion are verified by three-dimensional finite-element analyses.