Distortion of steel tub-girder with top flange bracing considering warping restraint effect

Abstract

During the construction stage of the steel-concrete composite box girder, the steel tub-girder has to support its weight, the self-weight of the wet concrete, and other construction loads. To improve the torsional stiffness of the steel tub-girder, the top flange bracing is often used to connect the top flanges to make “semi-closed” cross sections. The steel tub-girder with top flange bracing can form a closed shear flow under torsion, and thereby causing the distortion effect. However, the distortional behavior of this type of structure remains unclear. In this paper, the equivalent thickness of the fictitious top flange is first derived based on the restrained torsion theory. Subsequently, the errors of distortion loads given by the conventional load decomposition method which ignores the effect of warping restraint are derived, and are further verified by finite element analysis. It is demonstrated that the effect of warping restraint on the distortion is significant in the continuous tub-girder bridges with the top flange bracing. Therefore, the corresponding correction coefficients for the distortion loads caused by eccentric vertical loads and the girder axis curvature are proposed, and the simplified formulas for estimating these correction coefficients are established based on parametric analyses. Finally, the calculation method for the internal force of cross frames is modified and the accuracy of the proposed method is validated with results from finite element analysis.