Analysis of the Torsional Effects in Thin-Walled Box Girders Considering the Variations in Flange Thickness

To reasonably analyze the restrained torsion in concrete thin-walled box girders with variable flange thickness and quantify the impact of corner supports on torsional behavior, the torsional effect analytical theory is derived based on Umansky's second theory. Starting with the differential equation, initial parameter solutions for torsional internal forces are derived for simply supported box girders subjected to concentrated and uniformly distributed torques. A practical engineering example is used to compute the torsional generalized internal forces and stress distribution of the box girder. The results indicate that, compared to the solutions that ignore the flange thickness variation, those that account for it agree more closely with finite-element analysis results and demonstrate higher accuracy. The arrangement of corner supports increases the torsional stiffness and reduces warping internal forces to some extent. In this example, the torsional stress at the cross-sectional corner significantly decreases, with the warping axial stress at the upper flange–web junction decreasing by 31.6% and the torsional shear stress at the bottom flange–web junction decreasing by 36.9%. Additionally, the stress reduction effect induced by the corner supports is localized. Compared to web-type corner supports, flange-type corner supports more effectively suppress restrained torsion in the box girder, leading to a greater reduction in the torsional stress.