Interaction of bending, shear and torsion in U-shaped thin-walled RC girders

U-shaped thin-walled concrete girders have been widely applied in the urban construction of rail viaducts in China as well as worldwide, and these girders are usually subjected to the combined actions of bending, shear and torsion. However, the interactions of bending, shear and torsion in these girders are unclear. The behaviors of these girders under the combined actions are analytically studied, and a method predicting the failure modes and calculating the ultimate loads is developed here. Also, numerical models are built and calibrated with the test results. Then case studies are conducted based on the above calculating method and numerical models. It is found that when a specific set of U-shaped thin-walled RC girders are under the combined actions, with the increase of torque-bending moment ratio, the failure type tends to change from flexural failure to shear failure. Besides, shear failures are more likely to occur in the following cases: simply supported girders, girders with large or small span lengths, and girders with large reinforcement ratios of longitudinal bars to stirrups. Finally, the interaction relationships of bending, shear and torsion in these girders are derived. In a flexural failure, the relationship between bending moment and torque is basically linear; the relationship between torque and shear force is basically quadratic; and the relationship between bending moment and shear force is quadratic. In a shear failure, the relationship between torque and shear force is basically linear.