Experiments and calculation of U-shaped thin-walled RC members under pure torsion

•Four U-shaped thin-walled RC members with span of 6.65 m were tested in torsion.•Warping moment controlled the failure mode of such members.•Proportion between circulatory and warping torsion varied after yielding of bars.•Simple formulas for flexural cracking torque and ultimate torque were proposed.

U-shaped thin-walled RC (Reinforced Concrete) members have been widely applied in construction projects of urban rail viaduct in China. Researches about the torsional response of this kind of structures are extremely limited. Four large U-shaped thin-walled RC beams with both ends restrained were tested to investigate the pure torsional response. The detailed information includes torque–rotation curves, crack patterns, failure mode and steel bar strains. Typical flexural failures at support and mid-span where the warping moment predominates were observed in all four test specimens. And at quarter span, stirrups strain caused by circulatory torsion reached nearly 80% of total stirrup strains at failure stage, due to variable proportion between circulatory torsion and warping torsion after yielding of longitudinal steel bars. Finally, a simple method to calculate ultimate torque of such structures was proposed here. And it agrees well with tested results.