Torsional strength limitation of reinforced concrete beams

In order to promote stirrup yielding prior to concrete crushing and avoid over-reinforcement failure modes in reinforced concrete (RC) members, current design codes stipulate limits on the maximum amount of shear and torsional reinforcement. Studies have shown that the shear strength limits introduced based on the plane-truss approach estimate the maximum shear strength and shear failure mode with reasonable accuracy. However, the torsional strength limits derived based on the space-truss analogy and thin-walled tube theory generally overestimate the maximum torsional strength. In this study, the difference between the limiting values introduced in current design codes on the maximum shear and torsional strengths was evaluated by analysing the test results of 406 shear and 153 torsional members. Additionally, experimental tests were conducted on 22 RC beams subjected to torsional moments in order to measure the strain rate of the web concrete directly and investigate the torsional strength limits. The torsional strength limits derived based on the space-truss model overestimated the actual maximum torsional strength. Based on these observations, a lower limit for the maximum torsional strength is proposed in order to avoid over-reinforced torsional failure.