Torsional strength limitation of reinforced concrete beams

In order to promote yielding of stirrups prior to concrete crushing and avoid the over-reinforced failure modes in reinforced concrete (RC) members, the current design codes stipulate limits on the maximum amount of shear and torsional reinforcement. Studies show 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 which are 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 introducing the current design codes on the maximum shear and torsional strengths was evaluated by analyzing the test results of 406 shear and 153 torsional members. Additionally, experimental tests were conducted on 22 RC beams subjected to torsional moments to directly measure the strain rate of the web concrete and investigate the torsional strength limit values. The results indicate that f the torsional strength limit values derived based on the space truss model overestimate the actual maximum torsional strength. Based on these observations, a lower limiting value for the maximum torsional strength is proposed to avoid over-reinforced torsional failure