Maximum torsional reinforcement ratio of reinforced concrete beams

The amount of maximum torsional reinforcement is specified to prevent the failure of concrete compressive struts before the reinforcement yielding in reinforced concrete members subjected to torsion. The maximum torsional reinforcement presented in the design codes, however, differs by up to more than three times. This study therefore aimed to theoretically derive the maximum torsional reinforcement ratio. The proposed ratio was derived based on the equilibrium and compatibility conditions. The parametric study results of the torsional behavior analysis were also used to reflect key variables, such as tensile resistance of concrete, the cross-sectional area surrounded by shear flow, the concrete softening factor, and the average stress factor, in a simple form. In addition, the maximum reinforcement was presented not only in transverse direction but also in longitudinal direction to predict the failure modes of torsional members according to the yielding of reinforcements. The validation was performed by comparison with the experimental results of 98 beams under pure torsion collected from the literature, and the maximum torsional reinforcement ratio proposed in this study was found to reflect the failure modes of reinforced concrete beams subjected to pure torsion better than those suggested by other researchers and the design codes.