Constitutive relationships of concrete for hollow RC members in pure torsion

Abstract

The softened truss model (STM) has been used to analyse the torsional behaviour of concrete members since the 1980s. However, methods based on the STM neglect the effect of concrete tensile stress and hence can only predict the post-cracking responses. A recent rational torsion model, the softened membrane model for torsion (SMMT), successfully incorporates the effect of concrete tensile stress and can predict the entire torque–twist curve, including the pre- and post-cracking branches. However, the SMMT is limited to solid-sectioned reinforced concrete (RC) members and there are still no rational torsion theories available for predicting the entire torque–twist responses of hollow RC members. In this study, the SMMT is modified to circumscribe the shear flow zone in the wall thickness of a hollow cross-section. Based on 38 hollow RC beam specimens from previously reported experiments, the amplification factor for the pre-cracking stiffness and the peak-stress strain of the tensile stress–stress relationship of concrete is quantified to be 1·15, and a multiplier factor for the average tensile and compressive stresses of concrete is proposed for hollow RC members. The new constitutive relationships of concrete and the modified SMMT closely predict the cracking torques of the 38 hollow specimens and estimate the entire torque–twist curves of 27 eligible specimens very well; they are also validated using four large specimens from a recent experiment conducted as part of this study. A detailed comparison and discussion of the analyses and experiments is presented.