Improved Softened Membrane Model for Reinforced Concrete Circular Bridge Columns under Torsional Loading

This paper presents an improved softened membrane model for the analysis of reinforced concrete (RC) circular bridge columns under torsion loading. Concrete exhibits higher tensile strength and stiffness due to the strain-gradient effect under torsional loading. Results of previous studies suggest that the inclusion of strain-gradient and bidirectional stress effects improves prediction of the torsional behavior of concrete members. However, the influence of strain-gradient and bidirectional effect stresses on the torsional response of circular columns has never been investigated. With this paper, the authors aim to fill the existing knowledge gap in this important area of research by proposing a modified softened membrane model for torsion (SMMT) for circular columns. The authors also recommend a new tension-stiffening relationship of concrete for improved predictions. The estimated analytical response is compared with experimental data, and a good correlation can be seen. Predictions from the proposed model are also compared with the classical rotating-angle softened truss model (RA-STM) to illustrate the efficacy of the proposed model. Parameters such as postcracking stiffness, peak torque, and twist at peak torque are captured by the improved SMMT better than with the existing RA-STM. The improved model can be used also for developing simplified design equations.