Analytical Approach to the Torsional Behavior of Concrete Beams Reinforced with Fiber-Reinforced Polymer Bars

Fiber-reinforced polymer (FRP) bars have emerged as a promising alternative to conventional steel reinforcement for improving the durability of reinforced concrete (RC) members in corrosive environments. Despite increasing experimental research, analytical models capable of capturing the pure torsional response of FRP bar-reinforced concrete beams remain scarce. This study presents a modified softened membrane model for torsion (SMMT) for solid FRP-RC beams. The proposed formulation incorporates an FRP-compatible strain-coupling relationship through a modified Hsu/Zhu approach to account for poisson’s effect. The model is validated against sixteen rectangular FRP-RC beam tests reported in the literature. Predicted cracking torque, ultimate torque, torque-twist response, and selected stirrup strain responses show good agreement with experimental results. A parametric study is further conducted to quantify the influences of concrete strength, FRP elastic modulus, and longitudinal and transverse FRP reinforcement ratios. The proposed model provides a reliable analytical framework for evaluating the torsional behavior of FRP-RC beams.