Torsional Response of FRP-Strengthened Reinforced Concrete Beams Using a Modified Fixed-Angle Softened Truss Model

This study develops a mechanics-based fixed-angle softened-truss model (FA-STM) to predict the full nonlinear torque–twist response of reinforced concrete (RC) beams externally strengthened with fiber-reinforced polymer (FRP) sheets under pure torsion (FA-STM-FRP). The proposed model integrates Bredt’s thin-walled tube analogy with the FA-STM, explicitly incorporating the tensile contribution of externally bonded FRP as an additional resisting mechanism. Key advancements include the modification of concrete constitutive relationships to account for FRP effects on compressive softening and enhanced post-cracking tension stiffening. The FA-STM-FRP model was rigorously validated against a comprehensive database of 34 FRP-strengthened RC beams compiled from the literature, encompassing a wide range of concrete strengths, reinforcement ratios, section geometries, and FRP configurations. The model accurately captures the complete torque–twist curve, cracking torque, ultimate torsional strength, and governing failure modes. Statistical comparisons demonstrate predictive accuracy comparable to the established softened membrane model for torsion-FRP (SMMT-FRP), with mean ratios of 1.15 for cracking torque and 1.05 for ultimate strength, while offering superior physical interpretability and consistent treatment of FRP–concrete interaction effects. The proposed model thus provides a practical, mechanics-based tool for performance-based assessment and design of FRP-strengthened RC beams under torsion, supporting efficient retrofitting strategies in structural engineering practice.