Torsional response of FRP ‐strengthened reinforced concrete beams using a modified fixed‐angle softened truss model

Abstract This study develops a mechanics‐based fixed‐angle softened‐truss model (FA‐STM‐FRP) to predict the nonlinear torque–twist response of reinforced concrete (RC) beams externally strengthened with fiber‐reinforced polymer (FRP) sheets under pure torsion. The formulation couples Bredt's thin‐walled tube analogy with the FA‐STM and integrates established FRP‐sensitive constitutive relationships within a fixed‐crack equilibrium–compatibility framework. Externally bonded FRP is introduced as longitudinal and transverse tensile force resultants, while its influence on cracked concrete is represented through FRP‐modified compression softening and post‐cracking tension stiffening. The evolving effective shear‐flow thickness and effective FRP strain limits associated with debonding, peeling‐off, or rupture are incorporated into an incremental–iterative solution that traces the response up to the first governing terminal limit state. Validation was performed using a database of 34 rectangular RC beams strengthened with FRP and tested under pure torsion, as reported in previous experimental studies. The FA‐STM‐FRP formulation produced mean calculated‐to‐experimental ratios of 1.15 for cracking torque and 1.05 for ultimate torsional strength, with corresponding coefficients of variation of 20% and 10%, respectively. For ultimate torsional strength, the softened membrane model for torsion (SMMT)‐FRP benchmark yielded a mean calculated‐to‐experimental ratio of 1.04 and a coefficient of variation of 9%. The statistical results indicate that both formulations provide comparable overall predictive accuracy within the investigated database. In addition, the governing failure mode predicted by FA‐STM‐FRP was consistent with the experimentally reported classification for all 34 specimens, including concrete crushing, FRP debonding, and FRP rupture. The results support the use of FA‐STM‐FRP as a mechanics‐based fixed‐angle formulation for predicting the overall torsional response and failure mode of FRP‐strengthened RC beams within the range of the available experimental data.