Analytical model for torsional behavior of reinforced concrete beams strengthened with fiber reinforced polymer sheets

This study presents a new analytical model for predicting the full torsional response of reinforced concrete (RC) beams strengthened with fiber-reinforced polymer (FRP) composites. The proposed approach extends the Modified Softened Variable Angle Truss Model by incorporating the confinement effects of FRP materials on concrete and accounting for the softened compressive and tensile behavior of concrete. These enhancements improve the accuracy in predicting the torsional response. An iterative trial-and-error algorithm, implemented in MATLAB, based on the modified constitutive relationships of concrete under the influence of external FRP reinforcement and the equilibrium conditions of the beam. The validity and reliability of the proposed model were assessed using 38 experimental RC beam specimens strengthened with carbon-FRP (CFRP) or glass-FRP (GFRP) in various practical configurations reported in the literature. For comparative evaluation, the torsional strengths of the same specimens were also predicted using the fib Bulletin 90 strut-and-tie approach. The analytical results show closely match experimental results, capturing complete torque-twist curves of the tested beams. In contrast, predictions obtained from fib Bulletin 90 were found to be conservative, yielding an average strength ratio of approximately 0.81 relative to the experimental results.