A modified softened truss model and strength prediction formula for fiber-reinforced polymer reinforced concrete members under pure torsion

A modified rotating-angle softened truss model (RA-STM) is proposed for analyzing the torsional behavior of fiber-reinforced polymer-reinforced concrete (FRP-RC) members under pure torsion. In the modified model, a generalized tension stiffening model is developed with consideration of discrepancy in the modulus of reinforcement. Furthermore, a robust numerical model is developed by refining the RA-STM as a system of nonlinear equations instead of a trial-and-error technique, which can significantly improve computational efficiency. The proposed model is verified against the experimental results of FRP-RC members, yielding an average test-to-predicted torsional strength ratio of 1.04 with a coefficient of variation of 11.66%. Moreover, the predicted torque-twist response, reinforcement strain, concrete strain, and failure modes can closely agree with the experimental results. At last, the adapted model is utilized in a parametric study to investigate the influence of key variables on the torsional behavior of FRP-RC members. Nonlinear regression analyses are performed to propose a semiempirical strength prediction formula and balanced stirrup ratio for the FRP-RC members under pure torsion.