Model for determining the torsional resistance of reinforced concrete beams strengthened with externally bonded FRP sheets

Abstract

Reinforced concrete (RC) structural members, especially cantilever beams, bridge girders, curved beams, and eccentrically loaded beams, may be subjected to significant torsional loading. Although extensive research has been done on the strengthening of RC structures, fiber-reinforced polymer (FRP) laminates are commonly used to strengthen torsional structures. Therefore, some torsional design guidelines for RC structures strengthened with external FRP sheets were based on the equilibrium truss theory, such as fib Bulletin 90, NCHRP Report 655, and CNR-DT 200/2013. Two parameters that significantly influence the prediction of torsional capacity, namely the effective strain and the inclination angle of the main concrete crack, were also mentioned. However, there was no consensus among current standards and technical guidelines in determining the effective strain. In addition, the inclination angle was prescribed based on standards and technical guidelines for RC structures subjected to torsion. A strut-and-tie model proposed in this study was used to calculate this inclination angle. To evaluate the design methods in the standards and technical guidelines, 35 specimens strengthened with CFRP and GFRP sheets were collected in previous experimental studies. The calculation results showed that the calculation model, according to the standards and technical guidelines combined with the strut-and-tie method, was conservative in predicting the torsional resistance.