Design of reinforced concrete T-beams strengthened in shear with externally bonded FRP composites

Current design guidelines for concrete structures strengthened with externally bonded (EB) fibre-reinforced polymer (FRP) reinforcement do not differentiate between the shear design of rectangular and T-beams. Nonetheless, EB FRP shear-strengthened rectangular and T-beams can have significantly different behaviour. In rectangular beams, EB FRP reinforcement may be bonded to the full depth of the beam web and therefore can effectively join the tension and compression chords. On the other hand, the presence of the slab in T-beams limits the effective depth of the EB FRP reinforcement. This can result in current design guidelines overestimating the shear strength enhancement offered by the FRP reinforcement in the case of T-beams. This paper presents a design model for reinforced concrete (RC) T-beams strengthened in shear with EB FRP composites. In the proposed design model, the FRP contribution to shear resistance is based on the 45° truss analogy and the FRP strain at failure is derived from direct-pull test results from the published literature. The predictions of the proposed model, together with those of Concrete Society TR55 and ACI 440.2R-17, were evaluated using an experimental database comprising 48 RC T-beams. The proposed model had an average predicted-to-experimental shear strength enhancement ratio of 1.062 and a standard deviation of 0.470. The average predicted-to-experimental ratios of TR55 and ACI 440.2R-17 models were 1.139 and 1.277, respectively, with standard deviations of 0.498 and 0.642, respectively. Not only does the proposed model generate more consistent predictions, but it also has a greater conservative nature, providing fewer overestimated predictions compared to current international design guidance.