Method for calculating the bearing capacity of BFRP bar-reinforced concrete beams subjected to combined bending, shear, and torsion

: Replacing steel bars with fiber-reinforced polymer (FRP) bars can effectively address the durability issues of infrastructure caused by steel corrosion. Concrete members are often subjected to combined bending, shear, and torsion under seismic loadings, resulting in complex failure mechanisms and making their bearing capacity difficult to predict. However, current research on the performance of FRP bars-reinforced concrete (RC) members under combined loadings is limited, and the specimen sizes are much smaller than actual engineering components. Previous studies have shown that FRP bars-RC members exhibit significant size effects under single shear or torque; nevertheless, the current calculation methods for the combined bearing capacity of FRP bars-RC beams are based on small-scale specimens and fail to reflect the characteristics of full-scale members. Therefore, this study investigates the mechanical performance of BFRP bars-RC beams with different sizes under combined bending, shear, and torsion through a combination of experiments and simulations. The research focuses on the influence of beam height, stirrup ratio, and torsion-shear ratio, and ultimately establishes a combined bearing capacity calculation model that considers size effects. The results indicate that the beams’ size has a minor impact on failure modes, whereas loading conditions (torsion-shear ratio) significantly alter failure patterns. Specifically, as the torsion-shear ratio increases, the failure mode transitions from shear failure to shear-torsion failure, ultimately evolving into torsional failure. Loading conditions also profoundly affect the torsional/shear bearing and deformation capabilities of beams, demonstrating that torque and shear mutually weaken the BFRP bars-RC beams’ shear and torsional performance. Additionally, significant size effects exist in both torsional/shear deformation capacity and strength. When the beam size was proportionally scaled up by four times, reductions of up to 90% in torsional deformation capacity, 60% in shear deformation capacity, 60% in torsional strength, and 60% in shear strength were observed. The proposed calculation formulas for torsional and shear bearing capacities, based on the variable-angle space truss theory, effectively capture the interaction between shear and torsional capacities as well as the size effect of BFRP bars-RC beams under combined loading. The parameters in the formulas possess clear physical significance, facilitating practical application.