Influence of structure size on bending-shear-torsion combined mechanical properties of concrete beams with BFRP bars

To investigate the effect of beam size on the combined mechanical performance of concrete beams reinforced with Basalt Fiber Reinforced Polymer bars (BFRP-RC beams), the simulation model of BFRP-RC beams under bending-shear-torsion loading was established with the aid of a mesoscale simulation method. The effects of beam size, stirrup ratio and torsion-bending on damage mechanisms of BFRP-RC beams, as well as the size effect law (SEL) were analyzed. Finally, a prediction formula that can characterize the impact of the torsion-bending ratio and stirrup ratio on BFRP-RC beams' SEL under combined loadings was proposed. Research shows (1) the BFRP-RC beams' damage mode changes from shear to torsion with the torsion-bending ratio improving, and there is a mutual weakening effect between shear and torsion. (2) BFRP-RC beams have noticeable size effects under combined loading. Moreover, the torsional and shear size effects are first enhanced and then weakened as the torsion-bending ratio increases. (3) The beams' torsional and shear strengths increase with increasing stirrup ratio, and the corresponding size effects diminish. (4) The SEL proposed in this study is accurate in predicting the shear and torsional capacity of beams under combined loading.