Experimental study on shear performance of reinforced concrete beams under fire conditions

To investigate the shear performance of reinforced concrete (RC) beams under thermo-mechanical coupling, eight full-scale simply supported RC beam specimens were designed and fabricated based on the ‘strong bending and weak shear’ principle. A four-point concentrated loading at the top of the beam method was employed to simulate uniform loading, and static load tests were conducted on two specimens at the ambient temperature. Subsequently, utilizing a large-scale fire simulation test system, the remaining six beams were subjected to fire resistance tests under combined high-temperature and uniform loading, considering the influence of varying load ratios. Test results revealed that both RC beams with and without stirrups exhibited shear failure under static loading at ambient temperature. Under thermo-mechanical coupling, the simply supported beams without stirrups exhibited distinct shear failure characteristics upon reaching their fire-resistance limits. Furthermore, the combined effect of the dowel forces generated by the longitudinal reinforcement bridging the primary inclined cracks and the elevated temperatures led to severe splitting failure of the concrete cover along the longitudinal reinforcement. In contrast, beams with stirrups did not exhibit significant shear failure but showed fully developed flexural cracks and pronounced bending deformation, indicating a shear-flexure failure mode. The findings demonstrate that the load ratio has a major impact on the fire-resistance limit of RC beams with and without stirrups. As the load ratio increased, the measured fire-resistance limit decreased. The fire-resistance limit of beams with stirrups decreased linearly with increasing load ratios, while that of beams without stirrups followed a nonlinear trend. At lower load ratios, the rate of decrease was relatively low, whereas at higher load ratios, it became considerably larger.