EXPERIMENTAL STUDY ON THE INFLUENCE OF SHEAR-SPAN RATIO ON THE CRACK RESISTANCE AND SEISMIC PERFORMANCE OF RC FRAME BEAMS

To explore the influence of shear-span ratio on the crack resistance and seismic performance of RC frame beams, six frame beam specimens were designed with shear-span ratios ranging from 1.7 to 4.0. By conducting monotonic stepwise loading tests, the crack development in negative bending moment zones of the frame beams under vertical loads was investigated. And then by conducting low cycle reciprocating loading tests, the bearing capacity, deformation performance and failure mode of the frame beams under earthquake actions were studied. The test results show that under vertical loads, bending cracks first appear at the beam ends of the frame beams, while shear inclined cracks appear later but develop rapidly. Due to the influence of shear force, the width of cracks in the negative bending moment zone of frame beams exhibits significant variability, the tensile stress of longitudinal steel bars increases by about 25% compared with the calculation results according to the "Code for design of concrete structures" GB 50010−2010, 2015 ed, and the maximum width of bending cracks increases accordingly. Based on the characteristics of frame beam cracks and the measured stirrup stress, a formula for calculating the width of shear inclined cracks has been established. Under the action of reciprocating loads, as the shear-span ratio decreases, the proportion of inclined cracks in negative bending moment zone of frame beams increases, and the bending failure mode of concrete crushing and peeling at the beam end gradually changes to the shear failure mode of concrete fragments being squeezed upward and downward by the intersecting main diagonal cracks at the frame beam ends. The pinching phenomenon of the load-displacement hysteresis curves intensifies, the stiffness degradation accelerates, the equivalent viscous damping coefficients decrease, and the energy dissipation capacity decreases; the axial residual elongation of the frame beams gradually increases, and the maximum residual elongation can reach 0.86%.