Effect of axial compression ratio and beam end reinforcement ratio on the failure mode of RC frames

Numerous seismic damage investigations have demonstrated that reinforced concrete (RC) frame structures tend to exhibit a Strong-beam Weak-column failure mechanism, which contradicts the intended Strong-column Weak-beam design philosophy. To explore the underlying causes of this discrepancy and identify effective strategies to enhance the realization of the Strong-column Weak-beam behavior, the mechanical performance of RC frames with varying axial compression ratios and beam-end reinforcement ratios was analyzed using the finite element analysis software ABAQUS. Key structural characteristics of failure modes, such as deformation patterns, concrete tensile damage distribution, stress distribution in slab reinforcement, concrete compressive strain in columns, plastic hinge distribution, and structural displacement ductility were examined. The results indicate that merely reducing the amount of reinforcement at beam ends has limited effectiveness in altering the failure mode and improving displacement ductility, even reduce the amount of top reinforcement at beam end to 33 % of designed quantity which is below the minimum value specified in structural design codes, the failure mode and ductility of the RC frame still cannot be substantially improved. Whereas lowering the axial compression ratio significantly enhances structural performance, when the high axial compression ratio is adjusted to a low one, for instance, from 0.9 to 0.3, the ductility of the frame structure is significantly improved, and the plastic hinge at the column end of the upper floor is successfully shifted to that beam end. Furthermore, the influence of the monolithic slab on the failure behavior of RC frames is twofold: it not only involves the contribution of slab reinforcement parallel to the beam ribs, but also stems from the presence of the monolithically cast slab-beam-column system, which improves the overall structural integrity performance. Moreover, to better measure the participation effect of the cast-in-place floor slab and reflect the potential participation capacity of the monolithic slab, the calculation formulas for the bending resistance reserve of the exterior and interior the middle frame were respectively proposed. This formula breaks through the rigid limitation of the 6 times of the slab thickness (6t) as the width of the beam end flange suggested by the code, and better estimates the participation effect of the cast-in-place floor slab.