Two-stage dynamic shear response of square RC columns under lateral impact: Mechanisms, inertial force analysis, and punching shear resistance model

Reinforced concrete (RC) columns are critical components vulnerable to lateral impact loads (e.g., vehicle collisions, ship strikes), yet existing studies inadequately address the distinct shear resistance mechanisms between punching shear and diagonal shear failures under dynamic conditions, as well as their implications for assessment methodologies. To address these gaps, this study systematically investigates the dynamic shear behavior of square RC columns through high-fidelity finite element (FE) simulations validated against pendulum impact tests, with emphasis on two-stage shear mechanisms, inertial forces, and parametric effects. The simulation results indicate that the impact response of RC columns can be divided into two distinct stages: a local response stage characterized by the synchronous growth of the forces in the upper and lower sections at the impact point and a global response stage governed by flexural-shear interactions. The peak dynamic shear force in the shear plug decreases with increasing impact velocity beyond a critical threshold, which is essentially consistent with the transition between these two mechanisms. Parametric analyses reveal the sensitivity of shear resistance to impact velocity, shear-to-span ratio, boundary conditions, axial compression ratio, and cross-sectional dimensions. Crucially, the inertial force of the shear plug is proven to correlate with both the peak impact force and the shear plug mass, enabling the development of a semi-empirical inertial force formula. Furthermore, an analytical model for predicting the punching shear resistance in the local stage is proposed, explicitly incorporating inertial forces and validated against 80 experimental and numerical cases. This work advances the understanding of dynamic shear resistance mechanisms and provides a validated analytical framework for predicting punching shear failures in laterally impacted RC columns, offering both mechanistic insights and practical design implications.