Mechanism transition and dynamic strut-and-tie modeling of shear failure in RC columns under near-support impacts

Numerous collision incidents involving ships, vehicles, or rockfalls impacting bridge piers indicate that these collisions often occur near the base of columns. Owing to the small shear span, columns are more prone to shear failure. However, current studies have not yet clarified the shear failure mechanism and resistance of columns under near-support impacts. To address this gap, this study conducts an in-depth investigation into the shear failure mechanisms of reinforced concrete (RC) columns subjected to near-support impact. Through drop hammer impact experiments, the effects of impact location, speed, and shear reinforcement were analyzed. Using Digital Image Correlation (DIC) technology, the strain distribution characteristics and damage evolution of the columns during the entire impact process were clarified. Two distinct shear failure mechanisms were identified: punching shear and diagonal shear. The impact process can be divided into two stages: the "mass-related stage", where inertia effects are dominant in the early impact stage, and the "stiffness-related stage", where inertia effects diminish. Punching shear failure occurs during the mass-related stage, with the peak impact force also appearing at this stage, when the structural impact resistance mainly relies on inertia. Diagonal shear failure occurs during the stiffness-related stage. The study elucidated the transformation mechanism between these two types of shear failure and proposed a method for determining the failure mode. A Dynamic Strut-and-Tie Model (DSTM) was also established to describe diagonal shear failure under near-support impacts, and a formula was derived to calculate the shear capacity under such conditions. The validity of this DSTM was confirmed through experimental and literature data. This research not only fills the research gap regarding shear failure mechanisms under near-support impacts but also introduces a mathematical-physical model and calculation method for predicting the shear capacity of RC columns. • Revealed the characteristics of damage distribution and evolution throughout the impact process by the drop hammer tests. • Clarified the role of inertial effects during the impact process. • Determined the transition mechanism and identification method for punching shear and diagonal shear failures. • Proposed a Dynamic Strut-and-Tie Model (DSTM) to describe the diagonal shear failure under near-support impacts.