Dynamic response and damage mechanism of RC beam-column sub-assemblage under middle-joint drop-weight loading with different impact velocities

When a frame structure experiences a close-field explosion, an instantaneous tensile force may arise in the column due to significant lateral deformation. This force acts downward on the beam-column joint, generating an impact action in the affected span. Against this backdrop, this paper investigated the dynamic response and damage mechanism of an RC beam-column sub-assemblage subjected to middle-joint drop-weight loading with varying impact velocities. Firstly, a drop hammer was utilized to apply the impact load on the middle joint of the sub-assemblage with two distinct impact velocities, determined based on the equivalence of the impact force and axial tensile force induced by the blast scenarios. Then numerical model was established and validated through the test, followed by parametric studies covering a wider range of impact velocities. The study thoroughly examined the effect of impact velocity on the damage mode and internal force distribution of the beam of the sub-assemblage, elucidating the damage mechanism. It was found that in the dynamic response process, the beam got through a local response and global response in successive. There existed a critical velocity distinguishing two different damage mechanisms of the sub-assemblage. When the impact velocity was lower than the critical velocity, a flexure deformation appeared in the beam, resulting to a reverse arch action at local response stage. Compressive arch action and tensile catenary action emerged in success at global response stage. When it was higher than the critical velocity, a shear damage occurred at the beam end near middle joint at local response stage, followed by tensile catenary action at global response stage. An equivalent single degree of freedom (SDOF) model was employed to predict the peak middle joint displacement (MJD) of the sub-assemblage under different impact velocities. Two equivalent stiffness models were proposed respectively for the velocity lower and higher than the critical velocity. The prediction results were verified against the numerical model.