Responses of reinforced concrete frame structures under mainshock–aftershock earthquake sequences

Aftershocks significantly amplify structural damage and collapse risk. However, current seismic design codes ignore sequential effects that may lead to unconservative assessments. This study experimentally investigates the impact of mainshock–aftershock sequences on seismic performance of the existing reinforced concrete (RC) frame structures with distinct “beam-hinge” and “column-hinge” controlled mechanisms. Shaking-table tests were conducted on four experimental frame structures to evaluate the interstorey drift and floor acceleration responses under a series of designed mainshock-aftershock sequences. The experimental results indicate that the structural response is strongly governed by sequence-dependent resonance effects arising from the interaction between cumulative damage and the frequency characteristics of the input ground motions. Under unfavourable resonance conditions, aftershock-induced displacement and acceleration demands may exceed those generated by the preceding mainshock, even when the aftershock intensity is lower, with this phenomenon being particularly pronounced in “column-hinge” controlled frames. In addition, the influence of pre-existing damage induced by preceding earthquakes on subsequent structural response was quantitatively evaluated, providing direct insight into cumulative damage evolution. The results demonstrate that maximum displacement alone is insufficient to accurately evaluate cumulative damage evolution under sequential seismic loading. These findings highlight the necessity of explicitly incorporating mainshock–aftershock effects into seismic design provisions and post-earthquake performance assessment frameworks. The results provide experimental evidence to inform more resilient seismic design and risk evaluation frameworks for RC frame structures.