Structural pounding in buildings with vertical stiffness irregularity under earthquake sequences

Structures are typically designed for single seismic events, whereas real earthquakes often consist of multiple tremors, including foreshocks, mainshock, and aftershocks. Many buildings in seismic regions exhibit vertical stiffness irregularity, making this issue particularly critical. A six-story steel moment-resisting frame was designed as the Base Model, and variations in stiffness distributions were created to study pounding effects when placed adjacent to a heavier structure (Adjacent Model). The investigation considered repeated earthquakes (mainaftershock sequences) at two seismic risk levels. The results reveal that the seismic separation gap is the primary factor governing pounding risk, while stiffness distribution and cumulative damage are significant contributing factors. Code-based separation gaps prove generally conservative and sufficient to prevent pounding. However, structures with reduced stiffness in upper stories demonstrate particular vulnerability and require special consideration. Consequently, gap sizing should be case-specific and account for repeated seismic events through practical assessment methods. Furthermore, while structural pounding sharply amplifies peak relative floor accelerations, seismic force demand, maximum inter-story drift ratio, and residual displacement are governed primarily by lateral stiffness distribution rather than by pounding effects. These findings underscore the necessity of incorporating both multiple seismic events and vertical stiffness irregularities in seismic design practice to effectively mitigate pounding risks.