Torsional behavior of buildings induced by pounding of base-isolated systems under long-period and pulse-like ground motions

Structural pounding in base-isolated buildings poses severe threats to the superstructure, particularly when mass irregularities introduce accidental eccentricities that trigger torsional corner impacts. However, existing studies remain largely restricted to low-rise base-isolated buildings, leaving the coupled effects of building height and mass eccentricity under distinct ground motion characteristics unquantified. To bridge this gap, this study investigates the 3D torsional pounding responses of 5-story and 15-story base-isolated reinforced concrete frames incorporating code-specified accidental mass eccentricities of 0%, 5%, and 10% subjected to pulse-like and long-period ground motions. The results indicate that torsional pounding acts primarily as a kinematic constraint that restricts the torsional rotation and concentrates the seismic responses at the structural corners. While long-period ground motions trigger high frequency of pounding occurrences, low amplitude impacts, pulse-like ground motions generate severe seismic demands that increase corner story accelerations, resulting in severe localized damage. Specifically, pulse-like motions induce high-velocity impacts that yield large maximum displacement ductility factors on the structural elements. Conversely, long-period ground motions exhibit a highly repetitive, cyclic pounding mechanism, and accumulate extensive energy dissipation, ultimately yielding higher cumulative ductility demands. Furthermore, building height significantly affects damage patterns, where 5-story model experiences up to 130 plastic hinge occurrences, 15-story model escalate damage beyond 1000 beam hinges and 270 column hinges. These findings highlight the critical necessity of incorporating building height and mass eccentricity into performance-based design to maintain the safety of base-isolated structures under different ground motion characteristics.