Background: Conventional seismic design strategies based solely on ductility, stiffness enhancement, or mass modification cannot fully prevent structural damage during strong earthquakes. Although base isolation has proven effective for low- and mid-rise buildings, its application to high-rise structures is generally limited by excessive flexibility, longer vibration periods, and increased displacement demands. Inter-story seismic isolation (ISI) has recently emerged as a promising alternative; however, the influence of isolator location on the seismic performance of high-rise steel buildings has not yet been comprehensively established.
Methods: This study evaluates the seismic behavior of 20- and 25-story steel buildings with dual moment-resisting frame and eccentric bracing systems equipped with inter-story seismic isolators. Lead-rubber bearings with different stiffness and damping properties were modeled in OpenSees and installed at various elevations corresponding to changes in column sections, including the base level. Modal analysis and Incremental Dynamic Analysis (IDA) were performed using 22 far-field ground motions in accordance with FEMA P-695. Structural performance was assessed at the Immediate Occupancy (IO), Life Safety (LS), and Collapse Prevention (CP) performance levels using spectral acceleration, floor acceleration, and inter-story drift as evaluation criteria.
Results: The results demonstrate that the effectiveness of inter-story seismic isolation is highly dependent on its installation level. The optimum locations were identified as the 20th floor for the 25-story frame and the 15th floor for the 20-story frame. At these elevations, the isolated structures exhibited higher seismic capacities at both LS and CP performance levels while reducing floor accelerations by approximately 20% and 13%, and inter-story drifts by approximately 8% and 3.5%, respectively, in the isolated portion of the buildings. In contrast, placing the isolator at lower stories, including the base level, decreased structural capacity and adversely affected seismic performance. Furthermore, increasing isolator stiffness alone did not improve structural response, demonstrating that stiffness and damping must be optimized simultaneously to achieve the desired seismic performance.
Conclusion: Inter-story seismic isolation provides an effective seismic protection strategy for high-rise steel buildings when implemented at an optimum elevation. Properly positioned isolators enhance seismic capacity while significantly reducing acceleration and drift demands without adversely affecting the lower portion of the structure. Conversely, lower installation levels may compromise structural stability and are therefore not recommended. Based on the obtained results, a practical design flowchart is proposed to facilitate the identification of the optimum installation level of inter-story seismic isolators in future high-rise building designs.