Background: Conventional force-based seismic design may not adequately control structural damage, excessive deformation, and collapse, particularly under severe earthquakes. Steel special moment-resisting frames may experience considerable interstory drift and plastic-hinge rotations during ground motions. Therefore, the use of seismic isolation systems, such as Lead Rubber Bearings (LRBs), can be an effective approach for improving structural performance and reducing collapse vulnerability. This study investigates the effectiveness of LRB isolators in enhancing the seismic behavior and collapse capacity of steel special moment-resisting frames.
Methods: Three-, six-, and nine-story steel special moment-resisting frames were designed and evaluated in isolated and fixed-base configurations using a performance-based seismic design framework. Nonlinear static and nonlinear time-history analyses were conducted to assess the structural response at the Immediate Occupancy (IO), Life Safety (LS), and Collapse Prevention (CP) performance levels. Incremental Dynamic Analysis (IDA) was performed using selected ground-motion records, which were scaled and matched with the Standard 2800 design spectrum at the design-earthquake and maximum-considered-earthquake levels. Interstory drift, beam and column plastic-hinge rotations, seismic fragility, collapse probability, Collapse Margin Ratio (CMR), and Adjusted Collapse Margin Ratio (ACMR) were used as the main evaluation criteria. The collapse assessment was conducted in accordance with FEMA P695 criteria.
Results: The results demonstrate that the use of LRB isolators reduces interstory drift demands and beam and column plastic-hinge rotations at three performance levels. Isolated frames exhibited lower seismic fragility and collapse probability than their corresponding fixed-base frames. The CMR and ACMR values increased for the three-, six-, and nine-story isolated frames, indicating an enhanced collapse capacity. The beneficial effects of seismic isolation were pronounced in the low-rise, short-period frame; nevertheless, improvements were also observed in the taller frames. The isolated systems showed more uniform seismic responses and lower record-to-record dispersion, particularly under severe and near-fault ground motions.
Conclusion: LRB isolation is an effective and reliable strategy for improving the seismic performance of steel special moment-resisting frames. By reducing deformation and plastic-rotation demands, decreasing fragility and collapse probability, and increasing collapse capacity, LRBs can significantly enhance the safety and resilience of steel frames within a performance-based seismic design framework.