Seismic performance enhancement of a three-dimensional isolation system under near-fault ground motions: IDA-based fragility analysis

Near-fault earthquakes impose significant demands on building structures due to their intense vertical components and impulsive velocity pulses. These characteristics amplify vertical force fluctuations and floor accelerations, increasing the risk of structural failure. Traditional base isolation systems, primarily designed to mitigate horizontal seismic effects, are often less effective when subjected to strong vertical excitations. To address this challenge, this study proposes a novel three-dimensional isolation system (3D-DIS), which integrates lead rubber bearings (LRB) for horizontal isolation and a friction-based ring-spring vertical isolator (RSVIB-CR) for vertical isolation. A ten-story base-isolated reinforced concrete (RC) frame model is analyzed under near-fault and far-field ground motions using nonlinear time-history analyses. Incremental dynamic analysis (IDA) demonstrates that the 3D-DIS system significantly reduces inter-story drift ratio (IDR) and peak vertical floor acceleration (AZ). The derived fragility functions show increased median capacities ( μ ) and reduced dispersion ( β ) compared to the fixed-base structure, indicating a substantial enhancement in seismic performance in both horizontal and vertical directions. Additionally, parametric sensitivity analysis reveals that optimizing the ratio of horizontal to vertical stiffness plays a crucial role in improving system performance, offering valuable insights for future design optimization of 3D-DIS. This study illustrates that the 3D-DIS can significantly enhance the seismic performance of buildings under near-fault ground motions, effectively reducing the risk of structural failure. Furthermore, it provides performance-based guidance for the design and optimization of three-dimensional isolation systems, particularly for buildings located in near-fault regions.