## 1 Introduction
Finite element analysis results demonstrated that the device effectively reduces structural acceleration and deformation, significantly suppressing the dispersion caused by the randomness and type of input motion, thereby enhancing the predictability of structural performance. Despite the continuous optimization of isolation systems, which can effectively improve the seismic capacity of structures, the uncertainty of earthquake magnitudes can still lead to structural failure. The 2011 Fukushima nuclear power plant accident, where beyond-design-basis seismic events were a major cause [28], resulted in equipment damage and severe nuclear leakage, drawing the attention of researchers and nuclear industry professionals to seismic probabilistic safety assessment. Fragility analysis is a crucial component of seismic probabilistic safety analysis, providing the conditional probability of structural failure under different earthquake magnitudes to evaluate the seismic performance of structures. The Incremental Dynamic Analysis (IDA) method, as an important dynamic elastoplastic analysis method in seismic fragility analysis, can simulate the entire process from elasticity to elastoplasticity until collapse, used for structural collapse capacity analysis and performance evaluation. Many scholars have conducted extensive research on the theory and application of the IDA method [29]. The IDA method was first proposed by [PERSON] [30] in 1977, and over the following 40 years, it was widely studied by scholars and applied in many practical engineering applications. In 2002, [PERSON] et al. [31] provided a comprehensive summary of the IDA method. The PEER Center applied IDA for the seismic performance evaluation of the University of California's science building [32]. [PERSON] et al. [33] proposed an approximate IDA curve method based on the Pushover analysis method, termed Incremental Modified Pushover, and by comparing this method with IDA and modal dynamic analysis, they showed it to be the least time-consuming, albeit with more conservative results. [PERSON] et al. [34] used the IDA method to assess the exceedance probabilities of different limit states related to reinforced concrete superstructures and isolation layers, determining seismic fragility curves. Recently, fragility assessment has also been applied to nuclear power plant structures and related equipment [35]. [PERSON] et al. [36] conducted a fragility study on nuclear power plant piping systems, showing that when deriving fragility curves for pipe elbows, both open and closed modes should be considered separately, with the low-probability failure of straight pipe segments having higher confidence levels than that of elbow segments. [PERSON] et al. [37] performed a seismic fragility analysis on base-isolated structures of nuclear power plants, indicating that the deformation coefficient of isolation bearings can serve as a damage index to assess the seismic fragility of isolated nuclear power plant structures. [PERSON] et al. [38] established the finite element model of the nuclear power plant isolation structure through SAP2000, conducted nonlinear time-history analysis via IDA, defined the damage state based on the shear deformation of the isolation bearing, and carried out the seismic vulnerability analysis of the nuclear power plant isolation structure. Most of the aforementioned studies on nuclear power plant fragility focus on the structures and related equipment, with limited research on the fragility of isolated structures in nuclear power plants. Moreover, in beyond-design-basis seismic events, isolated structures are prone to tensile stress in the bearings, leading to structural overturning. Therefore, it is essential to conduct seismic fragility analysis on base-isolated structures in nuclear power plants, particularly addressing overturning issues. Our research group proposed a base isolation scheme for a diesel engine building in a nuclear power plant.