Introduction

Base-isolation technology has been widely applied to industrial and civil buildings, bridges, and major foundation works, and effectively decreases the seismic demands of structures and protect them from being damaged [1,2]. However, the applications of baseisolation technology in the nuclear power plants are limited. Among all the nuclear power plants currently in commercial operation worldwide, perhaps only Koeberg in South Africa and Cruas in France have adopted the base-isolation technology [3,4]. The application of base-isolation technology into the design of nuclear power projects can improve the safeties of integral structures and internal facilities, realize the standardized and modular design of buildings, internal structures and facilities, and strengthen the adaptability of their plant sites. One particular advantage of isolation technology lies in the capability to significantly mitigate the seismic damage of structures, and to improve the safety margin of structures subjected to extreme seismic events. On behaviors of isolation technology, Chen et al. [5] investigated on the response characteristics of base-isolated nuclear island building under a typical safe shutdown earthquake. Frano and Forasassi [6,7] studied seismic responses of reactor structures. Tamura et al. [8] investigated dynamic response of nuclear power plant piping systems and components with viscoelastic dampers subjected to severe ground motions. Perotti et al. [9] presented seismic fragility of base-isolated nuclear power plants structures. Liu et al. [10] studied the dynamic responses of nuclear structures and the influence of bearing parameters under beyond-design basis earthquake (BDBE) shaking, and concluded that isolation technology can significantly mitigate the seismic response of nuclear structures. Huang et al. [11] conducted a series of nonlinear time-history analysis to explore the influence of ground motion and the mechanical properties of seismic isolation system on the seismic responses of isolated nuclear power plants under 100% design basis earthquake (DBE) shaking and 150% DBE shaking. The analysis and design of nuclear structures should consider the influence of BDBE shaking. The American Society of Civil Engineers has two standards that are relevant to the seismic analysis and design of nuclear structures, namely, ASCE 43-05 [12] and ASCE 4-16 [13]. The objective in using ASCE 4 together with ASCE 43 is to achieve specified annual target performance goals. For seismic design, the target performance goals are prescribed in ASCE 43-05. Section 1.3 of ASCE 43-05 presents dual performance objectives for nuclear structures: (1) less than 1% probability of unacceptable performance against 100% DBE shaking and (2) less than 10% probability of unacceptable performance against 150% DBE shaking. In this paper, a series of nonlinear time-history analyses will be performed to study the impact of the variability in ground motion intensity on the seismic response characteristics of base-isolated nuclear shield building (BI-NSB), and to investigate the displacement control strategies for BI-NSB subjected to BDBE shaking. The finite element models of base-isolated and nonisolated nuclear shield buildings (NSBs) are created by using ANSYS (ANSYS, Inc., Southpointe 2600 ANSYS Drive Canonsburg, PA 15317, USA), and three groups of ground motions of various intensities will be discussed.