## 1 Introduction  
Base isolation has been known as one of the most effective seismic protection systems of which the desirable decoupling mechanism resulting from significant low lateral stiffness of isolators minimizes the seismic damage to superstructures [1; 2]. To recognize the advantages of the base isolation system in reducing the earthquake-induced forces to a structure, its practical applications are getting popular in many construction fields [3]. Various researches have been carried out to find out the dynamic characteristics of isolated buildings incorporating various types of isolation systems [4; 5] and the effects of isolation system properties on their seismic performances [6; 7; 8; 9; 10]. However, most of the existing research studies are focused on isolated buildings located at high seismicity regions. There is little information on the seismic performance and related design requirements of isolated buildings at low-to-moderate seismicity regions, although base isolation has been increasingly found in those regions [11], especially for buildings of which damage on contents or structural members should be strictly limited [12; 13]. Based on the previous research works and the design practices, seismic design codes prescribe general design requirements for providing promising decoupling mechanisms on a superstructure and restricting excessive deformations of its isolation systems [14; 15]. Current seismic design codes require that the isolation system should be designed to accommodate the displacement demands subjected to even the maximum considered earthquake (MCE) where they should be calculated by reflecting the property uncertainties of isolation systems. Regarding this, the property modification factors are utilized to determine the upper and lower bounds of properties of isolation systems, and both the boundary properties are used to calculate the seismic responses of isolated buildings. Although the lower lateral stiffness of an isolation system might be beneficial to increase decoupling effects on the superstructure, it should satisfy the lateral restoring force requirements and in turn possess a higher post-yield stiffness ratio to limit potential residual deformations after earthquake attacks. This study is motivated by the review of the current design requirements that could induce isolation systems under large lateral displacements designed to develop the larger post-yield stiffness, even if the desirable decoupling effects are lost. This is particularly true for isolation systems equipped into buildings built at low-to-moderate seismicity regions, since they are subjected to smaller imposed lateral displacements [13]. Therefore, such design requirements for restoring the mechanism of isolation systems in a structure built at low-to-moderate seismicity regions need to be revised with ensuring the excellent seismic performance of the superstructure and limiting excessive displacements and residual deformations of the isolators. The displacement and strength capacities of isolation systems governed by their design requirements can be sufficient to obtain the required seismic performance of isolated structures and to restrain the total system uncertainty expected for the designed isolated structures. Many sources of uncertainty contribute to variability in seismic capacity, and therefore, larger variability in the overall seismic design or evaluation procedures will necessitate larger performance levels of structures in order to control the seismic responses to an acceptable limit. For this reason, the possible uncertainty on seismic performance of isolated buildings plays an important role to determine the design requirements of isolation systems. The design requirements for isolation systems should be determined by reflecting the relation between the seismic performance of isolated buildings and the required level of total system uncertainty induced by the designed isolation systems.