## 2 Description of Prototype Seismically Isolated Buildings  
### Main Design Requirements for Isolation Systems  
The introduction of isolating systems into a building limits the inelastic behavior of structural members and in turn reduces potential seismic damage. This can be achieved throughout the lateral seismic displacement demands of a building being concentrated on isolators that are capable of developing significant large lateral displacement capacities without permanent damage. To assure this, a current seismic code prescribes several design requirements for isolation systems. In general, acceptable isolation systems shall (1) remain stable for required design displacements, (2) provide increasing resistance with increasing displacements, (3) not degrade under repeated cyclic loads, and (4) have quantified engineering parameters [14]. Thus, the design of isolation systems should be mainly confirmed by the codified criteria on the minimum lateral restoring forces--usually controlled by the post-yield stiffness--and the property modification factor for their nominal properties that are believed to be strongly related to the robust hysteretic behavior, as shown in Figure 1.  
The current seismic code requires that bounding analyses for the property modification factor, \(\lambda\) of an isolation system, be performed to determine design forces variations resulting from minimum and maximum values of both effective stiffness values and energy dissipation. The property modification factor of a prototype isolation bearing is obtained from a series of characterization tests that are planned to reflect the natural variability in its properties, the effects of temperature rise during cyclic motions, and the influence of aging, contamination, and loading history. Figure 1a is a graphic presentation depicting the upper and lower bounds of the yield (or friction level) and the second slope in the lateral-force displacement relation of an isolation system. The bound values are calculated by multiplying \(\lambda\) factors to nominal properties. Then, the effective stiffness and damping of the isolation system are determined as both the upper and lower bound properties at the MCE displacement on the monotonic force-displacement curve. With property variations, seismic design codes [14; 15] strictly regulate lateral restoring forces that should be developed in an isolation system to limit residual and excessive displacements and consequently to ensure the acceptable safety margin in the event of after-shock or future earthquakes. As presented in Figure 1b where \(D_{M}\) is the total design displacement and \(W\) is the seismic weight, the isolation system shall be configured, for both upper and lower bounds of isolation system properties, to produce the lateral restoring force such that the lateral force at the corresponding maximum displacement is at least \(0.025W\) of the superstructure greater than the lateral force at 50% of the corresponding \(D_{M}\). The monotonic force-displacement behavior of an isolation system is characterized with the three parameters of the yield strength, \(Q_{y}\), the yield displacement, \(D_{y}\), and the post-yield stiffness, \(k_{d}\). Of them, the first two parameters are closely related to the design wind load condition. The lower bound value of \(Q_{y}\) is usually determined to be equal to or greater than the design wind loads imposed to the building. The value of \(D_{y}\) would be determined by the requirement that the lateral displacements in an isolating story shall not be larger than those between floors in the superstructure when subjected to wind loads. These two requirements enable the isolation systems to avoid wind-induced permanent plastic deformations. The last parameter, \(k_{d}\) is strongly related to the criterion on lateral restoring forces which the isolation system has to be capable of developing.