Rigid body response of base‐isolated structures

The superstructure of a base‐isolated building is relatively rigid compared with the isolation system. This can lead to idealization of the superstructure as a rigid body, modelling the base‐isolated structure as a single‐degree‐of‐freedom system. Such analysis significantly reduces the computational efforts required to design the base‐isolated structure under seismic loading. In this study, a comparison is made of the seismic response of a multi‐storey base‐isolated building by idealizing the superstructure as rigid and flexible. Various isolation systems considered include elastomeric bearings (with and without lead cores) as well as sliding systems. The governing equations of motion of the isolated structural system are derived and the response of the system is obtained for a specified time‐history of earthquake ground motion. The top floor acceleration and bearing displacement of the system are plotted for different system parameters and compared with the corresponding response under rigid superstructure conditions to study the influence of superstructure flexibility. The comparison of the response is made under various isolation system parameters (i.e. isolation period, damping, yield strength of the elastomeric bearings and friction coefficient of sliding systems). It is observed that the bearing displacement can be accurately obtained by modelling the superstructure as a rigid body in a base‐isolated structure. Further, such approximation is also justified for floor accelerations of a relatively stiff superstructure isolated by elastomeric bearings. However, the rigid superstructure approach significantly under estimates the floor accelerations for the structure isolated by sliding systems.