On the use of base isolation for the protection of rigid bodies placed on a multi-storey frame under seismic excitation

The use of base isolation applied to rigid bodies placed on a multi-storey frame is considered with the aim of understanding whether or not seismic isolation is beneficial in preventing their collapse during an earthquake. The rigid body is placed on either a fixed or an isolated oscillating base. It may be subjected to sliding, rocking and sliding–rocking motions. When base isolation is considered, security stops capable of preventing the isolation system from breaking are always assumed to be present. The frame, modelled as a four-storey, shear-type system, is always considered to work in the elastic regime. The geometrical characteristics of the body are chosen so that a collapse event, such as overturning or falling out from the support, is obtained for an excitation for which the behaviour of the frame remains in the elastic regime. Overturning and falling-out curves are plotted against PGA (Peak Ground Acceleration) to demonstrate the role of the geometrical parameters characterising the body, of the spectral characteristics of the earthquake and of the level of the frame at which the object is placed. The analyses performed reveal that base isolation applied to a rigid body placed on a frame is not always appropriate in cases where the same body is placed on a fixed base.