Control of torsional modes in buildings under seismic excitation by adaptive base isolation

Passive base isolation systems used for the seismic response control of structures appear to be effective for small to medium strength earthquakes. Hybrid base isolation systems, which use an active system together with the passive base isolation system, may be used to control the response of structures subjected to larger ground motions created by larger magnitude earthquakes. A hybrid base isolation system which uses passive base isolation pads together with deflection inserting actuators is proposed. The system, placed between the foundation of the building and its superstructure, is used to minimize the forces imposed on the superstructure by the earthquake induced ground motion. A three dimensional structural model is developed to study the effectiveness of the adaptive base isolation system. The effect of the base isolation system on the flexible modes of the structure, including the torsional modes, is studied.