It has been demonstrated that active variable stiffness (AVS) systems may be effective for response control of buildings subjected to earthquake excitations. The applications of active variable stiffness systems involve nonlinear control in which control theories for linear systems are not applicable. Based on the theory of variable structure system (VSS) or sliding mode control (SMC), control methods are presented in this paper for applications of active variable stiffness systems to seismic-excited buildings. In addition to full-state feedback controllers, general static output feedback controllers as well as simple output feedback controllers using only collocated sensors are presented. The principle of active variable stiffness control is interpreted based on the concept of the dissipation of hysteretic energies. Simulation results indicate that the control methods presented are robust and the performance of static output feedback controllers is comparable to that of the fullstate feedback controllers. Simulation results further indicate that the active variable stiffness systems, using locking and unlocking devices, are effective in reducing the interstorey drifts of seismicexcited buildings. However, the floor acceleration of the building may increase significantly, depending on the structure design and earthquake excitation.
