This paper proposes a new semi-active variable stiffness and damping system (AVSD) for
vibration control of civil engineering structures. The AVSD system can alter rigidity and damping
of the controlled structure at each control interval to attenuate the structural responses. The
composition of two hydraulic cylinder-piston systems of the AVSD system is capable of
amplifying the interstory drift and velocity and make dampers dissipate more energy. In this study,
the control law of the AVSD system is established based on an instantaneous perform index. To
optimally utilizing the poposed control devices and economically improve the control performance,
a genetic algorithms is used to design the device location. Numerical results indicate the AVSD
system is a quite economical, reliable and effective semi-active control system
