It has been recognized that the semi-active control concept is promising for vibration suppression of flexible structures and that the electrorheological (ER) fluid-based device is a good candidate for such applications. In this research, a new control law is developed to maximize the damping effect of ER dampers for structural vibration suppression under actuator constraints and viscous-frictional-combined damping. Numerical simulations and experimental work are carried out to evaluate the semi-active concept. It is illustrated that the performance of the semi-active system is superior to those of the critical damping and maximum damping variety. It is also concluded that the actuator frictional effect is significant, and should be considered in the controller design process.