This paper deals with the development and validation of a backstepping controller to effectively reduce seismic responses of civil engineering buildings due to external disturbances such as earthquakes, heavy loads and strong winds. Semiactive control systems using magnetorheological MR dampers are especially studied by means of simulated and experimental tests. Two control strategies using backstepping control technique were studied. Experimental verification of semiactive controllers was achieved by means of laboratory tests using a 6-story experimental building model with shear-mode semiactive MR dampers installed at the two lower floors