A semi-active control strategy is developed to suppress vibration of flexible structures by on-line varying the characteristics of variable dampers. This novel algorithm is designed on the basis of sliding mode control theory. The approach is insensitive to the spillover problem encountered in the fully-active control case. The control law is synthesised and illustrated on a beam structure example. Computer simulations are performed to examine the theoretical predictions. The results of this fundamental study are essential for advancing the technology of designing and controlling complex structural and mechanical systems.