A semiactive mechanism, called a VDSA (variable damping semiactive device), is proposed to reduce the seismic response of structures. It is composed of two fixed-orifice viscous fluid dampers installed in the form of a V whose top ends are attached to a floor and their lower ends to a collar that moves along a vertical rod. By varying the VDSA position one obtains an optimal instantaneous damping added to the structure. The position of the moving end is calculated with an algorithm based on a variation of the instantaneous optimal control theory which includes a generalized LQR (linear quadratic regulator) scheme. This modified algorithm, referred to as Qv, is based on the minimization of a performance index J quadratic in the state vector, the control force vector, and an absolute velocity vector. Two variants of the algorithm are used to present numerical simulations of the controlled seismic response of a single and a MDOF (multi-degree-of-freedom) structure.

When two structures in close proximity and with different properties (heights, structural systems, materials) are subjected to a strong seismic ground motion there is the possibility that pounding between them may occur. The large impact loads induced by this phenomenon usually lead to catastrophic results. One way to overcome this effect is to couple the structures through elastic or damping elements. This article examines the use of a new variable damping device as a coupling element. The system, which is termed a variable damping semiactive (VDSA) device, consists of two dampers with constant parameters whose lower ends are attached to a common vertical rod whereas the upper ends are attached to the two structures. As the structures vibrate due to the ground motion, the lower end is moved up and down by means of an actuator. By changing the orientation of the dampers, the effective damping in the two structures can be changed in time in an appropriate manner to minimize the response. A new control law is used to calculate the optimal position of the dampers. The algorithm, referred to as Qv, is a variation of the Instantaneous Optimal Control and it is based on the minimization of a performance index J quadratic in the state vector, the control force vector, and a vector of absolute velocities measured at selected points. rithm includes a generalized LQR scheme where penalties are imposed on the state vector, on the control vector, and on the absolute velocity vector through three predefined matrices. A numerical simulation is used to verify the performance of the proposed protective system in reducing the seismic response to a series of historic earthquakes. The results show that the proposed device is able not only to eliminate the pounding effects but also to significantly reduce the response of the individual adjacent structures.