Passive Fluid Dampers
Passive fluid viscous dampers have been implemented in a number of structures for the primary purpose of dissipating energy from earthquake ground motion or sustained wind loading (Symans et al. 2008a, b). In the case of earthquake ground motion, the dampers prevent or limit structural damage whereas, for wind loading, the dampers reduce vibration levels to relieve occupant discomfort. The seminal testing of passive fluid dampers for seismic applications was performed by Constantinou and Symans (1993) and clearly demonstrated the ability of such dampers to improve the performance of building structures. Fluid viscous dampers consist of a metallic cylinder, filled with a low-viscosity fluid, and containing a piston head which separates the two sides of the cylinder. As the damper is stroked, fluid passes around and/or through the piston head due to a pressure differential across the piston head. The orificing of the fluid results in the development of heat which is dissipated through the metallic cylinder. For the range of frequencies typical of the fundamental mode of most structures, fluid dampers can be designed to exhibit insignificant restoring forces, resulting in behavior that is primarily rate-dependent (linear or nonlinear viscous) (Symans and Constantinou 1998). Fluid dampers also offer the advantage that they provide high-energy dissipation density (i.e., due to high internal fluid pressure, they are able to dissipate large amounts of energy for their size). The high-energy dissipation density results in physically compact dampers.

Variable Stiffness Devices
Various semi-active devices that have adjustable stiffness and damping properties have been developed (Spencer and Nagarajaiah 2003). The Variable Stiffness (VS) systems developed by Kobori et al. (1993) at Kajima Research Institute, Japan, maintain a non-resonant state under seismic excitation by altering the stiffness, and thus natural frequencies, of a building based on the nature of the earthquake. The stiffness is varied by engaging and disengaging the braces in each story of the structural framing system. The hydraulic devices connected between the chevron braces and the floor beams above are used to engage and disengage the bracing system in an on-off manner, thus producing abrupt (discontinuous) changes in stiffness.

Yang et al (2000, 2007) and Agrawal and Yang (2000a,b) have developed and shown the effectiveness of a Resetting Semi-Active Stiffness Device (RSASD). The RSASD consists of a cylinder-piston system filled with hydraulic oil and containing an on-off valve within the by-pass pipe connecting the two sides of the cylinder. During operation in the resetting mode, the valve is closed, and hence energy is stored in the RSASD in the form of potential energy. At appropriate time instants, the valve is pulsed to open and closed quickly so that the pressure differential across the piston head is eliminated, and the energy stored in the device is thereby released. Hence, by regulating RSASD at appropriate time instants, the structural response is reduced by withdrawing energy from the vibrating structure. It is not possible to continuously vary the stiffness with this device as it can only operate in lock or unlock mode.

Variable Damping Systems
Symans et al. (1994) have developed variable damping systems that utilize variable orifice fluid dampers for structural systems and experimentally tested them at both the component level (Symans and Constantinou 1997a) and within multi-story building frames (Symans and Constantinou 1997b) and base-isolated structures (Madden et al. 2002, Symans and Reigles 2004). The variable damper consists of a metallic cylinder containing a piston rod/head assembly, a piston rod make-up accumulator (to minimize restoring forces) and is filled with silicone oil. An external bypass loop containing a control valve is attached to the damper for modulating fluid flow. The pressure differential across the piston head, and thus the output force, was therefore modulated by the external control valve. Depending on the type of valve used, either two-stage (on-off) or continuously variable damping was generated. It was shown that a simple phenomenological model consisting of a linear viscous dashpot with a voltage dependent damping coefficient was sufficient for describing the dynamic behavior of the fluid dampers over the frequency range that is typically of interest in structural vibration reduction applications. More complex models for describing the damper behavior over a wide frequency range are presented by Symans and Constantinou (1997a) and include a Maxwell model of linear viscoelasticity and a fundamental model based on fluid mechanics principles. Shaking table tests were performed by Symans and Constantinou (1997b) on a reduced-scale model of a three-story steel structure. The structure was subjected to historical earthquake records and was controlled by variable fluid dampers located in the diagonal bracing of the structure. The shaking table tests demonstrated that the peak response of the uncontrolled structure could be significantly reduced with the use of the variable damper control system. In addition, it was demonstrated that it can be challenging to achieve appreciable response reductions with respect to well-designed passive seismic protection systems.

Combined Variable Stiffness and Damping Systems
Nagarajaiah et al. (2006a,b) have studied smart base-isolated structures with combined SAIVS device and MR dampers and have shown that significant response reductions are possible by independently varying stiffness and damping. However, in these studies, the SAIVS device has the same physical limitations as previously described (i.e., the SAIVS device is a relatively large mechanical device and thus can only be implemented at the base of a base-isolated structures or as a STMD at the top of a fixed-base building).