Structural frame systems designed using conventional seismic design techniques develop significant inelastic deformations under strong earthquakes, leading to inelastic hysteretic behavior, stiffness and strength degradation, damage to beams and columns, increased peak inter-story drifts, and residual drifts. Supplemental fluid viscous dampers have emerged as an effective approach for reducing seismic response and limiting damage by shifting the inelastic energy dissipation from the framing system to the dampers. Fluid dampers primarily dissipate energy and do not provide self-centering stiffness capability or counter stiffness degradation. On the other hand, supplemental viscoelastic dampers/springs can provide, in addition to damping forces, stiffness or restoring/recentering forces. Recent investigations have shown that a combination of Adaptive Stiffness and Damping (ASD) can provide substantial response modification, particularly during near-fault pulse-type earthquakes (Nagarajaiah et al. 2006a). Such devices offer structural response modification capability by varying the restoring forces (stiffness) in accordance with the frequencies of vibration and dissipative forces (damping) that govern the behavior of the structural dynamic system. To date, adaptive stiffness systems have received relatively little attention as compared to supplemental damping systems and represent a significant gap in earthquake engineering. One of the main reasons for the limited attention given to such systems is the lack of practical adaptive stiffness devices. Preliminary work for this proposal (Dyck et al. 2006) investigated practical stiffness devices, one of them employing an adjustable fluid spring and adjustable damper in a single compact unit. However, such devices have limited stiffness and damping variation capability and can only be adjusted manually (i.e., no feedback is possible). Hence, there is need to develop new ASD systems that have enhanced stiffness and damping variation along with automatic passive adjustment. Thus, the research objective of this project is to develop practical stiffness and damping modulation techniques under strong earthquake excitations and providing self-centering capability to structural framing systems, using adjustable passive fluid springs and dampers.