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Energy dissipation devices have been accepted as a promising alternative to dissipate the energy that earthquakes induce in structures; therefore, reducing displacement demands. A large number of new or retrofitted buildings are equipped with fluid dampers or other energy dissipation devices which are expected to suppress earthquake induced shaking. They can be incorporated in the isolation system of a building2 or within the skeleton of a structure7. When structures are built near a potential fault, the expected displacement demands in the structure are large and there is a tendency to equip structures with supplement damping -in most cases of nearly viscous type2. On the other hand, near source ground motions usually contain one or more distinct and coherent pulses; and it is known that for pulse motions the efficiency of viscous damping in reducing displacement demands is limited when the system is highly damped14 In a recent paper, the senior author showed that plastic damping is more efficient than viscous damping in reducing the response of a SDOF structure when subjected to pulse motion represented by cycloidal fronts with duration close to the natural period of the structure. Recent developments on controllable fluid dampers showed that both electrorheological (ER) and magnetrorheological (MR) dampers are capable of delivering the desirable plastic or viscoplastic behavior. In this paper an analytical study on the response of a 2 DOF isolated structure equipped with controllable fluid damper subjected to a variety of near source earthquakes is presented in order to understand the difference between viscous and friction damping, and to investigate possible advantages of controllable fluid dampers. The isolated structure is considered to be supported either on elastomenc bearings or sliding bearings.
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