This paper describes semi-active fluidic control devices, the successful use of such, devices in military applications, and the research efforts of the writers in transferring and adapting this technology to the field of earthquake hazard mitigation. The experimental testing of a semi-active continuously adjustable damping device which operates on the principle of fluid orificing is described. Furthermore, mathematical models which describe the behavior of the device are presented. 

The energy supplied by an earthquake is transferred through the foundation of a structural system and into the superstructure. A significant portion of the energy within the foundation and superstructure can be dissipated through the introduction of a supplemental energy dissipation system placed either within a seismic isolation system or as structural elements within a conventional construction. Although a variety of supplemental energy dissipation systems have been proposed for the purpose of mitigating the harmful effects of earthquakes, all such systems may be categorized under three basic headings: passive control systems, active control systems, and semi-active control systems.

Figure 1 shows the construction of a semi-active fluidic (fluid + logic) control device whose operation is similar to that of a passive fluid viscous damper except that, based on the status of the control valve, it can deliver damping at two distinct levels (two-stage) or over a wide range between an upper and lower bound (continuously adjustable). Its potential for use as a semi-active two-stage damper in seismic energy dissipation systems has been explored by Shinozuka et al. (1992). The device of Figure 1 can be modified to allow for the development of stiffness through removal of the accumulator. Furthermore, a semi-active damping and stiffness device which can modify both its damping and stiffness characteristics can be developed by including a control valve connected to an external accumulator. In fact, semi-active damping and stiffness devices have been used in numerous applications within the U.S. military. Examples of applications include the suspension system of armored vehicles, the suspension system of self-propelled Howitzers, and the Sikorsky Flying Crane Helicopter.

CONCLUSIONS

Semi-Active fluid viscous dampers have been investigated for use as supplemental seismic energy dissipation devices. The mechanical properties of a continuously adjustable semi-active damper were experimentally determined and the device was shown to be capable of delivering two distinct levels of damping (high and low) as well as any prescribed level in between. Furthermore, a detailed mathematical model based on fluid mechanics principles and a simplified fluid viscous dashpot model were developed to describe the dynamic behavior of the semi-active fluid damper. Both models were found to adequately predict the experimental results.