# Current Trends in Fluid Viscous Dampers with Semi-Active and Adaptive Behavior
## 4 Adaptive Fluid Viscous Damper  
A main drawback associated with semi-active devices is related to the complexity of the sensor system and the control algorithm, which increases the product manufacturing cost, making their use not affordable in construction projects [43]. Therefore, in the last two decades, a new typology of FVDs with an adaptive behavior was developed; these systems are able to adjust their response to different external excitations thanks to their self-sensing and self-controlling capabilities, gained by means of special valves or specific geometric properties [166]. [PERSON] et al. [167; 168] investigated an FVD provided with orifices in the piston head and a bypass tube, which are opened by precompressed conical valves, as shown in Figure 22a. This damper behaves as a passive FVD up to a certain force \(F_{crit}\), at which the relief valves open to allow the fluid to flow freely through the tubes. Each valve is connected to the  
Figure 21: Semi-active FVD equipped with servo-valve [159; 160]. cylinder by a precompressed spring, which guarantees that the valve remains closed when the fluid pressure is lower than the prestress of the spring. This FVD can be modeled exactly as a passive FVD by considering that the geometry of the orifices changes; in particular, the area of the orifices passes from \(A_{o}\), when relief valves are closed, to \(A_{b}\) (\(A_{b}\gg A_{o}\)), when the valves are open [167]. Therefore, this device is characterized by a linear piecewise behavior (Figure 22b) [168], and its response can be tuned by selecting different values of the precompression in the springs. Relief valves allow the maximum damping force, which is transmitted to the structure, to be limited, whatever the velocity. A damper similar to the one proposed by [PERSON] [167] was first applied to a helicopter's rotorcraft systems by [PERSON] and [PERSON] [169] and [PERSON] et al. [170]. [PERSON] [171] and KYB Corporation [172] proposed a device with relief valves (Figure 23), similar to the one of [PERSON] et al. [167; 168], to be combined with a base isolation system for the seismic protection of buildings. In such combined systems, a passive FVD is normally designed considering earthquakes correspondent to the Life Safety Limit State [15; 59]; consequently, the earthquake protective systems would be too hard, and, in the case of small seismic events, the property of reducing the induced accelerations into the structure may be lost. Therefore, this new damper was designed in order to develop a different response during earthquakes corresponding to either the serviceability limit state or the ultimate limit state. In particular, the device is characterized by the H-shaped force-displacement profile shown in Figure 23b, where a certain displacement threshold is set, at which the relief valves open, increasing the damping capacity of the damper [171]. The performance of the device was verified with wave vibration tests, and it was certified by the Japanese Minister as a seismic protection device [171]. However, the applications of this FVD in real structures have not been reported yet. All the passive, semi-active, or adaptive FVDs presented up to this point are characterized by a hysteresis loop that acts in all four quadrants of the force-displacement plane (Figure 24a), which determines an increase in the base shear demand due to the added  
Figure 23: Adaptive FVDs with relief valves [171]: (**a**) device configuration; (**b**) force–displacement profile. Figure 22: Adaptive FVDs with relief valves: (**a**) geometry scheme [167]; (**b**) damper’s pressure–velocity characteristic [168]. resistive force provided by the device [173]. To overcome this issue, [PERSON] et al.