# Current Trends in Fluid Viscous Dampers with Semi-Active and Adaptive Behavior
## 3 Semi-Active Fluid Viscous Dampers
### Variable Orifices Fluid Viscous Dampers
#### 3.2.2 FVDs with Servo-Valves  
While devices equipped with solenoid valves can implement only two different behaviors due to the on-off property of the valve itself, devices equipped with servo-valves are able to dynamically adapt their response depending on the intensity of the excitation. They provide the structure with values of damping ranging between a maximum and a minimum level [46], thanks to the servo-valve's property of actively controlling its opening. For this reason, they are also called variable damping dampers. The variable damping damper works as a passive FVD and is characterized not only by orifices through the piston head but also by a secondary orifice in a bypass pipe that connects the two chambers. The bypass pipe is equipped with a servo-valve that controls the flow of the fluid through the secondary orifice (Figure 21). When the valve is closed, the viscous fluid flows from one chamber to the other only through the orifices in the piston head, while if the valve is open, the fluid flows both through the piston head and the bypass pipe. The flow of the fluid through the bypass pipe varies depending on the opening of the servo-valve. Therefore, the damping coefficient ranges from a maximum \(C_{D,MAX}\). when the valve is completely closed to a minimum \(C_{D,min}\) when the valve is open.  
[PERSON] et al. [159] developed a variable damping FVD with a fail-safe property: if a power loss occurs, the servo-valve automatically closes, allowing the device to operate as a passive FVD with maximum damping characteristics. This device requires a small power source to be operative (around 3.5 W, similar to a battery).  
[PERSON] et al. [161; 162] and [PERSON] et al. [163] studied analytically and experimentally a device aimed to control the seismic response of bridges. They realized shake table tests on a model of a simple-span girder bridge and demonstrated that the variable damping force provided by the servo-valve prevented small vibrations induced by traffic and wind loads and was able to reduce by up to 44% the displacement at the peak acceleration. [PERSON] and [PERSON] [160] focused on the application of the device in Figure 21 inside a laboratory test of a three-story steel structure. They performed shaking table tests on that structure and noticed that the dampers were able to improve the structural response by generating a large range of damping. They compared the performance of the variable damping FVDs with that of an active tendon control system. The results demonstrated that the tested semi-active system provided a damping larger than that of an active tendon control system, with a consequent larger response reduction. In particular, the semi-active FVDs were able to reduce the top-floor acceleration and the story drifts by 47% and 51%, respectively, while the active systems produced corresponding reductions of 38% and 32%.  
[PERSON] et al. [164] employed variable damping devices for the seismic protection of a four-story steel office building in Shizuoka. Through dynamic tests, they verified that the devices, even with a small amount of electric power (70 W), were able to control the structural response of the case-study building during strong earthquakes, providing a damping force of 1 or 2 MN.  
[PERSON] and [PERSON] [165] adopted the damper proposed by [PERSON] et al. [159; 160] to numerically verify the seismic protection of a 10-story rigid frame structure and studied an optimum control strategy to adjust the opening of the servo-valve at each time step during an earthquake. They demonstrated that these systems were able to effectively reduce the maximum shear force and total accelerations in the case-study building.