# Current Trends in Fluid Viscous Dampers with Semi-Active and Adaptive Behavior
## 2 Passive Fluid Viscous Damper
### Passive Fluid Viscous Damper: Critical Review of Past Studies  
The evolution of FVDs began with the military and aerospace industry [17; 31]; the first device, similar to modern dampers, was introduced in the 1860s to reduce the recoil of cannons. During World Wars I and II, fluid dampers were largely used in large cannons, guns, and aircraft but also in the automotive field. With the advent of the Cold War, these devices were turned into various industrial applications, such as energy-absorbing buffers  
Figure 4: Model of a viscous dashpot (adapted from [71]). Figure 5: (**a**) Maxwell model and (**b**) Kelvin–Voigt model (adapted from [44]). in steel mills, canal lock buffers, and offshore oil leg suspension alongside shock and vibration isolation [17; 53; 74]. From the last decades of the 20 th century up to now, the technology of FVDs has been adapted to applications related to the protection of structures from earthquakes and wind. One of the first studies that tackled the use of FVDs encased in steel braces for the seismic protection of buildings was carried out by [PERSON] and [PERSON] [85]. They proposed a device (Figure 2) characterized by a linear behavior and able to operate over a wide temperature range (from \(-40\)\({}^{\circ}\)C to \(+70\)\({}^{\circ}\)C). A further step was made by [PERSON] and [PERSON] [54], who developed four different configurations of FVDs (Figure 6), characterized by a non-linear response and obtained by modifying the geometry of the orifices. Through an experimental investigation, they demonstrated that non-linear devices were able to produce a higher reduction in displacement response than linear devices [54]. Similar studies were conducted by the Taylor company [31], whose first device for civil application was characterized by a huge size (Figure 7) [31]. In the following years, the company produced smaller devices that could be installed in brace configurations as well as components of base isolation systems in buildings [48]. FVDs are commonly used as velocity-dependent passive energy devices in dissipative bracing systems around the world due to their exceptional damping performance, stable mechanical properties, simple installation procedures, and cost-effectiveness [32; 67; 86]. For these reasons, the FVDs have been widely used in the last decades in new reinforced concrete structures and steel ones, but also for retrofit intervention [87]. The FVDs can be effectively used on buildings of different heights [76; 88]. In tall buildings, the FVDs, especially non-linear ones, are very useful because they are able to dissipate high values of seismic force at high velocities [89]. A practical application of these devices on high-rise buildings is the 181 Fremont skyscraper built in San Francisco in 2017, which is a 56-story steel building (Figure 8a). The peculiarity of this project is that the FVDs are placed inside the structure through a mega brace system that is able to provide damping within the length of the braces spanning multiple floors [89]. Another example is the Allianz Tower, a 50-story concrete building built in Milano in 2015 (Figure 8b). Here,  
Figure 6: Four configurations of the passive FVDs of [PERSON] and [PERSON] [54]. Figure 7: A Taylor Company FVD [31]. a set of eight localized external viscous dampers was installed, with the primary goal of improving the service life behavior in terms of comfort under wind loads and, additionally, of guaranteeing a proper structural response under seismic loads correspondent to the Life Safety Limit State [34]. Thanks to the reduction in absolute acceleration and displacement, FVDs can be placed in strategic buildings such as hospitals. Indeed, hospitals are particularly sensitive to the effects of earthquakes, given the presence of medical equipment and other technological content (e.g., surgery rooms, gas pipes, etc.) that are highly susceptible to acceleration or deformations caused by earthquakes [3]. [PERSON] et al.