# 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
[90] and [PERSON] and [PERSON] [75] showed that FVDs have a considerable impact in reducing both structural and non-structural damage, hence improving the functionality and seismic resistance of the upgraded structure. FVDs have been widely adopted for the seismic protection of bridges. These devices are appropriate for both retrofitting existing vulnerable bridges and designing new ones [91,92]. Indeed, the viscous dampers efficiently dissipate energy and reduce the movement of the bridge induced by braking, wind, and seismic loading. In addition to that, FVDs do not provide any valuable restriction at slow velocities, accommodating service slow motions like the ones induced by thermal movements [91]. [PERSON] et al. [92] studied the seismic retrofit of the Seohae Grand Bridge in Korea, which is composed of three different types of bridges: a cable-stayed bridge (990 m), a free cantilever method (FCM) bridge (500 m), and a precast span method (PSM) bridge (5820 m). The PSM bridge consists of a series of continuous spans that were not able to satisfy the seismic safety requirement. Therefore, 54 viscous dampers were placed between the superstructure and the pier to protect the structure against strong earthquakes. The adopted devices were able to exert a force equal to 500 kN at a velocity of 240 mm/s. Moreover, these devices were operative over a wide range of temperatures, from \(-25\)\({}^{\circ}\)C to 40 \({}^{\circ}\)C.  
Another iconic application of FVDs on bridges is the retrofit performed on the London Millenium Bridge built over the river Thames in London (United Kingdom). When the bridge was opened, it exhibited large lateral displacement induced by pedestrian motion. Thus, 37 FVDs were installed in three different configurations to prevent lateral, vertical, and torsional modes. The additional devices prevented resonance of any mode and allowed the bridge to produce an equivalent viscous damping of 20% instead of the original 0.5%. Another example is the Rion-Antirion Bridge, which was designed to resist seismic excitations with a return period of 2000 years and a PGA equal to 0.48 g through a seismic protection system made by fuse restraints and FVDs [93]. This bridge, located in the Gulf of Corinth in Greece, consists of a 2252 m long suspension bridge with four pylons and two  
Figure 8: (**a**) 181 Fremont skyscraper, San Francisco; (**b**) Allianz Tower, Milano [34]. approach viaducts. The main bridge is protected by fuse restraints that work in parallel with the dampers. The first type of device protects the bridge from excitations induced by the wind or by moderate earthquakes. Under excitations equal to or stronger than the design earthquake, the fuse restraints are designed to fail so that the FVDs are able to dissipate the input energy. Figure 9a shows the completed Rion-Antirion Bridge, and Figure 9b shows the seismic protection system installed in each of the four pylons. The approach viaducts are designed to withstand the same seismic event as the main bridge, and their seismic protection systems are composed of elastomeric isolators and FVDs.