# Current Trends in Fluid Viscous Dampers with Semi-Active and Adaptive Behavior
## 1 Introduction  
Earthquake protective systems can be divided into two main categories, namely base isolation and energy dissipation systems [1]. Base isolation systems reduce the input energy by separating the movement of the building from that of the ground, thereby providing a substantial decrease in internal forces and deformations in the structure. These systems have been primarily adopted in high seismicity regions [2] and have been proven to be effective for those constructions where the target performance is to protect highly technological contents or to permit continuous operation during a seismic event and in the aftermath of an earthquake, such as hospitals or other structures of strategic importance [3; 4; 5]. However, these technologies are not suitable for high-rise buildings [6; 7] or for structures built on soft soils [8; 9; 10; 11]. For these structures, the use of energy dissipation systems, whose aim is to absorb and dissipate much of the seismic input energy, is a viable solution. Nonetheless, energy dissipation systems have also been used in many applications in combination with base isolation systems to increase damping and reduce displacements. These devices are classified based on their operating principles [12; 13; 14] and, according to the European standard on anti-seismic devices EN15129 [15], they can be divided into two main categories: displacement-dependent devices (DDDs) and velocity-dependent devices (VDDs) [1]. DDDs are energy-dissipating systems whose constitutive law mainly depends on the imposed displacement rather than on velocity [16]. These systems dissipate energy through their hysteretic behavior, and they can be distinguished into hysteretic steel dampers, friction dampers, elastomeric dampers, and metal extrusion dampers, just to name the most common types [17; 18]. During the earthquake, the deformation of the structure in which the damper is inserted triggers the damper, which dissipates energy and reduces the seismic forces in the building [19]. However, it must be noted that since the response is dependent on the displacements, the forces generated by DDDs are in phasewith the forces generated by the earthquake [1]. These devices have been largely adopted to protect ordinary structures, such as residential, industrial, and school buildings [20; 21; 22; 23; 24; 25], thanks to their cost-effectiveness and ease of manufacturing [18; 26; 27]. VDDs dissipate energy through the lamination of a viscous fluid forced by a piston to pass through an orifice or a valving system, and their behavior depends on the rate of deformation of the structure [1; 18; 28]. The forces generated by these devices are out of phase with the seismic forces [29; 30]; therefore, the maximum force generated by the damper is not simultaneous with the maximum internal force in the structure due to the seismic action effects [31]. Moreover, VDDs can dissipate the seismic energy without significantly affecting the inherent stiffness of the structure [30] and, therefore, without altering its fundamental period. Indeed, as shown in Figure 1b, the VDDs reduce accelerations and displacements of the structure without changing its period. On the contrary, DDDs introduce an additional stiffness in the structural system, shortening the fundamental period of the structure [25] (Figure 1a). VDDs are very versatile and have been employed in several applications [32; 33; 34]; however, they are particularly indicated for flexible structures, such as high-rise buildings and long-span bridges [33; 35]. Strategic buildings, such as 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.) highly susceptible to accelerations or deformations caused by earthquakes [36]. VDDs are also largely employed in bridges to accommodate uncontrolled slow motions (like thermal motions) and guarantee protection from seismic and wind loads [37; 38].