# Current Trends in Fluid Viscous Dampers with Semi-Active and Adaptive Behavior
## 5 Conclusions  
Many FVDs are currently available, and their number is constantly growing. For this reason, this paper does not present a complete state-of-the-art on the topic, but it aims to provide an overview of the current trends in the field of FVDs used as energy dissipation systems for the seismic protection of structures. The various types of FVDs and their different configurations have been briefly investigated, starting from the simplest ones--the passive FVDs--to the more complex semi-active and adaptive FVDs. FVDs are velocity-dependent devices, providing a force that is out of phase with respect to the structure's internal forces and displacements caused by seismic excitation. Therefore, these devices can dissipate energy without overloading the structure. The first passive FVDs to be developed were derived from the devices used as shock absorbers in the military and aerospace fields. Starting from the 1990s, FVDs were largely adopted for the protection of structures and infrastructures against earthquake and/or wind excitations. Passive FVDs provide a fixed response curve, whatever the intensity of the seismic action. Moreover, they are usually designed considering the seismic action effects at the ultimate limit states, where the target is to avoid the collapse of the structure and protect human life. But, on the other hand, FVDs tailored for the design earthquake may provide an insufficient response to earthquakes of lower intensity, typical of service limit states, which may not damage the structure but can jeopardize non-structural elements and reduce the serviceability of the building. For this reason, many semi-active and adaptive FVDs have been studied in the last decades. Semi-active and adaptive FVDs can change their properties depending on the input excitation, providing an optimal amount of energy dissipation during both strong and low-intensity earthquakes. The most popular semi-active dampers are the ER and MR ones, which are characterized by a fluid that, depending on the intensity of an applied electric or magnetic field, changes its rheological properties. Besides the ER and MR devices, there are variable stiffness and variable damping dampers equipped with special valves that permit changes in the fluid flow cross-section. All these devices require sensors, a control system, and a small energy source, like a battery. Therefore, even if these FVDs are largely studied in the research field, their application is still limited in construction projects because of the complexity of the sensing and controlling equipment, which makes these systems particularly expensive and difficult to manufacture. Adaptive FVDs represent a promising technology since they are able to provide performance comparable to semi-active FVDs without any special external equipment. New devices like direction- and displacement-dependent FVDs, variable-damping-coefficientviscous fluid dampers, and other devices have been demonstrated in theoretical and laboratory studies to provide superior performance compared to conventional FVDs over a wide range of practical situations. However, they are characterized by either a particular geometry or special valves that increase the complexity of the manufacturing process. As for semi-active FVDs, the application of adaptive FVDs is also limited, but the growing research contributions and the increasing body of literature suggest a promising evolution of such technologies, overcoming their limitations. Conceptualization, E.B. and V.Q. ; Investigation, L.Z. ; Supervision, E.B., V.Q., and S.C.; Writing--Original Draft, L.Z. ; Writing--Review and Editing, E.B., V.Q., and S.C. All authors have read and agreed to the published version of the manuscript.