# Current Trends in Fluid Viscous Dampers with Semi-Active and Adaptive Behavior
## 1 Introduction
Based on their reaction to seismic input, energy dissipation devices are further classified into three categories [39; 40; 41], namely active, passive, and semi-active seismic control systems. Active control systems are capable of actively counterbalancing seismic actions by adjusting their characteristics. They employ sensors and computerized control to monitor and respond to the structural excitations in each time instant. These systems require a large external power supply to operate [42]. On the contrary, passive control systems do not require any external power supply or equipment to operate. They are activated directly by the deformation of the buildings, and the force-velocity characteristic curve is fixed. Therefore, passive FVDs certainly improve the response of a structure, but the damping coefficient is a constant that limits the output of the damper within a certain range. Owing to the randomness and uncertainty of external loads, such as earthquakes and strong winds, the damper cannot adjust its working performance under different working conditions and different structural displacements to achieve reasonable energy dissipation and protect the structure. Consequently, classical FVDs cannot adapt to changes in the structural damping requirements. On the other hand, semi-active control systems combine the properties of the other two categories. Indeed, they use the motion of the structure, as passive devices, to trigger the control forces, but they can adjust the intensity of their response as active  
Figure 1: Effect of energy-dissipating devices on the Acceleration–Displacement Response Spectrum: (a) DDDs; (b) VDDs. devices through a small external power source. The control of the developed forces is achieved thanks to a sensing system that measures the structural response [14]. However, the presence of external equipment increases the complexity of the damper [43], which explains why these systems are still not employed in construction projects. To address this issue, adaptive FVDs have been recently developed. These devices do not require any external equipment to operate but can automatically adapt their behavior to external excitation through a mechanical system. This behavior requires a more complex design of the adaptive device with respect to the others, and their development is currently ongoing. The state of the art of seismic protective systems based on energy dissipation has been the subject of several reviews [13; 14; 40; 44; 45; 46]. [PERSON] et al. [44] conducted a review on passive Metallic Yielding Dampers and Fluid Viscous Dampers (FVDs); [PERSON] and [PERSON] [40], [PERSON] and [PERSON] [14], and [PERSON] et al. [46] focused their research on semi-active devices only, while [PERSON] and [PERSON] [45] presented a study on active, semi-active, and hybrid devices. An updated review of the technological development of FVDs, with a particular focus on semi-active dampers with adaptive behavior, is the subject of the present paper. The main types of FVDs proposed and studied in the past 30 years have been highlighted, showing their working principles and design characteristics, and then attention is focused on the most recent research in the development of semi-active and adaptive devices. The present state of the art is not intended to be comprehensive due to the continuous development of new devices and technologies. The paper is divided into three sections. Section 2 deals with passive FVDs by presenting their main components and the constitutive behavior. Then, a description of modeling techniques of FVDs and a historical review of their evolution is provided. Section 3 introduces semi-active FVDs by describing the constitutive law of electrorheological, magnetorheological, and variable stiffness/damping devices. Finally, Section 4 is focused on adaptive FVDs.