# Current Trends in Fluid Viscous Dampers with Semi-Active and Adaptive Behavior
## 3 Semi-Active Fluid Viscous Dampers  
Passive FVDs described in Section 2 provide a fixed response curve, whatever the intensity of the seismic action. They are typically designed with a focus on the ultimate limit states, aiming to prevent structural collapse and ensure the safety of occupants during a severe seismic event. The effects of earthquakes with lower intensities, which are more frequent during the service life of a building, may not be totally mitigated by passive FVDs, and this represents an important drawback of this technology. Indeed, weaker seismic events may not pose a significant risk of structural failure but can compromise the non-structural elements inside the building and adversely affect the overall serviceability and functionality of the structure. To address this issue, semi-active and adaptive FVDs have been actively investigated in the last few decades. These damping systems are designed to provide variable damping characteristics, allowing them to respond more effectively to a broader range of seismic intensities. By adapting their response according to the specific seismic excitation, these FVDs can enhance the protection of non-structural elements and improve the overall performance of the building during serviceability limit states.  
Semi-active energy-dissipating systems are generally based on the same working principle as passive FVDs, but they are provided with a sensing and a control system [14]. The sensing system detects either external excitations or structural response information and transmits them to the control system. Sensors can be placed inside the device or directly on the structure. However, sensors are a critical component, as their signal may be affected by external factors, such as magnetic or electric fields, as well as environmental noise [94]. Originally, the sensing system was placed on the structure [13; 95], but this configuration increased the complexity and the cost of the system [96; 97]. For this reason, in the last decades, devices equipped with self-sensing components able to detect accelerations, forces, and displacements have been studied [94; 98]. The signal from the sensing system is analyzed by the control system and feeds into a control law that allows efficient control forces to be developed. The control laws can be divided into two categories [99]: the first one comprises conventional control algorithms such as the optimal control algorithms [100], Lyapunov stability theory [101], skyhook, and continuous sliding mode [102]; the second category comprises the fuzzy control method [103; 104], neural network [105], and genetic algorithms [46]. The first category of control algorithms are model-based algorithms, and their efficiency relies on the accuracy of the model. The algorithms belonging to the  
Figure 9: (**a**) The Rion-Antirion Bridge; (**b**) the seismic protection system of the suspended bridge [93].  
second category are based on the measured response of the structure. The semi-active systems are indeed capable of adjusting their behavior under different conditions and different structural responses to achieve the amount of energy dissipation necessary to protect the structure [45, 46, 95, 106]; for this reason, they are also called \"controllable passive devices\" [14]. They operate with a small external energy supply (e.g., a battery) with a power of the order of tens of watts [107].  
The semi-active energy-dissipating systems developed in the last decades include magnetorheological (MR) dampers, electrorheological (ER) dampers, and variable orifice dampers [108].