# Current Trends in Fluid Viscous Dampers with Semi-Active and Adaptive Behavior
## 4 Adaptive Fluid Viscous Damper
[173; 174] proposed a semi-active system, called semi-active direction- and displacement-dependent (D3) FVD, characterized by two sets of orifices in the piston head, where the inner set can be controlled according to three control laws, namely: (i) the 1-4 control law, in which the orifices are closed in all quadrants and the device behaves as a passive FVD (Figure 24a); (ii) the 1-3 control law, in which the orifices are closed only in quadrants 1 and 3, and the device exerts a resistive force only when the piston moves from its original position toward the peak displacement, while no force acts when the piston moves back to zero (Figure 24b); and (iii) the 2-4 control law, in which the orifices are closed in quadrants 2 and 4, and the resisting force applies only when the piston moves from peak displacement back toward zero (Figure 24c). Figure 24 illustrates how a semi-active device, governed by the three different control laws, affects the base shear force when installed within the structure. [PERSON] et al. [173; 174] demonstrated that only the 2-4 control law can effectively reduce the structural response without increasing the base shear. Indeed, using the 2-4 control law, the retrofitted structure's base shear (\(F_{b}\)) is equal to that of the undamped structure (\(F_{S}\)). In order to avoid the complexity and the expenses typically associated with semi-active dampers, [PERSON] et al. [175] implemented this behavior on a passive device called passive D3, which is able to provide a damping force in single or multiple quadrants by modifying the geometry of the system and specifically by combining the use of one-way valves and different diameters of the internal cylinder. Figure 25 shows a single first-quadrant D3 device, which is composed of a piston head equipped with one-way valves and a cylinder characterized by a larger inner diameter on its left part. One-way valves are opened or closed by an annular plate that is linked through screws to the piston head. When the piston head is in the right half of the cylinder moving toward the positive direction, the annular plate closes the valves, and the device shows a resistive force in the first quadrant of the force-displacement hysteresis loop (Figure 25). When the piston moves in the opposite direction, the annular plate opens the orifices, and the device provides only a small amount of damping. The damping is even reduced when the piston is in the left half of the cylinder since the same mass flow rate of fluid is free to pass through the holes and the gap between the piston head and the cylinder. Figure 24: Schematic force–displacement hysteresis loop for (**a**) a standard FVD, (**b**) 1–3 D3 FVD, and (**c**) 2–4 D3 FVD [173]. Figure 26 shows the 2-4 quadrant D3 FVD developed by [PERSON] et al. [175]. Indeed, passive devices characterized by the 2-4 control law [173; 174] were designed by combining two pistons provided by one-way valves and a shared shaft in a single cylinder characterized by two stepped portions with different inner diameters, as shown in Figure 26. In this configuration, the movement toward the positive direction closes the valves of Piston 1 and opens the valves of Piston 2, while the opening and closing of the pistons is inversed in the negative direction (Figure 26). Even if these devices are promising, they were investigated only in laboratory tests [176], and, to the Authors' knowledge, neither numerical tests on actual structures nor applications on real structures have been reported. [PERSON] and [PERSON] [166] proposed the adaptive viscous damper (AVD) depicted in Figure 27. The device is composed of an external cylinder filled with viscous fluid, a piston rod, and a piston head made of two movable pieces. In particular, the piston rod is linked to a twin cone, which is connected through eight springs to a ring (Figure 27b).