PSI - Issue 62

L. Zoccolini et al. / Procedia Structural Integrity 62 (2024) 669–676 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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seismic protection for bridges. Many different FVDs are available, but the primary focus of this article will be to spotlight the most noteworthy achievements and address the principal challenges that have shaped the use of FVDs in ensuring the seismic resilience of bridges. In Section 2, the behavior and application of passive FVDs are presented, while in Section 3, the semi-active devices and their applications are discussed with a focus on the magnetorheological FVDs, which are the most largely used in bridge applications. 2. Passive Fluid Viscous Damper According to the European standard on anti-seismic devices EN15129, a FVD is an anti-seismic device whose output is an axial force that depends on the imposed velocity only; its principle of functioning consists of exploiting the reaction force of a viscous fluid forced to flow through an orifice and/or valve system (EN 15129 Anti-seismic devices, 2009). Indeed, FVDs are generally composed of a hollow cylinder filled with viscous fluid, like silicon oil, and by a piston head characterized by orifices (Guo et al., 2015), see Fig. 1. The device is filled with a compressible viscous fluid that must be fire-resistant, nontoxic, thermally stable, non-flammable, non-combustible and not degrading with age. Usually, the employed silicone fluids are characterized by a flash point over 340 °C (EN 15129 Anti-seismic devices, 2009). They exploit the viscous behavior of the fluid to obtain the damping force needed by the bridge to resist earthquake excitations. The FVDs are usually placed between the bridge deck and the pylons (Farahpour & Hejazi, 2023). Whenever earthquakes or winds excite the bridge, the deck starts moving, and the FVDs are triggered. As the piston moves, the fluid passes from one chamber to another through the orifices of the piston head. When the fluid subsequently expands into the volume on the other side of the piston head, it slows down and loses its kinetic energy into turbulence (Apetsi K. & Zhao, 2019). This pressure difference between the downstream (low) and upstream (high) sides of the piston produces a significant force resisting its motion and generates a damping force (Constantinou & Symans, 1993; Lee & Taylor, 2001).

Fig. 1. Scheme of a passive FVD (Guo et al., 2015).

FVDs simultaneously reduce shearing and bending stress (Narkhede & Sinha, 2012). Moreover, thanks to the dependency on the velocity, the presence of FVDs do not affect the period of the structure (Martinez-Rodrigo & Romero, 2003). Indeed, the force generated by the device is out of phase concerning the maximum internal force generated during the excitation of the structure (De Domenico et al., 2019; Shen & Kookalani, 2020). The study of Seleemah and Constantinou (Seleemah & Constantinou, 1997) shows, through experimental testing, a suitable mathematical model to describe the behavior of viscous fluid dampers. The force provided by a FVD to counteract the motion of the structure is proportional to the relative axial velocity between the piston and the cylinder, and it is given by the non-linear force-velocity relation shown in Eq. 1. ( ) = ∙ | ̇( )| ∙ [ ̇( )] (1)