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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restraint that works in tandem with the FVDs. The fuse restraints initially safeguard the bridge against wind-induced vibrations and moderate seismic events. In the event of an excitation equal or greater to the design earthquake, these fuse restraints are designed to yield so that the seismic energy is transferred to the FVDs, and they can effectively dissipate the seismic energy. Fig. 2b shows how the seismic protection system was installed on the deck in correspondence with each of the four towering pylons. The FVDs, in combination with elastomeric isolators, were also adopted to protect the approach viaducts from earthquakes of the same intensity as the one selected for the design of the main deck.

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Fig. 2. (a) The Rion-Antirion Bridge; (b) seismic protection system of the main deck (Infanti et al., 2004).

3. Semi-Active Fluid Viscous Damper Semi-active energy dissipating systems are generally based on the same working principle as passive FVDs, with the additional capacity to adapt their behavior to the intensity of the external excitations. In particular, these devices are provided with a sensing system and a control system (Symans & Constantinou, 1999). The first one detects external excitations or structural response information and sends this information to the control system. This information is then analyzed through a control law that allows efficient control forces to be developed. 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 (Housner et al., 1997; Soong & Spencer, 2000; Xu et al., 2017; Yi et al., 2001) ; for this reason, they are also called “controllable passive devices” (Symans & Constantinou, 1997). A small external energy supply (e.g., a battery) is required for semi-active FVDs to work properly, and it usually has a power of the order of tens of watts (Symans & Constantinou, 1995). Different typologies of semi-active FVDs have been studied in the last decades, such as magnetorheological (MR) dampers, electrorheological (ER) dampers, and variable orifice dampers (Dyke et al., 1996; Zoccolini et al., 2023). However, the most used semi-active FVDs in bridges are the MR ones because of their large damping force, fast response, and simple structure (M-G. Yang et al., 2011). MR dampers are FVDs filled with magnetorheological fluids that can change their viscosity in the presence of a magnetic field. This process is entirely reversible, and the fluids return to their original viscosity as soon as the magnetic field is removed (Deng et al., 2022). The MR fluid particles have a peculiar structure, and when a magnetic field is applied, they polarize and align in columns following the direction of the applied field. The formation of the column structure allows the shear forces that are applied perpendicularly to the magnetic field to be sustained, thereby increasing the viscosity of the fluid (Davis, 1992; Wen et al., 2008). Each time the column structure is engaged, the fluid particles can be moved only if a minimum force is exerted. The force needed to mobilize these fluid particles increases proportionally to the increase of the magnetic field. As soon as different intensities of input are detected, a magnetic field with a specific strength is created, and the device is able to develop a damping force to mitigate the excitation effects.