PSI - Issue 64

Sabatino Di Benedetto et al. / Procedia Structural Integrity 64 (2024) 983–990 S. Di Benedetto / Structural Integrity Procedia 00 (2019) 000 – 000

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energy by crushing its internal core composed of layers of aluminium foam. Furthermore, a wedge system with a spring mechanism has been incorporated into the device to absorb permanent deformations following an earthquake. The device consists of concentrically perforated cylindrical layers (AFM layers in Fig. 5) tailored to accommodate the size of the brace. Meanwhile, the wedge system involves the utilization of an upper wedge with a threaded hole to house the brace, which interfaces with a lower wedge via a surface treated with a thermal spray coating ensuring a friction coefficient exceeding 0.6 (Ferrante Cavallaro et al., 2017). A bearing plate facilitates the transfer of axial force in the brace to the foam layers and also serves as a support system for accommodating the displacement of the lower wedge. A dual spring pre-stretching system ensures the connection between the bearing plate and the lower wedge. These elements are housed within an external tube welded to end plates. Additionally, the system includes an inner tube welded to the lower end plate to restrict interactions between the brace and other components. This damper activates when tensile axial forces arise in the brace, causing compression actions in the aluminium foam layers, resulting in plastic deformations that create a gap subsequently filled by the activation of the wedge mechanism. This kinematic mechanism prevents a pinching response. The aluminium foam layers serve as the dissipative components of the device, and thus, in accordance with the capacity design procedure outlined in EN 1998-1 (2004), they should be designed considering the maximum design axial force of the brace. All other components should exhibit an elastic response, taking into account the overstrength of the dissipative component. Regarding this, tests conducted on the aluminium foam layers have indicated a stable overstrength factor of 1.5. Further details on the design of this damper are provided in de la Peña et al. (2022).

Fig. 5. Aluminium foam damper.

As previously mentioned, the retrofitting strategy for the CBFs has involved replacing the existing bracings with diagonals equipped with aluminium foam dampers. The primary advantage of adopting these innovative devices lies in the enhanced energy dissipation capacity and, consequently, in the behaviour factor used for linear static analyses compared to the original structural configuration. Specifically, current provisions allow for a maximum behaviour factor of 4 for structures with CBFs. However, it is important to note that this factor should not induce a pseudo acceleration value at the LS lower than that at the DL. Therefore, for the analysed case, the behaviour factor can be defined as = (4; , / , )=3.7 . Based on this premise and considering the maximum actions observed in the diagonals during linear static analyses, the aluminium foam dampers are designed to withstand axial tensile forces of 460 kN and 235 kN at the first and second storeys, respectively. Since these forces are withstood by the dissipative components, all remaining non-dissipative elements must be designed/checked for forces increased by 50% in the bracings, resulting in 690 kN and 353 kN at the first and second levels, respectively. Consequently, replacing the plates of the diagonals with 130x25 mm and 100x20 mm plates at the two levels made of S235 steel grade was necessary. This led to the definition of dampers with diameters of approximately 26 cm and 19 cm at the two levels, capable of accommodating bracing elongations of about 135 mm.