PSI - Issue 78

Alessandro Fulco et al. / Procedia Structural Integrity 78 (2026) 2054–2061

2057

3. Improvement of the seismic safety The achievement of the performance levels established in compliance with the client requirements consists in reaching the seismic adaptation conditions in accordance with NTC (2018) by means of the seismic base isolation technique, Mezzi et al. (2013). The designed interventions are aimed at two main objectives: elimination/mitigation of local criticalities and global seismic retrofitting (adaptation) of the structure. In consideration of the significant level of vulnerability of the existing structure, the adopted design choice provided for the introduction of an isolation system at the base allowing the reduction from 5 to 10 times of the response accelerations with the reduction of: the forces on the structural elements; the inter-story drifts; the floor accelerations responsible for the damage of non structural elements and contents. Base isolation, concentrating the works at the basement, made it possible not to intervene in the superstructure, ensuring the continuity of the functionality of the strategic building, accounting for the request to eliminate or strongly limit the interference between the adaptation works and the building operations. Furthermore, the intervention ensures the Operation conditions even for high intensity seismic events. In order to achieve the seismic adaptation conditions of the structure, no reinforcement work was necessary in the superstructure. With regard to the seismic isolation system, the use of sliding devices with a double curved surface with radius of curvature R = 3100 mm was provided. The operation of the curved surface sliding isolators ("pendulum") can be traced back to that of the simple pendulum, in which the period of oscillation does not depend on the mass but only on the length of the pendulum, i.e. the radius of oscillation. It is planned to install 39 devices at all vertical structural elements, columns and core. The isolation plane is located in correspondence with the basement elements at about 1.00 m below the extrados height of the level 1 deck as far as the columns are concerned, while it is foreseen at level 0 for only the 5 isolators below the stairway/elevator shaft. It is planned to install three types of isolators differing in the friction coefficient μ: Type 1 with μ = 2.50%; Type 2 with μ < 1,00 %; Type 3 with μ = 3,80%. Fig. 2 a), b) show the carpentry corresponding to the isolation Level 0 and Level 1 respectively. Type 1 devices are shown in blue colour, type 2 devices in orange colour and type 3 devices in green colour. Fig. 2 c), d) shows the isolation plane (dashed red line) in the two sections of the structure (longitudinal and transverse).

Fig. 2 Carpentry at the isolation levels: a) Level 0; b) Level 1; c)-d) Seismic isolation plane: longitudinal section; transverse section

3.1. Numerical model Linear modal analyses with response spectrum were first carried out in order to calibrate the parameters of the isolation devices and to compare the results with subsequent nonlinear dynamic (time histories) analyses.The equivalent linear parameters of the isolation devices were defined by means of an iterative procedure based on the convergence of the target displacement S ob . Table 1 shows the parameters of the isolation system corresponding to the last iteration and thus to the convergence of the displacement parameter S d,iso . The remaining parameters given in the table are as follows: μ 0 = mean friction coefficient of the isolation system ; K eff = equivalent stiffness of the isolation system; T iso = fundamental period of the isolated system; ξ eff = equivalent viscous damping of the isolation system; S a,iso = spectral acceleration corresponding to the isolated period.

Table 1 Equivalent parameters of the isolation system S ob μ 0 K eff T iso

S a,iso

S d,iso

ξ eff

mm

%

kN/m

s

%

g

mm

148,00

2,30

291,00

2,90

21,00

0,114

149,000