PSI - Issue 78

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

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The linear analyses carried out confirmed the values of the estimated parameters reported in Table1, providing oscillation periods of 3.00 s for both longitudinal and transverse main directions with participating masses greater than 95% and displacements at the SLC of 150 mm. The base shear at the SLC in both directions is 850 kN and it is congruent with the spectral acceleration corresponding to the isolated period reported in Table1. The results of linear analyses show a reduction of the seismic shear at the base of approximately 8 times compared to the ante-operam conditions. The spectral deformations obtained from the linear analyses in both longitudinal and transverse directions are shown in Fig. 3. For the analysis and verification of the structure, nonlinear time-history analyses were carried out ; the non-linear behaviour was only assigned to the isolating devices, while the superstructure elements perform in the elastic range. The non-linear model of the devices was defined using friction pendulum type library elements characterized by the friction model based on hysteretic behaviour proposed by Park et. al (1986). For each limit state, seven spectrum fitting acceleration time-histories of the horizontal component were generated having duration of 30 s. Seven nonlinear time-history analyses were carried out for each limit state, applying acceleration time-histories in the two orthogonal directions..

Fig. 3 SLC deformed shape from linear analyses: longitudinal (above) and transverse (below) view

As an example, considering one of the SLC time-history analyses,Fig. 4a shows an instant deformated shape of the structure, while Fig. 4b shows the history of displacements in the transverse direction of the mass center of the isolation plane. Maximum displacements of the isolators lower than 150 mm results from the dynamic analyses, compatible with the feedback evaluations previously carried out with linear analyses. The nonlinear analyses confirm an oscillation period of approximately 3.00 s at the maximum displacement. Fig. 4c shows, for one of the analyses, the diagram of the response accelerations of the isolated deck of building (black line) compared with the diagram of the input base accelerations (red line). It is evident how the input accelerations (PGA), with maximum equal to about 0.26 g, are filtered by the seismic isolation system obtaining a reduction from 3 to 5 times at floors (peak values between 0.05g and 0.08g). On the contrary, in the ante-operam conditions (fixed base building) the input acceleration should be amplified until values of 0.80g. Therefore the overall reduction in response acceleration for the isolated structure with respect to the fixed-base one is close to 10 times. As an example Fig. 4d shows, the nonlinear force- displacement response diagram of a Type 1 isolating device, having μ = 2.50%, for one of the SLC time-history analyses carried out.

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Fig. 4 SLC time histories analyses: a) instant deformed state; b) transverse displacement history at the mass centre of isolation floor; c) histories of input ground acceleration (red line) and isolation floor response acceleration (black line); d) force-displacement diagram of an isolator The isolating devices have maximum displacements equal 145 mm at SLC and no tensile forces, they satisfy all the checks at the SLC in accordance with EN15129 (2009). The storey drifts observed in the post-operam conditions are of the order of 0. 3‰ at the SLO, guaranteeing the practical absence of inter-storey deformations and therefore the fulfilment of the required verifications. The drifts with respect to the SLV and SLC also present very contained values, lower than 1‰, compatible with a substant ial absence of damage to structural and non-structural elements and to the contents even for the maximum expected seismic events. Therefore the safety evaluation in the post-operam status