PSI - Issue 78

Fabio Mazza et al. / Procedia Structural Integrity 78 (2026) 33–40

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1. Introduction Base isolation systems are recognized as an effective solution for the seismic protection of structures, especially when medical facilities are taken into account (Mazza et al. (2024b)), with the aim of mitigating the structural response and the damage to nonstructural elements (NSEs) as masonry infills (MIs), whose in-plane (IP) and out-of-plane (OOP) collapse is responsible for the downtime in the aftermath of a seismic event (Mazza (2021)) and for the elevated reparation costs (Filiatrault and Sullivan (2014)). Nevertheless, conventional base-isolation systems are generally designed to reduce only the seismic demand due to horizontal components of strong ground motions, whose vertical component is directly transmitted to the superstructure as in a fixed-base building. This can have significant effects on the structural response when near-fault ground motions are considered, given their notable values of the vertical-to horizontal peak ground accelerations ratio combined with amplification of spectral acceleration ratio in the range of low vibration periods (Mazza (2016)). In particular, high values of the vertical acceleration are responsible for the formation of plastic hinges at the mid-span section of beams (Di Sarno et al. (2011), Mazza et al. (2024a)), also affecting the functionality of rigid and flexible NSEs (Merino et al. (2023)). In the present work, the effectiveness of a double seismic isolation system, combination of horizontal and vertical base-isolation, for the vertical isolation against near-fault ground motions is investigated. Specifically, an in-series assembly of a high-damping rubber bearing (HDRB) and high-damping rubber layers (HDRLs) is considered, the latter independent on the horizontal and vertical (in tension) responses of the HDRB (Mazza and Braile (2024)). A five-storey reinforced concrete (RC) pavilion of the hospital campus in Avellino, Campania (Italy), is assumed as test structure and retrofitted with the double base-isolation system designed in line with the current Italian seismic code (NTC18 (2018)). MIs with two leaves of equal thickness symmetrically arranged along the interior bays of the perimeter frames are considered. A purpose-built C++ code is implemented for the nonlinear seismic analysis of the test structure, including a lumped plasticity model for RC members and shear, compression and tension nonlinear modelling of the double elastomeric isolation system. Additionally, the IP-OOP nonlinear mutual interaction of MIs is considered and modelled as in Mazza (2021). A homemade MATLAB (MATLAB (2018)) code is also developed for the wavelet analysis of the vertical seismic floor structural accelerations. Nonlinear seismic analysis of the test structure is performed, assuming fifteen near-fault ground motions extracted from the PEER database (PEER (2004)), with and without accounting for their vertical component, with horizontal components rotated in the direction of the strongest pulse (Shahi and Baker (2014)). 2. Test structure A fixed-base five-storey RC pavilion of the hospital campus in Avellino (Campania, Italy), retrofitted by means of a base-isolation system, is chosen as test structure. The original fixed-base framed building is first designed according to an old Italian seismic code (DM96 (1996)), assuming a medium-risk seismic zone at the ultimate limit state, and a drift ratio threshold (Δ/h, where Δ and h represent the inter -storey drift and the storey height, respectively) equal to 0.4% for limiting nonstructural damage of MIs at the serviceability limit state (Mazza (2021)). A cylindrical compressive strength of 25 MPa and a yield strength of 450 MPa are assumed for concrete and steel, respectively. Four frames along the X direction and five along the Y one characterize the rectangular plan shape (Figure 1), with bays of different length and all the storeys 4 m high. MIs with two leaves of equal thickness are arranged symmetrically along the interior bays of the perimeter (red boxes in Figure 1), with the lowest (i.e. L X /h=1) and highest (i.e. L Y /h=1.75) aspect ratios (i.e. width-to-height ratio). The retrofitting of the original building is performed for a high-risk seismic zone and moderately-soft subsoil class (i.e. class C, site amplification factors S H and S V equal to 1), considering both horizontal (PGA H =0.499g) and vertical (PGA V =0.476g) seismic actions in compliance with current Italian standard (NTC18 (2018)). Details regarding cross sections of structural elements (beams and columns) and floor masses are reported in Figure 1, with the latter obtained assuming live loads equal to 3 kN/m 2 at the first and second levels (medical rooms), 2 kN/m 2 at the third and fourth levels (hospital rooms) and on the roof, and 5 kN/m 2 (emergency area) at the ground level above the grid of rigid beams placed at the top of the isolation level, as prescribed by NTC18 (2018). An additional load of 5.5 kN/m is considered on the perimeter beams corresponding to the infilled bays. Further details can be found in Mazza and Braile (2024).