PSI - Issue 44

Fabio Mazza et al. / Procedia Structural Integrity 44 (2023) 1172–1179 Fabio Mazza / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction Records of strong earthquakes worldwide have confirmed that the ratio of the vertical-to-horizontal peak ground accelerations may be very high in the near-fault area, with an amplification of the corresponding ratio of spectral accelerations in the range of low vibration periods (Mazza (2016); Mazza et al. (2017)). Renewed attention on the effects induced by the vertical component of near-fault earthquakes on the nonlinear response of base-isolated structures with elastomeric bearings (e.g. high damping rubber bearings, HDRBs) stems from experimental observation of the notable decrease of horizontal stiffness of an isolation system for high vertical stress (De Luca et al. (2022)). Note that purely elastomeric isolation systems are generally unable to provide effective vertical isolation. In fact, a conventional base-isolation system may amplify the vertical seismic response of the superstructure in a way similar to that experienced in a fixed-base structure, extending plastic hinge formation at the mid-span sections of beams at the upper storeys and accelerating the collapse of nonstructural elements (Di Sarno et al. (2011)), while low values of shape factor of the HDRBs might cause rocking instability (Lee and Constantinou (2018)). Initial attempts of vertical seismic isolation by means of HDRBs can be found in critical nuclear power plants, involving large-scale complex solutions (Zhou et al. (2016)): e.g. the elastomeric bearing-air spring and bearing hydraulic spring systems. On the other hand, solutions for use in the vertical seismic isolation of multi-storey framed structures have been recently proposed: e.g. in-series combinations of horizontal HDRB and inclined lead RB (Liu et al. (2018)) and HDRB and HDR layer (HDRL), the latter only subjected to vertical deformation (Pourmasoud et al. (2020)). However, there is a lack of information on the isolation ratio required in the vertical direction in order to obtain effective protection against the significant vertical component of near-fault earthquakes. To further complicate matters, the fundamental vibration period of an inelastic structure in the vertical direction may elongate in a range matching the predominant frequency content of the vertical ground motion thereby inducing a moving resonance effect (Naga and Eatherton (2014)). The first part of this study provides insight into the weakness of conventional base-isolation systems in order to improve structural performance and functionality of nonstructural elements of vital facilities related to significant near fault vertical accelerations. A fixed-base hospital pavilion with a five-storey reinforced concrete (RC) framed structure is retrofitted by the insertion of HDRBs, in line with the provisions of European codes (EC8 (2005); EN 15129 (2009)) and considering vertical seismic loads. To this end, four sets of design properties of the isolation system are evaluated in line with nominal properties and upper-bound, lower-bound and mixed values accounting for ageing, environmental and production variability conditions. Then, effects of vertical stiffness variability of the base-isolation system are investigated with reference to an in-series combination of previously defined HDRB and HDRL, so that vertical isolation is achieved without affecting the horizontal one. Specifically, HDRL is designed for eight different values of the stiffness ratio of the isolation system, defined as the ratio between the vertical and horizontal stiffnesses. Nonlinear seismic analysis of the test structures is carried out with reference to near-fault earthquakes selected from the Pacific Earthquake Engineering Research center database (PEER (2014)) in accordance with criteria specified in FEMA P695 (FEMA (2009)). Finally, wavelet analysis including the moving resonance effect induced by inelastic deformation of the superstructure is carried out, in order to obtain the vertical isolation ratio capable of shifting vertical response of the superstructure away from vibration periods of maximum amplification. 2. Horizontally and vertically base-isolated test structures A five-storey fixed-base RC pavilion of the hospital complex in Avellino, Campania (Italy), with regular plan and elevation, is supposed to be retrofitted with a base-isolation system constituted of HDRBs acting alone (Figures 1a,b) or in combination with HDRLs (Figures 1c,d). A high-risk seismic zone (i.e. peak ground acceleration on rock, a g =0.499g at the collapse prevention limit state), moderately soft subsoil (i.e. class C, site amplification factor S=1.022) and reference structural life V R =150 years are provided by Italian code (NTC18 (2018)). The dead loads are assumed equal to 7.23 kN/m 2 , for the first four floors, and 5.93 kN/m 2 , for the roof, with an additional dead load of 5.5 kN/m for masonry infills on the perimetral beams. Live loads provided by NTC18 are also considered on the floor levels, as function of their destination: i.e. 5 kN/m 2 at the ground level, as emergency area; 3 kN/m 2 at the first and second levels, for labs; 2 kN/m 2 at the third and fourth levels, for hospital wards, and on the roof. Interior flat beams are oriented