Base-isolation against out-of-plane seismic damage of masonry infills in existing hospitals

Masonry infills (MIs) are often prone to out-of-plane (OOP) collapse mechanisms during earthquakes, with devastating consequences for public buildings such as hospitals. Base-isolation systems represent one of the most effective techniques currently used for the seismic protection of structural parts and reducing the risk of the in-plane (IP) damage of MIs, but no attention has been paid in the literature to their influence on improving the OOP behaviour of these nonstructural components. To full this gap, a medical centre with a five-storey reinforced concrete (r.c.) framed structure is designed (as fixed-base) in compliance with a former Italian seismic code, for a medium-risk zone. Four infill aspect ratios (i.e. width-to-height ratio equal to 1, 1.25, 1.5 and 1.75) are examined, combining bays of different lengths with exterior (i.e. configuration C1) and interior (i.e. configuration C2) arrangements of MIs. Four structural models are considered, assuming: i) and ii), bare structures with nonstructural MIs, constructed so as to avoid affecting structural stiffness, fulfilling provisions of the former and current Italian seismic codes for limiting nonstructural damage; iii) and iv), infilled structures, with the C1 and C2 configurations of structural MIs in contact with the frame but not structurally connected, applying only provisions of the former Italian code. Then, these structures are retrofitted with a base-isolation system of high-damping-rubber bearings (HDRBs), to meet the requirements of the current Italian code in a high-risk seismic zone. The same values of the fundamental vibration period and equivalent viscous damping ratio in the horizontal direction are considered for all retrofitted structures. A five-element macro-model comprising four diagonal nonlinear beams and one (horizontal) central nonlinear truss for the prediction of OOP and IP behaviour of MIs, respectively, is implemented in a C++ computer code for the nonlinear dynamic analysis of the infilled r.c. framed structures. The proposed algorithm addresses the issue of nonlinear interaction by modifying stiffness and strength values of the MI in the OOP direction on the basis of simultaneous or prior IP damage. R.c. frame members of the superstructure are described by a lumped plasticity model, with hardening ratio equal to 3%, in which the axial load and biaxial bending moment interaction of the r.c. cross-sections is computed by a piecewise linearization of the limit surface. An advanced three-spring-three-dashpot model is adopted to take into account the observed behaviour of the HDRBs during severe earthquakes: high vertical forces significantly affect the horizontal response; softening occurs in the vertical direction with notable lateral deformations; horizontal stiffness lessens with increasing horizontal displacement; the equivalent viscous damping in the horizontal direction depends on the amplitude of displacement the bearing is subjected to and, ultimately, on the amplitude of the shear strain. Finally, bare and infilled models of the fixed-base and base-isolated hospitals are subjected to biaxial spectrum-compatible far- and near-field artificial accelerograms scaled at the level of the life-safety provided by the current Italian seismic code.