The restoring (or re-centring) capability is an important feature of any isolation system and a fundamental requirement of current standards and guideline specifications for the design of seismically isolated structures. In this paper, the restoring capability of spherical sliding isolation systems, often referred to as friction pendulum systems (FPSs), is investigated through an extensive parametric study involving thousands of non-linear response history analyses of SDOF systems. The dynamic behavior of the isolation system is described with the visco-plastic model of Constantinou et al. (J Struct Eng 116(2):455–474, 1990), considering the variability of the friction coefficient with sliding velocity and contact pressure. Numerical analyses have been carried out using a set of approximately three hundred natural seismic ground motions recorded during different earthquakes and differing in seismic intensity, frequency content characteristics, magnitude, epicentral distance and soil characteristics. Regression analysis has been performed to derive the dependency of the residual displacement from the parameters governing the dynamic response of FPS. The influence of near-fault earthquakes and the accumulation of residual displacements due to real sequences of seismic ground motions have been also investigated. Finally, the restoring compliance criteria proposed in this study are compared to the lateral restoring force requirements of current seismic codes. Based on the results of this study, useful recommendations for a (more) rational design of FPSs are outlined

Themechanical properties of elastomers can change significantly due to air temperature variations. In particular, prolonged exposure to subzero temperatures can result in rubber crystallization, with a considerable increase in the shear stiffness of the material. As a result, the seismic response of structures with elastomeric isolators can be strongly influenced by air temperature. Current seismic codes, indeed, require an upper and lower bound analysis, using suitable modification factors, to account for the changes in the cyclic behavior of elastomeric isolators due to air temperature variations. In this study, the sensitivity of the cyclic behavior of elastomeric isolators to air temperature variations is investigated based on the experimental results of an extensive test program on six different elastomeric compounds for seismic isolators, characterized by a shear modulus ranging from 0.5 to 1.2MPa at 100% shear strain and 20◦C. The cyclic tests have been performed on small-size specimens, subjected to shear strain amplitudes and frequency of loading typical for elastomeric seismic isolators, at seven different air temperatures, ranging from 40 to−20◦C. The effects of rubber\ud crystallization due to prolonged exposure to low-temperatures have been also investigated. A finite element model for the evaluation of the temperature contour map inside a full-size\ud elastomeric isolator exposed to low air temperatures has been also developed. In the paper, the experimental outcomes are compared with the modification factors provided by the current\ud seismic codes to account for the temperature effects on the mechanical properties of elastomeric isolators

A comprehensive parametric study on the inelastic seismic response of seismically isolated RC frame buildings, designed for gravity loads only, is presented. Four building prototypes, with 23 m by 10 m floor plan dimensions and number of storeys ranging from 2 to 8, are considered. All the buildings present internal resistant frames in one direction only, identified as the strong direction of the building. In the orthogonal weak direction, the buildings present outer resistant frames only, with infilled masonry panels. This structural configuration is typical of many existing RC buildings, realized in Italy and other European countries in the 60s and 70s. The parametric study is based on the results of extensive nonlinear response-time history analyses of 2-DOF systems, using a set of seven artificial and natural seismic ground motions. In the parametric study, buildings with strength ratio (Fy/W) ranging from 0.03 to 0.15 and post-yield stiffness ratio ranging from 0% to 6% are examined. Three different types of isolation systems are considered, that is, high damping rubber bearings, lead rubber bearings and friction pendulum bearings. The isolation systems have been designed accepting the occurrence of plastic hinges in the superstructure during the design earthquake. The nonlinear response-time history analyses results show that structures with seismic isolation experience fewer inelastic cycles compared with fixed-base structures. As a consequence, although limited plastic deformations can be accepted, the collapse limit state of seismically isolated structures should be based on the lateral capacity of the superstructure without significant reliance on its inherent hysteretic damping or ductility capacity

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