Sensitivity to modelling and design of curved surface sliding bearings in the nonlinear seismic analysis of base-isolated r.c. framed buildings

The main object of this study is to investigate the influence that different modelling assumptions of the curved surface sliding (CSS) bearings may have on the lateral-torsional response of irregular base-isolated structures located in near-fault area, characterized by fling-step and directivity effects with large amplitudes and long period horizontal velocity pulses. The second aim is to evaluate the effects of different design assumptions of the CSS system. To this end, a six-storey base-isolated reinforced concrete (r.c.) framed building, with an L-shaped plan and setbacks in elevation, is designed assuming low- and medium-type friction properties, both with two in-plan distributions of the dynamic-fast friction coefficient, corresponding to: (i) the same value for all isolators; (ii) a different value for each isolator. Four additional cases are compared reducing the friction coefficient in accordance with a temperature increase up to 250 °C during ground motions. A computer code for the nonlinear dynamic analysis is developed, in order to compare five models of the CSS bearings that consider constant and variable axial loads combined with constant and variable friction coefficients as function of sliding velocity, axial pressure and stick-slip effect. A lumped plasticity model is used to describe the inelastic behaviour of the superstructure, including a 26-flat surface modelling of the axial load-biaxial bending moment elastic domain at the end sections of r.c. frame members. Near-fault ground motions with significant horizontal components are selected and scaled in line with the design hypotheses adopted for the test structure