COMPARISON OF FRICTION MODELS FOR THE CURVED SURFACE SLIDING SYSTEM IN THE NONLINEAR SEISMIC ANALYSIS OF BASE-ISOLATED BUILDINGS

Abstract eng:
Current seismic codes allow for the use of simple bilinear curves to describe the nonlinear response of the curved surface sliding (CSS) bearings. However, the CSS bearings show a complex nonlinear behavior with variations in the friction coefficient, depending on the sliding velocity, axial pressure, heating at the sliding surface and stick-slip phases at the initial motion and motion reversals. Moreover, the friction force and lateral stiffness during the sliding phase are proportional to the axial load. The main object of this study is to investigate the influence that different modelling assumptions of the CSS bearings may have on the lateral-torsional response of irregular base-isolated structures located in near-fault area, characterized by ground motions with large amplitudes and long period horizontal velocity pulses. 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, with a lumped plasticity model to describe the inelastic behaviour of the superstructure, is developed in order to compare eight structural solutions through five models of the CSS bearings that consider: i) constant axial load and constant friction coefficient; ii) constant axial load and variable friction coefficient with velocity; iii) variable axial load and variable friction coefficient with velocity; iv) variable axial load and variable friction coefficient with velocity and pressure; v) variable axial load and variable friction coefficient with velocity, pressure and stick-slip effect. To this end, near-fault ground motions are selected from the Pacific Earthquake Engineering Research center database.

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