NONLINEAR DYNAMIC BEHAVIOUR OF BASE-ISOLATED BUILDINGS WITH THE FRICTION PENDULUM SYSTEM SUBJECTED TO NEAR-FAULT EARTHQUAKES

In spite of the fact that avant-garde sliding bearings have been proposed, the application of the single friction pendulum (FP) bearing is increasing due to its conceptual simplicity; yet there are still important aspects of its behaviour that need further attention. More specifically, the FP system presents spatial variation of friction coefficient, depending on the sliding velocity of the FP bearings. Moreover, the frictional force and restoring stiffness during the sliding phase are proportional to the axial load. Long duration intense velocity pulses in the horizontal direction and high values of the ratio between vertical and horizontal peak ground acceleration are expected for near-fault earthquakes. Torsion with residual displacement and uplift of the FP system need to be better understood for base-isolated structures located in near-fault areas. To this end, a numerical investigation is carried out with reference to a six-storey reinforced concrete (r.c.) framed building, characterized by an L-shaped plan with wings of different length and setbacks at different heights along the main in-plan directions. Twelve base-isolated test structures are designed in line with the Italian seismic code, considering (besides the gravity loads) the horizontal seismic loads acting alone or in combination with the vertical ones. Three design values of the radius of curvature for the FP system and two in-plan distributions of dynamic-fast friction coefficient for the FP bearings, ranging from a constant value for all isolators to a different value for each, are assumed. A nonlinear force-displacement law of the FP bearings in the horizontal direction, depending on sliding velocity and axial load, is considered, while a gap model takes into account the vertical uplift of the FP bearings. The nonlinear seismic analysis is performed on two sets of seven near-fault earthquakes, both sets with significant horizontal or vertical components selected and normalized on the basis of the design hypotheses adopted for the test structure.