Nonlinear seismic analysis of irregular r.c. framed buildings base-isolated with friction pendulum system under near-fault excitations

The application of the friction pendulum (FP) system is increasing due to its conceptual simplicity; yet there are still aspects of its behaviour that need further attention. Long duration intense velocity pulses in the horizontal direction and high values of the ratio between vertical and horizontal peak ground acceleration are expected in the near-fault areas. Base-isolated structures subjected to near-fault earthquakes present variation of friction force and lateral stiffness of the FP system during the sliding phase which can induce torsion with residual displacement and uplift. To investigate these effects, a nonlinear dynamic analysis is carried out considering 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 in-plan principal 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 and two in-plan distributions of dynamic-fast friction coefficient are assumed for the FP bearings, ranging from a constant value for all isolators to a different value for each. A nonlinear force-displacement law of the FP bearings is considered in the horizontal direction, depending on sliding velocity and axial load, while a gap model takes into account the vertical uplift of the FP bearings. The nonlinear seismic analysis is performed on near-fault ground motions with significant horizontal or vertical components, selected and normalized on the basis of the design hypotheses adopted for the test structure.