Nonlinear Dynamic Response of RC Buildings with Different Base Isolation Systems Subjected to Horizontal and Vertical Components of Near-Fault Ground Motions

Abstract

Near-fault ground motions are characterized by long-duration horizontal pulses and high values of the peak vertical acceleration, which can become critical for a base-isolated structure. In order to check if current code provisions can be considered adequate for the design of base-isolated structures located in a near-fault area, base-isolated five-storey r.c. framed buildings with elastomeric bearings acting alone (“Base Isolation” system) or combined in parallel or in series with sliding bearings (“Base Isolation and in-Parallel Sliding”, BIPS, or “Base Isolation and in-Series Sliding”, BISS, systems) are studied. The base-isolated structures are designed assuming the same values for the fundamental vibration period and equivalent viscous damping in the horizontal direction. Different values of the stiffness ratio, defined as the ratio between the vertical and horizontal stiffness of the elastomeric bearings, are considered; moreover, different values of the sliding ratio, defined as the global sliding force corresponding to an examined BIPS or BISS system divided by the maximum sliding force (in the case of a sliding bearing under each column), are also assumed. The nonlinear analysis of the test structures subjected to strong near-fault ground motions is performed using a step-by-step procedure based on a twoparameter implicit integration scheme and an initial-stress-like iterative procedure. At each step of the analysis, plastic conditions are checked at the potential critical sections of the girders (i.e. end sections of the sub-elements in which a girder is discretized) and columns (i.e. end sections), where a bilinear moment-curvature law is adopted. The response of an elastomeric bearing is simulated by a model with variable stiffness properties in the horizontal and vertical directions, depending on the axial force and lateral deformation, and linear viscous damping. Finally, a rigid-plastic (with friction variability) law is assumed to simulate the behaviour of a sliding bearing.