The 14 th World Conference on Earthquake Engineering October 12-17, 2008, Beijing, China EFFECTS OF HORIZONTAL AND VERTICAL NEAR-FAULT GROUND MOTIONS ON THE NONLINEAR DYNAMIC RESPONSE OF RC BUILDINGS WITH DIFFERENT BASE-ISOLATION SYSTEMS F. Mazza 1 and A. Vulcano 2 1 Researcher, Dept. of Engineering Modeling, University of Calabria, Rende (Cosenza), Italy 2 Professor, Dept. of Engineering Modeling, University of Calabria, Rende (Cosenza), Italy Email: fabio.mazza@unical.it, vulcano@unical.it ABSTRACT: Near-fault ground motions are characterized by long-duration horizontal pulses and high peak values of vertical acceleration, which can become critical for a base-isolated structure. The objective of this work is to study effectiveness and limitations of base-isolation systems obtained considering High-Damping-Laminated-Rubber Bearings (HDLRBs) acting alone or either in parallel or in series with steel-PTFE sliding bearings. A numerical investigation is carried out with reference to base-isolated five-storey reinforced concrete buildings designed according to the European seismic code (Eurocode 8). The design of the base-isolated test structures (in case of HDLRBs only) is carried out in a high-risk seismic region considering the horizontal seismic loads acting in combination with the vertical ones and assuming different values of the ratio between horizontal stiffness and vertical stiffness of the HDLRBs. The behaviour of the frame members is simulated by a bilinear law, checking plastic conditions at potential critical sections. A viscoelastic model with variable stiffness properties in the horizontal and vertical directions, depending on the axial force and lateral deformation, simulates the response of a HDLRB. A rigid-plastic (with friction variability) law is assumed to simulate the behaviour of a steel-PTFE sliding bearing. The nonlinear dynamic behaviour of the test structures is studied under near-fault earthquakes. KEYWORDS: base-isolated building, base isolation, elastomeric bearing, sliding bearing, nonlinear seismic analysis, near-fault earthquake 1. INTRODUCTION In the last two decades the seismic isolation technique proved to be very effective for the seismic protection of new constructions as well as for the seismic retrofitting of existing ones. Design guidelines for seismically isolated structures have been developed in many countries with a high seismic hazard (ASSISi, 2003) and, lately, suitable code provisions have been drafted also in Europe (Eurocode 8, 2003: EC8). The considerable horizontal deformability of a base-isolated structure may amplify the structural response under strong near-fault ground motions, which are characterized by long-duration horizontal pulses and so large displacements that an oversizing of the isolation system could be required (Kelly, 1999). Specifically, the frequency content of the motion transmitted by the isolators to the superstructure can become critical when the pulse intensity is so strong that the superstructure undergoes plastic deformations. Strong near-fault ground motions are also characterized by high values of the ratio α PGA (=PGA V /PGA H ) between the peak value of the vertical acceleration, PGA V , and the analogous value of the horizontal acceleration, PGA H . High values of the acceleration ratio α PGA can notably modify the axial load in reinforced concrete (r.c.) columns, producing undesirable phenomena (Papazoglou and Elnashai, 1996): e.g., buckling of the longitudinal bars, brittle failure in compression, bond deterioration or failure under tension. In addition, plastic hinges are expected along the span of the girders owing to the vertical acceleration, especially at the upper storeys, where the effects of the gravity loads generally prevail over those of the horizontal seismic loads and an amplification of the vertical motion is expected also depending on the vertical stiffness of the isolators (Mazza and Vulcano, 2006a, 2006b). To overcome the above problems, many solutions using different isolation systems were proposed (e.g., Jangid and Kelly, 2001, Mazza and Vulcano, 2004). The objective of this work is to compare different base-isolation techniques, evaluating their effects and applicability limits through the study of the nonlinear dynamic response of base-isolated five-storey r.c. framed buildings subjected to horizontal and vertical near-fault ground motions.