Base-isolated structures are often designed using spectra based on far-fault ground motions and neglecting the contribution of the vertical seismic loads. However, 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. The considerable horizontal deformability of a base-isolated structure may amplify the structural response under strong near-fault ground motions; critical conditions can be reached when the pulse intensity is so strong that the superstructure undergoes plastic deformations. High values of the vertical acceleration can notably modify the axial load in reinforced concrete (r.c.) columns, producing undesirable phenomena (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 due to the vertical acceleration, especially at the upper storeys, where the effects of the vertical (gravity and seismic) 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.

The objective of this work is to check if current NTC08 provisions can be considered adequate for the design of base-isolated structures located on a (high-risk) near-fault area, using different isolation systems. For this purpose, High-Damping-Laminated-Rubber Bearings (HDLRBs) and Lead Rubber Bearings (LRBs) are considered acting alone or combined in parallel or in series with steel-PTFE sliding bearings. The design of base-isolated five-storey r.c. buildings (test structures) is carried out in a high-risk seismic region considering the horizontal seismic loads acting alone or in combination with the vertical ones and assuming different values of the ratio between the vertical and horizontal stiffnesses of the HDRBs (LRBs). More precisely, the following assumptions were made according to the Italian seismic code (NTC08): elastic response of the superstructure in the horizontal and vertical directions; soft subsoil.

A numerical investigation is carried out considering the nonlinear seismic response of the above isolated structures subjected to strong near-fault and far-fault ground motions recorded on soft subsoil. Specifically, accelerograms with a peak ground acceleration (PGA) value approximately comparable, at least for one of the two horizontal directions, with the one prescribed by NTC08 are selected. For each ground motion the attention is focused on both the horizontal component showing the largest PGA value and the vertical component. Moreover, according to recent seismological studies (Baker, 2007) which allow the extraction of the largest (horizontal) pulses from a near-fault ground motion, the residual motion after the pulse extraction is assumed as the corresponding far-fault ground motion.

The nonlinear seismic analysis of the test structures is performed using a step-by-step procedure based on a two-parameter 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 effect of the axial load on the ultimate bending moment (M-N interaction) of the columns is also taken into account. 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 an HDLRB, while a bilinear model is considered for a LRB. Finally, a rigid-plastic (with friction variability) law is assumed to simulate the behaviour of a sliding bearing.