Seismic demand of base-isolated irregular structures subjected to pulse-type earthquakes

Base-isolated structures may be subjected to severe seismic demand in the superstructure and/or in the isolation system at sites located near an active fault. Forward directivity effects with long-period horizontal pulses in the fault-normal velocity signals are the main cause of this behaviour. However, recent studies have identified pulses in arbitrary orientations along with false-positive classification of pulse-type ground motions. The aim of the present work is to evaluate the reliability of elastomeric (i.e. high-damping-laminated-rubber bearings, HDLRBs) and sliding (i.e. curved surface sliding bearings, CSSBs) base-isolation systems for the seismic retrofitting of in plan irregular buildings located in the near-fault area. To this end, a five-storey reinforced concrete (r.c.) framed structure, with an asymmetric-plan and bays of different length, is chosen from benchmark structures of the Re.L.U.I.S. project. Attention is focused on the pulse-type and non-pulse-type nature of near-fault earthquakes and moderately-soft and soft subsoil conditions. First, a comparison between algorithms based on wavelet signal processing, that can identify pulses at a single (e.g. fault-normal) or arbitrary orientation in multicomponent near-fault ground motions, is carried out to classify records of recent events in central Italy and worldwide. Then, nonlinear seismic analysis of the fixed-base and base-isolated test structures is performed by using a lumped plasticity model to describe the inelastic behaviour of the r.c. frame members. Nonlinear force-displacement laws are considered for the HDLRBs and CSSBs, including coupled bi-directional motions in the horizontal directions and coupling of vertical and horizontal motions.