Reinforced concrete(r.c.) framed buildings designed in compliance with inadequate seismic classifications and code provisions present in many cases a high vulnerability and need to be retrofitted.To this end,the insertion of a base isolation system allows a considerable reduction of the seismic loads transmitted to the superstructure.However,strong near-fault ground motions,which are characterised by long-duration horizontal pulses,may amplify the inelastic response of the superstructure and induce a failure of the isolation system.The above considerations point out the importance of checking the effectiveness of different isolation systems for retrofitting a r.c.framed structure.For this purpose,a numerical investigation is carried out with reference to a six-storey r.c.framed building,which,primarily designed(as to be a fixed-base one) in compliance with the previous Italian code(DM96) for a medium-risk seismic zone,has to be retrofitted by insertion of an isolation system at the base for attaining performance levels imposed by the current Italian code(NTC08) in a high-risk seismic zone.Besides the(fixed-base) original structure,three cases of base isolation are studied:elastomeric bearings acting alone(e.g.HDLRBs);in-parallel combination of elastomeric and friction bearings(e.g.high-damping-laminated-rubber bearings,HDLRBs and steel-PTFE sliding bearings,SBs);friction bearings acting alone(e.g.friction pendulum bearings,FPBs).The nonlinear analysis of the fixed-base and base-isolated structures subjected to horizontal components of near-fault ground motions is performed for checking plastic conditions at the potential critical(end)sections of the girders and columns as well as critical conditions of the isolation systems.Unexpected high values of ductility demand are highlighted at the lower floors of all base-isolated structures,while re-centring problems of the base isolation systems under near-fault earthquakes are expected in case of friction bearings acting alone(i.e.FPBs) or that in combination(i.e.SBs) with HDLRBs.
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