Base-isolation systems for the seismic retrofitting of r.c. framed buildings with soft-storey subjected to near-fault earthquakes

More studies are needed to evaluate the effectiveness of the base-isolation in seismic retrofitting of reinforced concrete (r.c.) framed structures in the case of masonry infills (MIs) not uniformly distributed in elevation. Moreover, amplification of the inelastic demand is generally expected for base-isolated structures located in a near-fault area, in the event of long-duration velocity pulses. In order to understand the nonlinear seismic behaviour of masonry-infilled base-isolated r.c. framed structures, first a six-storey r.c. framed building is primarily designed (as fixed-base) in compliance with a former Italian seismic code, for a medium-risk zone. Then it is retrofitted by the insertion of a base-isolation system with elastomeric and sliding bearings to meet the requirements of the current Italian code, in a high-risk seismic zone. Failure mechanisms of totally and partly infilled structures are compared by considering three structural models: (i) bare structure with nonstructural MIs; (ii) infilled structure with in-elevation uniform distribution of structural MIs; (iii) infilled structure with in-elevation uneven distribution of structural MIs. Nonlinear dynamic analysis of the original (fixed-base) and retrofitted (base-isolated) structures is carried out by a lumped plasticity model describing the inelastic behaviour of the r.c. frame members, while nonlinear force-displacement laws are considered for the elastomeric and sliding bearings. A pivot hysteretic model is assumed to predict the nonlinear force-displacement law of the equivalent diagonal strut adopted for modelling the MIs. Finally, near-fault ground motions with significant horizontal pulses are selected and scaled on the basis of the design hypotheses adopted for the test structures.