Intermediate Isolation System for the seismic retrofit of existing masonry buildings

This paper deals with the potentials of Intermediate Isolation Systems (IIS) for retrofitting masonry buildings by means of a vertical extension, isolated at its base, and realized on the rooftop of the existing structure. By carefully defining the dynamic properties of the superstructure and isolation system, in previous papers the authors have demonstrated that this passive technique is able to reduce a priori the seismic demand on the existing building. For this reason, the non-linear behavior has been only assumed for the isolation system, with the masonry structure in the elastic field. This result has been validated by carrying out spectrum-compatible time history analyses on the 3D FE models of some case studies, and design criteria have been also derived. However, ground motions of intensity larger than the design level cannot be excluded, and the hysteretic behavior of the masonry structure should be taken into account. For this aim, Pivot-type hysteretic models are examined for assessing the representativeness of masonry behavior. By utilizing a case study, 3D FE models are developed, and non-linear time history analyses are carried out for assessing the effect of masonry nonlinearities on the retrofit solution designed according to linearity assumptions.