Effetti delle non-linearità costitutive dei sistemi di isolamento sismico e metodi semplificati per la valutazione della risposta

As other seismic codes do, italian Ordinanza 3274 considers a combined use of linear and non-linear devices for base isolation (hybrid isolation). In the 3274 code the sections on seismically isolated structures are generally very similar in contents to corresponding sections of other codes, except for a simplified non-linear analysis, presented in section 10.7.6, that appears inadequate. In this paper the main theoretical aspects influencing the seismic response of structures isolated using non-linear systems are analysed. To these aspects and to prescriptions of various codes concerning the use of static analyses, some experimental results are added. Release tests carried out on a real scale building isolated using a highly non-linear system, revealed the influence of higher modes on the overall response. The numerical results of some non-linear analyses presented herein, demonstrate how, in the case of “hybrid isolation”, the simplified non-linear analysis proposed by Ordinanza 3274, leads to considerably underestimating shear forces over the height of a structure. Consequently, it is not adequate for describing the higher mode response of a regular base isolated structure.