ANALISI NON LINEARE DELLA RISPOSTA SISMICA DEI TERRENI - IL CASO DEL CENTRO STORICO DI FABRIANO

Non-linear seismic response analysis of soil deposits This paper describes a newly developed computer code for performing one-dimensional, non-linear dynamic analysis (ONDA) of soil deposits. Non-linearity is modelled by assuming 1) a "backbone" curve to describe the initial loading stress -strain behaviour, and 2) a "rule" for modelling the unloading- reloading behaviour as the seismic excitation progresses (KRAMER 1996). The backbone curve can be obtained from quasi-static monotonic loading tests. Concerning the rule to model the unloading -reloading soil behaviour a common choice is the so-called "Masing rule" (MASING 1926) by which it is assumed that the unload-reload branches of the stress -strain curve have the same shape of first loading curve with a scale factor (n) equal to 2. In this work the 2° Masing rule has been modified by assumi ng a factor (n) not necessarily equal to 2 (TATSUOKA et al. 1993). In particular, in case of cyclic hardening, n will be greater than 2, while in the case of cyclic softening n can become even smaller than 2. ONDA gives then the possibility of computing permanent strains that are developed during seismic excitation and hence of taking into account the phenomenon of material degradation. ONDA and the equivalent linear code SHAKE (SCHNABEL et al. 1972) have been used to evaluate the seismic response at Fabriano. Differences between the results obtained by ONDA and SHAKE are discussed in detail. SOMMARIO: Analisi non lineare della risposta sismica dei terreni