GNGTS 2017 - 36° Convegno Nazionale

328 GNGTS 2017 S essione 2.2 The Rayleigh damping factors α and β for the soil and the structure have been computed as α = D · ω and β =D / ω , being D the damping ratio and ω the pulsation of the system. As for the loading conditions, the computation of gravitational loads to be applied on the structure has been performed as suggested by the Italian technical code (NTC, 2008). Results: effects of soil- nonlinearity in the 2DFEM modelling. Fig. 2c shows the amplification ratios R a for the seven accelerograms at the ground surface under the structure (SSI alignment), for the two approaches used to take into the soil non- linearity, in comparison with the amplification value S calculated by the Italian technical code (NTC, 2008) for soil type E and so equal to 1.34. It is evident that the values obtained are greater than the value provided by the regulations. The smallest value of R a is achieved for 1990 accelerogram, but however it is greater than the same provided by NTC08. Figs. 2b and 2c shows the ratios between the input frequencies and the soil frequency evaluated considering the SSI. In particular, the first three soil fundamental frequencies and the first two input frequencies have been considered. Ratios between 0.5 and 1.5 (shown by blue lines) indicate the frequencies for which a resonance phenomenon may be; 1693(1), 1693(2) e 1818(1) accelerograms are the more severe, because their ratios are in the above-mentioned range. Fig. 3 shows the results in terms of amplification functions A(f) for the seven accelerograms used and for the two approaches used. The soil fundamental frequency is about 4 Hz; this value is very close to the value obtained by the HVSR test (4 Hz). The presence of the structure has modified the frequency content of the soil; the change of the frequency content to values of 2 Hz and 4 Hz. Conclusions . The studies reported in the present paper highlight the important role of the soil non-linearity under dynamic load in response seismic local analysis. With reference to a Catania case-history, different 2D response analyses are performed for two different approaches to take into the soil non-linearity. The results have been presented in terms of amplification ratios R a and amplification function A(f). The results show differents value of amplification ratios for the two approaches used. Furthermore, surface amplification values have been recorded higher than the amplification ratios S given by technical regulation. In the frequency domain, the presence of the structure caused a variation of the frequency, recording a second peak of 2 Hz.