GNGTS 2014 - Atti del 33° Convegno Nazionale

440 GNGTS 2014 S essione 2.3 with important public or strategic functions, also in regard to the management of civil protection in case of calamity. Industries with activities particularly dangerous for the environment [...]”. Therefore, the conventional design life of the tank, obtained by the product of the nominal life expectancy and the importance factor (2, in this case), turns out to be 100 years. This value for the design life leads to the following reference return periods for each limit state: no – collapse requirement 1950 years, life safety requirement 949 years, damage limitation requirement 101 years and operational requirement 60 years. From the velocity model based on the geophysical analysis, the Vs30 parameter is equal to 430 m/s, which suits ground type B, but the spectra have been calculated on a ground type A, as the site effects have been taken into account through the software Pshake (Sanò and Pugliese, 1991). The T1 topography category has been adopted, since the tank is located on a flat ground. Despite several attempts it was not possible to find ground motion records compatible, nor within the Italian or European database. However, the national seismic code allows to scale the selected accelerograms in order to satisfy the target spectrum. We proceeded this way through the software SeismoMatch (Seismosoft, 2013a) developed by Abrahamson (1992) and then updated by Hancock et al. (2006). The software modifies an acceleration time history in the time domain in order to make it match with a specified spectrum using the technique proposed by Lihanand and Tseng (1987, 1988). This technique employs waves forms called wavelets, that uses the answer of elastic SDOF systems (Single Degree of Freedom) in the time domain. In the software procedure three phases can be distinguished: 1) the calculation of an elastic SDOF answer under the action of the acceleration time series, for each reference period and for each defined damping ratio; 2) the comparison between the peak of each SDOF answer with the target amplification and the calculation of the difference between the two; 3) the addition of wavelets with appropriated amplitudes to the ground motion data, suiting the phase in order that the peak of each answer corresponds to the target amplitude. A single wavelet is used to match each single SDOF answer. The matched accelerograms were then processed again with SeismoSignal (Seismosoft, 2013c) in order to get three spectra with a damping ratio of 5%, 2% and 0.5% for each accelerogram. The average between the 5 spectra for each damping value has then been calculated and used as input for the 1D model through Pshake. The amplification is considerable for periods between 0.15 and 0.5 s (Fig. 3). Fig. 3 – comparison between spectra obtained with accelerograms and amplified with PShake software and the non amplificated ones (n.a.), with and the spectrum obtained in accordance to the legislation (diagram cut at 1.5 s for a clear display of results and comparisons).