GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.1 55 well below the elastic response spectrum of NTC08 as well as the UHRS calculated in the present work, both referring to a 1950-year return period. In the case that the real accelerogram does not fully satisfy the selection criteria, it can be scaled through the addition of wavelets. To adapt the real accelerogram to a specific target response spectrum, the software SeismoMatch (Seismosoft, 2013a), developed by Abrahamson (1992) and later updated by Hancock et al. (2006), has been used. This program allows changing, in the time domain, an acceleration time-history making it compatible with a specified reference spectrum using the technique of Lilhanand and Tseng (1987, 1988), which employs wavelets based on the elastic response in the time domain of a Single Degree Of Freedom (SDOF) system. In the software procedure, three phases are distinguishable: 1) the calculation of the elastic response of a SDOF system under the action of the time series of acceleration, for each defined period and damping level; 2) the comparison between the peak of each SDOF response with the target and the calculation of the difference; 3) the addition of wavelets to the accelerogram with appropriate amplitudes, adjusting the phase so that the peak amplitude of each response matches the target. A single wavelet is used to fit a single SDOF response. This method preserves the phase characteristics, the temporal variability (non-stationarity) and the frequency content. The result is such that each record has an elastic response spectrum that coincides, within a certain tolerance, with the target spectrum. In the present study, we have selected as target spectrum: 1) the spectrum of the Italian building code NTC08 (blue line in Fig. 3a), 2) the average horizontal UHRS obtained in this work (red line in Fig. 3a), and 3) the average spectrum of horizontal and vertical (not shown in the present paper) components of the 5 selected real recordings (black line in Fig. 3a). The match between the modified time histories and the target spectra for the NTC08 (Fig. 3b) and UHRS (Fig. 3c), both referring to a 1950- year return period, is satisfactory for all frequencies. Synthetic time histories . The use of real accelerograms and their matching to a specific target spectrum is recommended by the Italian legislation for obtaining a set of recordings to be used in the dynamic non-linear analysis of the structures. However, if the access to real accelerograms presents, for any reason, difficulties, it is allowed to generate synthetic accelerograms to be matched to a specific target response spectrum. Despite a sufficient number of real accelerograms have been collected for each site, an additional set of synthetic accelerograms have been calculated using the SeismoArtif v2.1 software (Seismosoft, 2013b). This software, based on the work of Halldorsson and Papageorgiou (2005), first generates a synthetic accelerogram, whose characteristics of duration, amplitude, envelope and phase distribution are determined according with the magnitude ( M ) and the (Joyner-Boore) distance of the seismic event to be simulated. The soil type (based on V S30 ) and the tectonic regime (intra-plate, inter-plate; active extensional tectonic regimes) of the site under study are also taken into account. In a second step, an iterative procedure modifies the distribution of the amplitudes in the frequency domain to obtain the spectrum convergence to the desired response (Mucciarelli et al. , 2004). The result is an accelerogram that fulfills all the legislation requirements. In the present study, we have selected as target spectrum: 1) the average spectrum of horizontal components of the 5 selected real accelerograms (black line in Fig. 3a) and Fig. 3d shows the resulted match, that is satisfactory for the whole frequency range. Additional elaborations . Another set of analyse, useful to better define the seismic background of the dam, has consisted in the monitoring of accelerations that theoretically may have affected the dam site in the past, resulting by historical and instrumentally recorded earthquakes. The accelerations have been estimated using the Sabetta and Pugliese (1996) GMPE, which is suitable for events in the Italian territory. Moreover, the accelerations caused by recent earthquakes that the dams have actually experienced after their construction and instrumentally recorded have been also considered. Conclusions . The present study is an example of integration between disciplines traditionally associated with both the research and industrial worlds, with a direct practical application. In