GNGTS 2013 - Atti del 32° Convegno Nazionale

dampers to add damping at each of the nodes at the base of the finite model. Moreover, the software includes a method for the introduction of damping matrices to reduce the damping at highest frequencies, commonly associated with the Rayleigh damping formulation (Lanzo et al. , 2003). The latter one establishes two control frequencies that define the frequency interval where the damping can be assumed free from numerical bias. For the 1D and 2D numerical analyses preliminarily five different accelerograms (“DET1”, “DET2”, “DET3”, “NTC08”, “PROB LADE”) selected for the seismic microzonation studies (MS–AQWorking Group, 2010), were used as input ground motions applied on the outcropping bedrock. In short, this paper, illustrates only the results obtained by “DET1” accelerogram together with those achieved by a simple richer pulse. In particular, “DET1” is compatible with the deterministic spectrum obtained from Sabetta and Pugliese (1996) attenuation relation for the moment magnitude M w – epicentral distance R epi pair ( M w = 6.7, R epi = 10 km) established by means of disaggregation analysis. In addition, this accelerogram is scaled for the site of L’Aquila, to a peak ground acceleration PGA = 0.261 g, for a return period T R = 475 years and a ground type “A”, according to European building code (CEN, 2003) and Italian building code (NTC, 2008). The richer pulse is characterized by a length of about 0.2 s and a PGA = 0.261 g. It is important to highlight that “DET1” is an artificial accelerogram, not admitted by NTC (2008) and CEN (2003) in order to provide an elastic response spectrum. Nonetheless this accelerogram was used to check the reliability of the geotechnical model proposed in linear- equivalent approximation. At first numerical analyses were run by applying a richer pulse, as input motion. Due to the characteristic of the input source (impulsive and short induration), the richer pulse didnot allowto reach significant strain-compatible values, able to produce important non-linear effects. On the other side, the use of this pulse do extend the range of frequencies not affected by numerical bias. In fact, in this 2D modeling the first frequency of the soil deposit is 0.4 Hz, while the seismic input is characterized by a predominant period of 0.18 s for “DET1” and 0.06 s for the richer Fig. 3 – Response spectra obtained from 2D numerical modeling in linear and linear equivalent approximation. For comparison, it is shown the elastic response spectrum obtained in 1D modeling. 176 GNGTS 2013 S essione 2.2