ABSTRACT

The Italian building code (NTC08) and Eurocode 8 (EC8-1) take into account the influence of local site conditions on seismic site response by defining, for different soil classes, an appropriate elastic design spectrum. The so-called "soil factor" S represents ground motion amplification with respect to outcrop conditions. The main parameter used in site classification is V_s,30, but this assumption is a rather simplified hypothesis, which may be misleading in many cases. Therefore new site classifications have been recently proposed, considering other site parameters. In order to analyze the influence of these parameters and to obtain a reliable soil classification, a comprehensive stochastic site response study has been conducted using numerical procedures. In detail, the activity has been focused on the set up of a database of 1D equivalent linear elastic site response analyses, carried out using the software SHAKE91, considering twelve different real records of acceleration time histories as input. The adopted stochastic method is based on a Monte Carlo approach: for each simulation the input parameters (i.e. thickness of each layer, V_s and PI values, water table depth, unit weight, shear modulus reduction and damping curves) are randomly selected according to some previously defined probability distributions. In order to have meaningful results, 320000 deterministic predictions of site response have been carried out for each seismic input. Then, using a multi-variable analysis and allowing for the main site parameters, it has been proposed a new soil classification that may improve seismic code provisions.