According to the current Italian Building Code (NTC08) the seismic input in stable (i.e. non-liquefiable) soils can be defined using a simplified approach to account for local site effects. For the horizontal component of ground motion this is based on the introduction of five different soil categories. Specific soil factors (SS and CC), associated to each soil category, allow modifying amplitude and shape of elastic acceleration response spectra computed at rocky (i.e. stiff) sites (reference spectra). Questions have been raised about the accuracy of this approach to predict site-specific ground motion and specifically on the ability of soil factors to properly quantify local site amplification due to the large variability of geotechnical parameters that may be associated to a given soil category, the uncertainty of the reference seismic input and the frequency range of interest of most structures. With the objective to clarify and, at the same time, quantify the relevance of the above-mentioned problem, a large number of 1D linear-equivalent, ground response analyses have been carried out using Monte Carlo simulations coupled with the Latin hypercube sampling technique. From this fully stochastic ground response analysis, new parameters SS and CC have been derived from the shape of site-specific, 84th percentile, free surface, acceleration responsespectra and new hazard-dependent empirical correlations are proposed to define upgraded soil factors.
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