Abstract
In this paper the site categorization criteria and the corresponding site amplification factors proposed in the 2021 draft of Part 1 of Eurocode 8 (2021-draft, CEN/TC250/SC8 Working Draft N1017) are first introduced and compared with the current version of Eurocode 8, as well as with site amplification factors from recent empirical ground motion prediction equations. Afterwards, these values are checked by two approaches. First, a wide dataset of strong motion records is built, where recording stations are classified according to 2021-draft, and the spectral amplifications are empirically estimated computing the site-to-site residuals from regional and global ground motion models for reference rock conditions. Second, a comprehensive parametric numerical study of one-dimensional (1D) site amplification is carried out, based on randomly generated shear-wave velocity profiles, classified according to the new criteria. A reasonably good agreement is found by both approaches. The most relevant discrepancies occur for the shallow soft soil conditions (soil category E) that, owing to the complex interaction of shear wave velocity, soil deposit thickness and frequency range of the excitation, show the largest scatter both in terms of records and of 1D numerical simulations. Furthermore, 1D numerical simulations for soft soil conditions tend to provide lower site amplification factors than 2021-draft, as well as lower than the corresponding site-to-site residuals from records, because of higher impact of non-linear (NL) site effects in the simulations. A site-specific study on NL effects at three KiK-net stations with a significantly large amount of high-intensity recorded ground motions gives support to the 2021-draft NL reduction factors, although the very limited number of recording stations allowing such analysis prevents deriving more general implications. In the presence of such controversial arguments, it is reasonable that a standard should adopt a prudent solution, with a limited reduction of the site amplification factors to account for NL soil response, while leaving the possibility to carry out site-specific estimations of such factors when sufficient information is available to model the ground strain dependency of local soil properties.
Highlights
In modern seismic codes, the actions for design are generally based on a seismic hazard assessment at the reference ground
In the 2021-draft of Part 1–1 of Eurocode 8 (CEN/TC250/SC8 2021), the site categorization criteria and site amplification factors were modified with respect to EC8-1 (CEN 2004), with the main objective to reduce classification ambiguities and to provide a continuous formulation of SAFs that could lead to a smooth transition of amplification levels from class to class
The continuous formulation of short and intermediate period SAFs in the 2021-draft (Table 4) was introduced to approach the median period-dependent values obtained by earthquake records and expressed through the site-dependent coefficients of Ground Motion Model (GMM)
Summary
The actions for design are generally based on a seismic hazard assessment at the reference ground. Because of the previous limitations, the EC8-1 criteria were reassessed within the comprehensive work in progress by the Committee CEN/TC250/SC8 "Eurocode 8; Earthquake resistance design of structures" for drafting the new version of Eurocode 8 This implied a new definition of elastic design spectra, where spectral amplitudes are introduced based on two parameters related to seismic hazard, i.e. the constant acceleration branch at short periods (Sα,RP); and the spectral acceleration at T = 1 s (Sβ,RP). The non-linear correction factors to site amplification, introduced within the 2021-draft, are checked through the detailed analysis of ground response at several Kik-Net stations, i.e., some of the few worldwide stations that recorded both weak and strong motions, at both ground surface and borehole levels In this way, it is possible to obtain a quantitative estimation of the non-linear reduction effects on the response spectral ordinates
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