Vertical Seismic Action for Design: Developments for the Next Generation of European and Italian Norms

his paper addresses the definition of the vertical seismic action for design, within the progress studies for the next generation of the European and Italian seismic norms, partly still in progress. Besides presenting the background of the vertical design spectra according to the second generation of the Eurocode 8, a period-dependent factor for the Vertical-to-Horizonal (V/H) response spectral ratios is introduced, following the findings from recent empirical studies. The proposed V/H factor is expressed in a simple format as a function of relatively few parameters (i.e., seismicity level and site conditions), which are typically adopted in a normative framework, being, at the same time, suitable to identify the main physical factors affecting the V/H ratios. This factor is found to explain reasonably well the results of probabilistic seismic hazard analyses for both H and V components conducted for selected test sites in Italy, with different approaches to account for the V component. A favorable comparison with probabilistic results is also obtained for the V/H spectral ratios according to the new Eurocode 8.