Realistic Modelling of the Seismic Input: Site Effects and Parametric Studies

The work done in the framework of a large international cooperation, showing the very recent numerical experiments carried out within the framework of the EC project "Advanced methods for assessing the seismic vulnerability of existing motorway bridges" (VAB) to assess the importance of non-synchronous seismic excitation of long structures have been illustrated. The definition of the seismic input at the Warth bridge site, i.e. the determination of the seismic ground motion due to an earthquake with a given magnitude and epicentral distance from the site, has been done following a theoretical approach. In order to perform an accurate and realistic estimate of site effects and of differential motion it is necessary to make a parametric study that takes into account the complex combination of the source and propagation parameters, in realistic geological structures. The results for the final local model, characterized by an exaggeratedly thick and low velocity layer, demonstrate that a deep source excites lower frequencies than a shallow one and that the effect of increasing the epicentral distance is to attenuate high frequencies, making the resonant peaks, present at frequencies around 0.8 Hz, the dominant features of the entire spectra. The main practical conclusion of our analysis, verified by laboratory experiments, is that the Warth bridge is likely to well stand the most severe seismic input compatible with the seismic regime of the Eastern Alps.