Design actions for Continuous Deck Bridges considering Non Synchronous earthquake Motion

This paper aims to develop and validate structural design criteria which account for the effects of earthquakes spatial variability. In past works (Nuti & Vanzi, 2004 & 2005; Carnevale et al., 2010) the two simplest forms of this problem were dealt with: differential displacements between two points belonging to the soil or to two single degree of freedom structures. Seismic action was defined according to Seismic Codes and structure was assumed as a linear elastic sdof oscillator. Despite this problem may seem trivial, existing codes models appeared improvable on this aspect. For the differential displacements of two points on the ground, these results are now validated and generalized using response spectra of both EC8 and new Seismic Italian Code (Ministero Infrastrutture, 2008). The problem of statistically defining the differential displacement among any number of points on the ground (which is needed for continuous deck bridges) is approached. The model is used to compute the differential displacements of points on the ground, both for two and multiple points cases, and with different response spectra shapes. Preliminary results indicate that the design Codes can be strongly improved on this topic, both for the two points (e.g. simply supported decks) and the multiple points (e.g. continuous decks on multiple piers) cases. The results, in terms of differential displacements, have further shown sensitivity to the spectral shape, an aspect which must be carefully investigated. So the earthquake spatial variability does appear to be a significant problem for failure modes governed by differential displacements, also for structures of minor importance like small bridges. Since its inclusion in the design phase brings about small or no extra cost for most situations, it is worth to stress the importance of a rapid Code update on this subject.