The study aims to further develop, with respect to previous findings, and validate structural design criteria which account for the effects of earthquakes spatial variability. In past works [Nuti, C. and Vanzi. I. (2004) & (2005); Carnevale, L. 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. Existing codes appear indeed improvable on this aspect. For the differential displacements of two points on the ground, these results are generalized with different response spectra and validated using (indeed a small set of) real recordings. For the experimental validation, the first obtained results point towards an acceptable agreement of model vs. experimental results [Tropeano, G. et al. (2011)]. In any case, results indicate that the design codes can be 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.