To simplify the analysis, three-dimensional soil structure interaction problems are often modelled by considering a two dimensional slice without changing the material properties of the soil. This procedure, although convenient, is of questionable validity because two dimensional modeling inherently overestimates the radiation damping for translational and rocking motions. To make matters worse, two dimensional modelling always entails an underestimation of the dynamic spring coefficient for the translational motions. The damping ratio of the two dimensional case, which is proportional to the ratio of the damping coefficient to the spring coefficient, will thus be even larger. Thus, reliance upon a two dimensional analysis based on an equivalent slice of a strip foundation may result in a dangerously non conservative design.