The purpose of this work is to investigate the role of the non-linear soil behavior on the structural dynamic response. Several soil-structure analyses are performed, assuming a linear-equivalent approximation for non-linear soil behaviour into a coupled FEM-BEM approach. A parametric study is carried out including the mechanical properties of both the soil foundation and the structure as well as the characteristics of the input motion. The differences found between the linear and the equivalent-linear dynamic SSI approach demonstrate the importance of a proper soil behaviour evaluation. A technique to derive the soil portion influenced significantly by the structure is presented. This limit defines a neighbourhood of the soil where non-linear behavior may be decisive for the global response of the structure. The results show the dependency of this influenced zone due to structure characteristics and giving a tool to establish when a most refined non-linear soil evaluation must be done.