Soil-Structure Interaction (SSI) phenomena and foundation rocking can modify the structural response signifi-
cantly with respect to the response predicted adopting the fixed-base assumption. The importance of SSI and rocking depends,
among other factors, on the structural mass and the distribution of static stresses at the soil-foundation interface. Within this
context, an experimental campaign was carried out aiming to investigate the SSI effects on the response of a 3m x 3m x 5m steel-
framed structure. The prototype structure, called EUROPROTEAS, was founded on a shallow footing at the well-characterised
Euroseistest site, while its mass was either 18Mgr or 9Mgr. The present study simulates free vibration experiments, placing
particular emphasis on soil nonlinearity and soil-foundation interface. A novel approach to simulate gaps at the soil-foundation
interface, foundation rocking and to manipulate interface stresses under static conditions is presented. The three aspects are
shown to significantly affect the response, while they are found to be more important for the lighter structure.