This paper presents the motion and structural responses of a model that is structurally and dynamically similar to a typical semi-submersible platform. The model is fabricated of high impact polystyrene sheets, 0.78 mm (0.03 in.) thick, and cellulose acetate butyrate (CAB) tubes (for bracings), 1.58 mm (0.06 in.) thick. The response of the model to regular and irregular waves under operating/survival conditions and impacts of modeled bergy-bits (prototype weights of 1000 t and 2000 t traveling at speeds of 1.0 to 4.0 m/s) was determined. In addition, the response of a damaged-corner-column semi-submersible to survival wave conditions was also examined. The motion and structural responses were monitored using 30 strain gages, an accelerometer and four potentiometer devices. The impact loads were simulated by weights swinging in a pendulum mode in still water. The motion RAOs, stress RAOs, PSDs of stresses, impact loads versus time histories, etc., are computed and compared with published values. There is a very good agreement between the present experimental/analytical results and other results published earlier for the motion response. It was found that the theoretical models tend to overestimate the structural response at short wave periods and underestimate it at long periods.