The authors have developed a new composite steel-concrete sandwich system for the strength members of huge offshore structures, where concrete is placed in between steel plates. In the previous papers, a series of experiments under static and repeated loadings, and theoretical analyses using the nonlinear finite element method were carried out. It was clarified that the composite sandwich structure has various excellent properties. In the present paper, the authors carried out two types of experiments. The first type of experiment was to study the strength of the composite structure under distributed loading which corresponds to ice loadings acting upon offshore structures in the Arctic. The second type of experiment was carried out using the composite structure reinforced by various types of shear connectors. It was found that T-stiffeners and long stud bolts are quite effective to increase the ultimate strength and the amout of deformation.