The authors have developed a new composite steel-concrete structure for the strength members of huge offshore structures, where concrete is placed between steel plates. In the previous papers, a series of experiments under static loading and theoretical analyses using the nonlinear finite element method were carried out. It was clarified that the composite has various excellent properties and the nonlinear analyses accurately represented the behavior of the composite. In the present paper, the authors carried out experiments under repeated loadings. The effect of the repeated loading on the strength of the composite was clarified. Experimental and theoretical investigations were also carried out on the strength of the composite structure with a girder plate. It was found that the composite with the girder is very effective for shear force.