Offshore structure should have a high degree of structural safety not only under normal conditions but also extreme conditions even under collision loadings, as shown in Fig.1. The authors carried out both experimental and theoretical investigations on the toughness of the sandwich composite structures. Experiments were carried out for the twodimensional models of composite structures under pure bending and combined shear and bending as well. A nonlinear analysis was developed to predict the toughness of sandwich beam under pure bending. In the analysis the material nonlinearity of both concrete and steel plate were taken into consideration. The analysis were found to be very close to the experimental results.
Recently various kinds of conceptual designs have been proposed for arctic offshore structures. The authors have developed a composite steel/concrete structure to eliminate, as much as possible, the various drawbacks of both concrete and steel. In the previous studies (M. Matsuishi and S.lwata 1977, 1980, 1986, 1987, 1990), the composite structure was found to be able to endure high levels of deformation and be able to absorb a great deal of energy until failure, because the steel plate having high tensile strength and ductility, suppresses the development of surface cracks. In designing, a composite steel/concrete structure, there is especially in consideration of collision loading. Ductility implies the ability to sustain significant inelastic deformations after the ultimate load without a significant variation in the resisting capacity prior to collapse. This ability is, generally quantitatively described by a parameter called the ductility ratio and the toughness which paraphrase is deformation energy. Fig.2 shows the typical load - deflection curve for sandwich concrete beam. The ductility ratio is defined as the ratio of ultimate deformation to yield deformation. The more ductile structure which has larger toughness and ductility ratio is capable of absorbing greater energy before rupture than the less ductile structure. (Fig 1,2 are shown in the paper)
