1. Introduction It is suggested that an earthquake-resistant design for buildings such that the effects of an earthquake on a building are evaluated as energy and the energy input by the earthquake is compared with the energy absorbed by the structure is possibe. It is also clear that the distribution of damage caused by an earthquake to each story of a multi-story building mainly depends on the yield-shear force coefficient distribution to each story and that the damage is concentrated on any story with relatively lower strength. Therefore it is possible that the limiting of damage to a specific story and the addition of a specific inelastic deformation capacity only to that story may allow earthquake-resistant considerations to be lessened for other stories, the improvement of the earthquake-resistant capacity of the whole structure and the production of an economical design. It is known that steel is naturally extremely highly ductile and that it has a large energy absorption capacity due to its large inelastic deformation capacity. 2. Proposal of Our Structure System From the standpoint of aseismic design methods based on energy theory, the authors propose a new earthquake-resistant steel structure system consisting of flexible and stiff members. The strength of the first story is intentionally made lower than that of the second and higher stories so that most of the energy input by an earthquake is absorbed by the first story and thus the result is a vast reduction in energy input by the earthquake to the second and higher stories. In the first story, the energy input is absorbed as cumulative inelastic strain energy by yielding specific steel members of the first story into the inelastic range while columns and girders, which are the major members of the structure, remain in the elastic range even when a severe earthquake occurs. An example of this structural system is shown in Figure 1. This structural system employs materials consisting of flexible members and stiff members on the first story as shown in Figure 2. The flexible members are meant to ensure the restoring force by making all the members remain within the elastic range when a severe earthquake occurs, In this structural system, the main columns are constructed using flexible members. Highstrength steel is used to ensure large elastic deformation and bearing capacity. The required stiffness is added in order to prevent the members from deteriorating due to the P-Δ effect against expected large deformation. As a result, the members are less yielded in only one direction and are less deformed. The stiff members absorb most of the energy input by an earthquake as cumulative inelastic strain energy. The use of mild steel with large inelastic deformation capacity and a design which considers the width-to-thickness ratio so as not to cause local buckling are required. 3. Study of Parameters of Flexible and Stiff Members The fundamental properties of this system are subsequently studied by inelastic response analyses in order to apply this structural system to actual buildings. The analytical model used is assumed to be a five-story moment resistant frame with a first story constructed of flexible-stiff mixed frames (Fig. 3). The yield-shear force coefficient of the second and higher stories is 1.5 times the optimum yield-shear force coefficient distribution, as shown in Figure 4, in order for most of the energy input by an earthquake to be absorbed by the first story. The yield-shear force coefficient of the stiff members in the first story _sα_1, the yield deformation ratio _fδ_y/_sδ_y and the yield strength ratio _fQ_y/_sQ_y of the flexible and stiff members are selected as parameters. Three earthquake waves, El Centre 1940 NS, Taft 1952 EW and Hachinohe 1968 EW, are employed as input earthquake waves. The input acceleration is normalized by the total energy input, giving an equivalent velocity of V_E-150

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