Abstract
In the ultimate limit state design and seismic design of steel structures, it is postulated that the structural members have sufficient deformability or rotation capacity. In this context, the relationship of the plastic deformation capacity of steel structural members and the stress-strain curves of their materials are made clear. Namely, to secure the sufficient deformation capacity of structural members, the yield-ratio (the ratio of yield stress to tensile strength) of material should be reasonably low. For flexural members such as beams and beam-columns, the rotation capacity can be evaluated by the complementary energy of materials more exactly. Scattering of yield stress could give an adverse effect on the strength and deformability of steel structures subject to horizontal loading. In this viewpoint, the upper limits of the yield stress and of the yield ratio should be specified in the material standard.
