In order to estimate easily the elasto-plastic behavior of a certain story of a multi-span multi-story unbraced steel frame, an approximate horizontal load-deflection relationship of "a fundamental subassemblage" is proposed using the stability limit strength formula obtained by authors previously. This relationship is composed of four straight lines; 1. Elastic line, 2. Yield line, 3. Stability limit strength line and 4. Mechanism line. Applying the relationship to multi-span one-story subassemblages, it is verified that the approximate stability limit strengths and deformations agree well the results of precise numerical analysis, inspite of the simplicity of the proposed method.
