<p>1. Introduction</p><p>The objective of this study is to examine the relations between the simple superposed strength and generalized superposed strength of CFT cross sections subjected to axial load and flexural moment, regarding the allowable stress design. AIJ CFT Recommendations specify the simple superposed strength for calculating the allowable strength of CFT sections, whereas reference 4) it is supposed to use the generalized superposed strength. About the allowable strength, the differences between two superposed strengths are not clarified, and the calculation method of generalized superposed strength has not been shown explicitly.</p><p>2. Analytical work</p><p>In addition to the simple superposed strength, the equations to calculate the generalized superposed strength is presented. The equations to evaluate the generalized superposed strength for the square and circular CFT sections are Eqs. (29) ~ (33) and (34) ~ (38), respectively. It is shown that the differences between two superposed strengths appear in low axial load level, that is the limiting axial loads are obtained by Eqs. (21) and (26) for square and circular section, respectively.</p><p>3. Results and discussions</p><p>The comparison between two superposed strengths was carried out taking the width (diameter) -to-thickness ratio and strengths of materials as the analytical parameters. Moment-axial load relations for the allowable strength are shown in Figs. 5 and 6. The differences between the generalized superposed strength and simple superposed strength are shown in Figs. 7 and 9, and the maximum differences are shown in Figs. 8 and 10. It is observed that the difference becomes large when the width (diameter) -to thickness ratio and the allowable stress of concrete is large and the allowable stress of steel is small. The maximum differences can be estimated by the Eqs. (39) and (40).</p><p>4. Conclusions</p><p>The conclusions derived from this study are as follows:</p><p>1) The equations to evaluate the generalized superposed strength for the square and circular CFT sections are presented as the Eqs. (29) ~(33) and (34) ~ (38), respectively.</p><p>2) The differences between two superposed strengths appear in low axial load level. The limiting axial loads are obtained by Eq. (21) and (26)for square and circular section, respectively.</p><p>3) The maximum differences are estimated by the Eqs. (39) and (40).</p>