曲げ座屈するコンクリート充填鋼管圧縮材の耐力評価

 1. Introduction<br> The concrete filled steel tubular columns subjected to axial compressive force can be designed by using "Recommendations for Design and Construction of CFT Structures". Nowadays, as the strengths of materials are increasing and a wide variety of cross-section composition are observed in the actual structure, extension of the range of application is desired. The objective of this study is to propose the formulas for evaluating the ultimate or allowable compressive strength of CFT columns.<br><br> 2. Strength of CFT Columns by the Current AIJ CFT Guide<br> The current "Recommendations for Design and Construction of CFT Structures" is examined and the characteristics of the CFT Guide are described. The design formulas are shown in Eqs. (2)-(20). In the formulas, as the slenderness index, the buckling length to the section depth ratio is adopted. The strength of the concrete ranges from 18 to 90 N/mm², and that of the steel tube ranges from 235 to 440 N/mm². In this paper, the normalized slenderness ratio of a steel tube is adopted as the slenderness index, eliminating the categories of short, intermediate and slender columns, and the strength of concrete is allowed up to 150N/mm².<br><br> 3. Strength Estimation of Concrete Column and Steel Tubular Column<br> The buckling strengths are calculated by superposing the strengths of concrete column and steel tubular column. At first the buckling loads of concrete columns are calculated by using the characteristic Equation (25), and approximate equations are presented as Eqs. (26)-(28). The buckling strengths of a steel tube are obtained by Eqs. (32)-(34) specified by the AIJ "Recommendations for Limit State Design of Steel Structures".<br><br> 4. Proposed Strength<br> The strength formulas are shown in Eqs. (47) and (51) for the ultimate and the allowable strength, respectively. For the circular short columns (normalized slenderness ratio _sλ₁<0.15), the confined effect can be considered as Eq. (50). In Figure 14, the proposed strengths are compared with those by the CFT guide. As shown in Figure 15, the value of (the current CFT Guide strength) /(proposed strength) ranges from 0.94-1.04 for square CFT columns and 0.94-1.15 for circular CFT columns. In Figures 16, 17 and 18 the experimental results are compared with the proposed strength. As shown in Table 3, the value of (experimental strength) / (proposed ultimate strength) ranges from 0.96-1.04 for square CFT columns and 0.97-1.15 for circular CFT columns. The proposed strength is in good agreement with the experimental maximum load.<br><br> 5. Conclusions<br> The conclusions derived from this study are as follows:<br> 1) The formulas for evaluating the ultimate or the allowable compressive strength of CFT columns are presented as Eqs. (47), (50) and (51).<br> 2) Strengths obtained by the proposed formulas are compared with those of current CFT guide, and it was shown that the value of the current CFT Guide strength/proposed strength ranges from 0.94-1.04 for square CFT columns and 0.94-1.15 for circular CFT columns.<br> 3) Strengths obtained by the proposed formulas are compared with those of experimental results, and it is shown that the proposed strength is in good agreement with the experimental maximum load.<br> 4) Proposed formulas are easy to use, because the categories of short columns, intermediate columns and slender columns are eliminated.