コンクリート充填角形鋼管柱材の曲げ耐力について

－全塑性耐力を期待できる条件と耐力設計の枠組と課題－

抄録

 1. Introduction

 The objective of this study is to propose the axial load ratio n_y-slenderness ratio _sλ₁ relations which assure that the CFT beam-columns attain the full plastic moment M_pc. While the CFT beam-columns can be designed by using the strength formulas specified by the “Recommendations for Design and Construction of Concrete Filled Steel Tubular Structures”, the required conditions for attainment of full plastic moment are not apparent. In this study, the strengths of concrete filled steel square tubular beam-columns are obtained by an analytical work, and the required conditions are proposed. Moreover, the framework of strength design are presented.

 

 2. Analytical work

 Analytical model is shown in Figure 1, and concrete filled steel square tubular (CFT) beam-columns are subjected to constant vertical load N and monotonic increasing moment M₁ and _κM₁. The stress-strain relations of steel tube and concrete are assumed as shown in Figure 2. As the analytical parameter, the moment gradient ratio κ, the value of α defined as Eq. (8) and strengths of materials are selected. The load-deflection relations and the maximum strengths of CFT beam-columns are obtained by the shooting method.

 

 3. Results and Discussion

 After showing the moment M₁-slope θ₁ relations in Figure 5, effects of end moment ratio (Figs. 6 and 7), effects of the strength of steel tubes (Figs. 8 and 9) and effects of the strength of concrete (Figs. 10 and 11) on the maximum strength M_1max are discussed. It has been shown that the maximum strengths increase when the end moment ratio and/or the strength of steel increases and the strength of concrete decreases.

 

 4. Required conditions for attainment of full plastic moment

 Axial load ratio n_y-slenderness ratio _sλ₁ relations are shown in Figures 12 and 13. In these figures, white circle ○ marks indicate the condition where the maximum strengths M_1max are greater than 0.95M_pc. In the case of the α is smaller than 0.05, beam-columns can attain the full plastic moment when the axial load ratio is lower than 0.75. Required conditions for attainment of full plastic moment M_pc are examined and consequently summarized as Eqs. (12)-(15). Moreover, an example to calculate the maximum axial load ratio _limitn_y which assures M_pc is presented.

 

 5. Conclusions

 The conclusions derived from the analytical work are as follows:

 1) Required conditions for the attainment of the full plastic moment M_pc as the strength of CFT beam-columns are proposed as Eqs. (12)-(15). These equations are related to axial load ratio n_y, end moment ratio κ and slenderness ratio _sλ₁ of steel tubular column.

 2) In the range α≦0.05, the full plastic moment M_pc can be expected under the conditions of end moment ratio κ≧-0.5 and axial load ratio n_y≦0.6.

 3) It becomes easy to exert the full plastic moment M_pc, as the end moment ratio κ and the strength of steel tube _sσ_y becomes large, and the strength of concrete _cσ_B and axial load ratio n_y becomes small.

 4) The framework and issues to solve of the strength design are presented.