Hysteresis characteristics of concrete filled steel tubular columns have been proposed. However, it has not been clarified if the skeleton curve could be applied for slender columns, and there are few studies about the unloading stiffness of the hysteresis loop. The purpose of this study is to compare the skeleton curves of square CFT columns, which have been proposed by the moment-rotation angle relationships and the experimental data by the previous research. The relationship between the initial stiffness obtained by the theoretical calculation and the unloading stiffness obtained by the experimental study are shown. Also the effects of the axial force ratio and the width-thickness ratio on the above mentioned relationships are discussed. The loading condition of the experimental study is monotonic and cyclic loading. It is found that the skeleton curve evaluates conservatively the bending moment-rotation angle relationships obtained by the test in both loading conditions, except for the width-thickness ratio 43 specimen with cyclic loading. The ratio of unloading stiffness exKr obtained from the experimental data and calculated theoretical stiffness K was shown. The effect of the axial force ratio on the exKr/K was observed when the rotation chord angle was more than 1% and the width-thickness ratio increased as exKr/K decreased.
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