This paper addresses the problem of plastic hinge length in the evaluation of ductility performance of square concrete filled steel tubular (CFST) column that was largely used in earthquake-resistant structures. Physical tests were conducted on a series of square CFST stocky columns having different cross-section widths (B = 200 mm & SIM;800 mm). The curvatures of each column were measured, serving as the basic building blocks for establishing the linkage between the plastic hinge length and the cross-section width. From the experiments, it was found that the failure region of all columns is concentrated in the bottom of the CFST columns, which is located in the plastic hinge region. The plastic hinge length of square CFST stocky column increases linearly with the increase of the cross-section width. Meantime, an analytical approach is developed based on the equivalent plastic hinge model and cross-section analysis, by which the analytical formulas of plastic hinge length were proposed. The important parameters involving axial ratio, strengths of concrete and steel plate, slenderness ratio as well as the width-to-thickness ratio on the plastic hinge length of square CFST column were studied. The results indicate that the plastic hinge length tends to increase with the increase of the axial load ratio, however, decreases with the increase of the aspect ratio and the compressive strength of concrete. No significant effects of the width-to-thickness ratio and the steel strength were found on the plastic hinge length of the square CFST column.
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