Concrete filled steel tubular (CFT) columns are rational columns in which local buckling of the steel tube is suppressed by the filled concrete and the filled concrete is confined by the steel tube; the steel tube and concrete complement each other’s disadvantages. Therefore, CFT columns have structural characteristics of high strength and high toughness. For this reason, CFT columns are widely used in high-rise buildings because they allow the column section to be reduced and the effective space of the building to be increased compared to steel reinforced concrete columns or reinforced concrete columns.
Recently, taller building and wider living space have required, sustained load and seismic force of column are increasing. Therefore, the application of high-strength materials to CFT columns is desired. Some studies on CFT columns using high-strength materials have been conducted and useful data is being accumulated. However, there are still many unclear points about the basic structural performance required for the design, such as ultimate flexural strength, limited displacement and so on.
To grasp the structural performance of square CFT columns using high-strength materials (590N/mm2 class steel and Fc150 N/mm2 concrete), six specimens were tested in this study. The test variables were width-to-thickness ratio and clear span-to-depth ratio. Using the results from this test and past tests, ultimate flexural strength, limit displacement and so on are discussed. In addition, a formula for evaluating the ultimate flexural strength and limit displacement was proposed. The major findings from this study are follows:
1) In the high-strength square CFT column, after the maximum flexural moment, axial displacement progressed due to local buckling of the flange/web of the steel tube at the hinge region and crushing of concrete. In addition, the flexural moment gradually decreased accordingly.
2) The lower limit of ultimate flexural strength can be evaluated by correcting the concrete strength reduction coefficient using the normalized width-to-thickness ratio of the steel tube as a parameter.
3) As an index to evaluate the limit displacement, the ratio of the concrete stress at the ultimate flexural strength (cNu) and the axial compressive strength of the column (N0) was proposed. In addition, a formula for evaluating the limit displacement using cNu / N0 as a parameter was proposed.
