Structural Performance of Welded Built-Up Mega Composite Columns Subject to Compressive Force and Moment

The width thickness ratio of a concrete-filled steel tube column is limited in order to prevent the local buckling of the tube. Determined by column width and steel tube thickness, width-thickness ratio decides whether the cross-section is compact, non-compact or slender. Different compressive strength formulas are applied depending on the type of the cross-section. This study suggests the shape of binding frames which are placed at a constant distance inside the steel tube of a mega column having large cross-sections to improve the composite effect and confinement effect. Structural tests are conducted on 9 welded built-up and generic CFT columns with different steel thicknesses to evaluate their structural performance under compressive force and moment applied simultaneously. The maximum load of the welded built-up specimens with compact, non-compact and slender cross-sections is approximately 38%, 30% and 20% higher when compared with their generic CFT counterparts. While the effective stiffness of the members of concrete-filled composite columns is calculated by one formula, the difference among the specimens is greater than 10% depending on the width-thickness ratio. Therefore, it is deemed that effective stiffness needs to be categorized depending on the width-thickness ratio as in the case of compressive strength and bending strength.