Concrete filled double section steel tube columns

Concrete filled double-tube (CFDST) columns developed as new configuration of columns construction. Concrete filled double-tube comprises of two hollow steel tubes, one inside the other, concentrically coordinated. In this study two combination of cross section were proposed: double-skin and double-section column. In case of double-skin column only the shell between two steel tubes was filled with concrete, and for double-tube the inner of steel tube also filled with concrete. This combination provides perceptible advantages in structural building such as the steel hollow section works as formwork and that led to reducing the extra costs for this purpose. Furthermore, the inner steel tube compensates the commercial steel reinforcement and that led to eliminating time spending time to construct the reinforcement and stirrups. Also, the core concrete increase the bending stiffness and ductility under compression strength. The important advantage of columns with dual tube is the inner tube provide more confinement of concrete which enhancing ductility and increase maximum buckling load. In this dissertation two concrete classes were proposed and 24 composite columns divided into two set with four of corresponding CFST as reference of each group. The division of these sets depending on type of concrete strength. The parameters were the class of plain concrete, thickness of steel tubes and inner concrete contribution ratio. The influence of these parameter was investigated on compressive loading. Moreover, the relation between load and deflection extracted. Based on experimental results found that when the thicker tube located in the outer part of the section, the performance of CFDST greater than those with thinner outer steel tube. The strength enhancement index SI increases with the increases of the section confinement. Moreover, it was found the average strength index for columns specimens filled with normal strength concrete NSC were greater than thos filled with high strength concrete HSC. This expreimental study showed that the developing forms such as double-skin and double-section columns validats the effectiveness of CFDST concept by enhancing the mechanical response compared with common CFST.