Performance evaluation of double skin steel tube filled by concrete as composite beam

This study is presented in order to show how Concrete-Filled Double Skin Steel Tube (CFDSST) beams behave under pure bending. Laboratory investigations were made on thirteen test specimens that involved twelve units of the CFDST specimens and one unit of Concrete-Filled Steel Tube (CFST) beam as a control. Six of the CFDSST specimens were having a square and rectangular inner tube in the bottom side of the outer tubes, while the other six CFDSST specimens had a centric square and centric rectangular inner tubes. Two major and important parameters relied on this study, namely, the inner tube position and inner tube cross-section. Based on these parameters, bending moment against mid-span deflection, deflection curves, flexural stiffness, flexural capacity, ductility and self-weight differences were investigated. The results of laboratory work showed clear improvement in moment capacity and strength for the CFDSST beams with inner tubes in the bottom side compared to those with centric inner tubes, in addition to showing the advantages of CFDSST beams to CFST beams. Also, this study confirmed that the effect of inner tube cross-section (square to rectangular and vice versa) for the same section area was slight.