Overall buckling behaviour of circular concrete-filled dual steel tubular columns with stainless steel external tubes

Recently, a short composite column consisting of dual steel tubes (external and internal stainless and carbon steel tubes, respectively) with concrete filled in the entire tubular section has been introduced. This column, called as the concrete-filled dual steel tubular (CFDST) column, proved that a lower cost as well as weight can be achieved compared with the concrete-filled stainless steel tubular column (CFSST). This is mainly attributed to the increased strength of the concrete fill, inside the internal tube, that is surrounded by both the external and internal steel tubes. Based on the fact that the columns are generally slender in practice, this paper investigates numerically, by means of finite element (FE) analyses, the axial compressive behaviour of the CFDST slender columns, which has been rarely investigated in literature. The external tubes are currently made of the lean duplex stainless steel material recently attracted the structural community because of its relatively lower cost. The FE models carefully consider the compressive and tensile nonlinear behaviour of the concrete. The FE models are verified for their different material models. This is followed by examining the accurate overall buckling behaviour of the slender columns. This has been made through FE comparisons with tested columns of different cross-section types existing in literature. The fundamental behaviour of the CFDST slender columns under the effect of the slenderness ratio, the concrete confinement effect and the concrete compressive strength is then investigated. The study additionally addresses the differences in behaviour between the intermediate length and long CFDST columns. Moreover, the comparison between the design strengths calculated by Eurocode 4 from one side and the FE and experimental ultimate strengths from the other side shows generally that Eurocode 4 gives unsafe predictions. Accordingly, a modified European design model is suggested at the end to predict accurately the resistance of the CFDST slender columns under axial compression.