Global buckling behaviour and design of square cold-formed concrete-filled dual steel stiffened slender columns

This paper investigates concrete-filled dual steel stiffened tubular (CFDSST) slender columns with a cold-formed stiffened outer tube, a square inner tube and concrete infill between the two steel sections and also inside the inner section. To the best of the authors' knowledge, the behaviour of these types of slender column has not previously been studied, although the behaviour of short columns has been previously explored. Accordingly, the details and development of a finite element model are presented, as well as the validation against available test data. The model is employed to conduct an extensive parametric study to examine the behaviour of CFDSST slender columns with different geometric and material properties. Based on an analysis of the strain distributions in the cross-section, a method for defining intermediate-length and long columns is proposed. It is shown that the slenderness ratio is a very important parameter in the behaviour of CFDSST columns, affecting both the capacity and the failure mode. Other important variables for intermediate-length columns are shown to be the yield strength of the outer steel tube, the strength of the sandwiched concrete and the width-to-thickness ratio of the outer tube whilst only former of these is shown to be very influential for long columns. The ultimate strengths are compared with a number of different design methods, which were suitably adapted for the section types, owing a lack of specific guidance currently available. Based on the results it was recommended that the AS/NZS 2327 design standard provides the most accurate reflection of the real behaviour for CFDSST slender columns.