Numerical research on full-scale lean duplex stainless steel material rectangular concrete-filled double skin steel tubular (CFDST) axial slender columns and slender beam-columns had not been considered in previous works in the literature. CFDST columns possessed a number of advantages like the high strength and bending stiffness, fire performance and constructability. Lean duplex stainless steel grade (EN 1.4162) had recently gained major attention for its higher structural performance and corrosion resistance compared to the austenitic type. The current paper dealt with the behaviour of axial slender CFDST columns and CFDST beam-columns with rectangular cross-section and using the lean duplex stainless steel material for the outer tubes. Three dimensional finite element (FE) models were developed and verified against the existing tests in literature. A parametric study was conducted to investigate the behaviour of rectangular axial slender CFDST columns and beam-columns with respect to different key parameters. The effect of different classes of the outer rectangular hollow sections on the behaviour of the CFDST columns were considered. The depth-to-thickness (D/t) ratios of the outer tubes were chosen to produce fully-effective (Feff), slender (S) and very slender (VS) cross-sections. The applied loads applied at different location from the center of columns to account the effect of eccentricity. It was found that for short and intermediate columns the strength of VS and S cross-section was higher than Feff cross-sections in all load positions. For slenderness ratio equal to or more than 70 the strength of all cross-sections was nearly the same. For axial intermediate and long columns, the ductility of VS sections was found to be higher than S sections and the ductility of Feff was the least one. Alternatively, for beam-column specimens it was found that S and VS section were nearly the same trend in terms of stiffness and ductility but different with Feff which has less stiffness and large ductility.