Buckling resistance of concrete-filled cold-formed steel (CF-CFS) built-up short columns under compression

The steel-concrete composite columns (SCCC) are widely used in the construction sector. The buckling phenomenon of the steel part (especially thin-walled steel) of the SCCC due to compressive loading is the governing characteristic of these columns. The present paper reports an experimental and numerical study on the compressive behavior of innovative concrete-filled (CF) closed built-up cold-formed steel (CFS) columns to investigate the applicability of EN 1994-1-1 for the prediction of the buckling resistance. Twelve CF-CFS short columns were tested. The finite element (FE) design, test setup, procedure, and results, including the load-carrying capacities, load-deformation, and buckling modes, were fully reported. Additionally, the numerical results and EN 1994-1-1 predictions were compared assessing the validity of the EN 1994-1-1 formulations for the estimation of the contributions of each component. Since the closed built-up cross-sections are composed of Class 4 individual cross-sections, both gross and effective sectional areas were considered to estimate the steel contribution to the overall resistance of the composite columns. Based on the FE results it was observed that using the gross area the EN 1994-1-1 formulation overestimates the steel contribution and underestimates it when the effective area is used. Therefore, a modification was proposed for determining the effective cross-sectional areas of Class 4 closed built-up CFS cross-sections, by considering the overlapping plates, fastener positioning, and the concrete encasement of the individual plate elements resulting in a close agreement between the proposed methodology and experimental results.