Analytical prediction of the axial capacity of concrete-filled cold-formed steel (CF-CFS) built-up columns

Cold-formed steel (CFS) built-up sections are being increasingly used in the construction sector, especially as compression members. One of the challenges of these compression members is their sensitivity to early buckling under compression. The CFS closed built-up columns can be filled with concrete to overcome the early buckling and to increase its axial load-bearing capacity. As a consequence, the applicability of the design prediction in these sections should be investigated; this is, therefore, the purpose of this paper. First, the results of twenty-four experimental specimens, including four full and partial concrete-filled cold-formed steel (CF-CFS) section configurations with two different lengths under pure compression, are presented. Then, a finite element modelling approach is calibrated against the experimental data, and consequently, a parametric study is used to investigate a larger range of slenderness. Finally, the axial load-bearing capacity of the CF-CFS built-up composite columns is compared to the design predictions according to EN 1994-1-1 and the AISC Specification. Reliability analysis is also performed to assess and compare the available and proposed methodologies. The results show that both standards present an unconservative prediction for the partial CF-CFS columns and a highly conservative for the full CF-CFS composite columns. Therefore, this paper modifies EN 1994-1- 1 and the AISC analytical procedures to be applicable in predicting the buckling resistance of the CF-CFS composite columns. The paper first suggests extending the constant range of the buckling curve of EN 1994-1-1 from a non-dimensional slenderness of 0.2 to 0.4, then defines two new buckling curves by proposing two imperfection factors, specifically for the partial and full CF-CFS columns.

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