Concrete-filled cold-formed steel (CF-CFS) built-up columns subjected to elevated temperatures: Test and design

This paper evaluates the behaviour of an innovative type of lightweight concrete-filled cold-formed steel (CF-CFS) built-up composite long column in the fire. Twelve fire resistance tests were performed on CF-CFS columns. The experimental results were discussed, including test observations, buckling modes characterization, vertical and horizontal deformations, temperature evolution, and fire resistance in terms of load-bearing capacity. A finite element modelling approach was developed and validated against the test results. Then, the experimental data were compared against analytical predictions following EN1994-1-2, assessing the suitability of existing design methodologies. For this purpose, two analytical approaches available in EN1994-1-2 were used, including Annex-H and the general method (Clause 4.3.5.1). The results showed that the fire resistance prediction according to EN1994-1-2, Annex H, was remarkably conservative. However, the general method of EN1994-1-2 (Clause 4.3.5.1) provided unsafe predictions. Therefore, a modification to the analytical prediction of EN1994-1-2 (Clause 4.3.5.1) was applied to account for the CFS effective cross-sectional area, buckling curve d, and experimental degradation of CFS mechanical properties at elevated temperatures, resulting in an excellent agreement with the test results. Reliability analysis and safety evaluation criteria proposed by Kruppa were also carried out to assess the suitability of the design methodologies.