Sectional resistance of built-up concrete-filled cold-formed steel (CF-CFS) columns under compression: Design proposal for EN 1994-1-1

This study investigates the compressive resistance of innovative concrete-filled cold-formed steel (CF-CFS) built-up composite columns through an extensive numerical analysis of 1084 finite element models. The research evaluates the individual contributions of the built-up CFS cross-section and the concrete-filled component within the composite column system. The finite element models were validated against experimental tests, demonstrating excellent agreement. Then, the parametric study was performed based on calibrated and validated models. The results show that the axial contribution of the concrete can be accurately represented as its compressive strength to its cross-sectional area. For the built-up CFS section, the axial resistance was determined using a modified effective width method, which considers the gross area for encased plates, the effective area for outer plates, and adjusted buckling factors k_σ based on plate location and fastening conditions. The proposed method accounts for the buckling behavior of built-up CFS sections, which differs from the assumptions in EN 1993-1-3. The modified method accurately predicts the axial capacity of CF-CFS composite stub columns, with an average ratio of FE-to-predicted results of 1.04 and a coefficient of variation (COV) of 0.036. These findings offer a robust and pragmatic design approach for CF-CFS composite columns, paving the way for an expanded application of EN 1994-1-1 in future structural engineering practices. The study highlights the importance of considering the confinement effect of concrete and the interaction between CFS plates in composite systems, offering a robust framework for future design standards.