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

The concrete-filled cold-formed steel built-up composite (CF-CFS) column is an innovative and versatile solution that may be used as an alternative structural element in construction. One of the main advantages of composite concrete-filled columns is their fire behavior, which has not been assessed for this specific solution (CF-CFS). Moreover, the available design codes for composite structures do not present a straightforward analytical procedure for concrete-filled cold-formed steel built-up columns. The present paper reports an experimental investigation on the fire resistance of four CF-CFS built-up columns. Twelve CF-CFS short columns were tested. The fire test setup, procedure, and results were fully reported and discussed, including the temperature evolution, load-temperature capacities, and load-deformation. A sequential thermal-stress finite element analysis (FEA) model was developed and calibrated against the test results. Experimental results were compared with design predictions according to the EN 1994-1-2 for composite steel-concrete sections, considering the simplified method (Annex H) and the general method. Moreover, the paper discussed the applicability of EN1994-1-2 to predict the fire resistance of the concrete-filled thin-walled CFS built-up composite columns. The results showed that the predictions according to the simplified design method (Annex H) tend to be highly conservative; but, the predictions based on the general method tend to provide slightly unconservative results, still requiring additional adjustments to improve its accuracy.