Behaviour of foamed concrete-filled steel hollow column under fire

Abstract

Reduction in self-weight and achievement of full fire resistance requirements are some of the important considerations in the design of high-rise structures. Lightweight concrete filled steel tube (CFST) column provides an alternative method to serve these purposes. Recent studies on lightweight CFST columns at ambient temperature have revealed that foamed concrete can be a beneficial and innovative alternative material. Hence, this study investigates the potential of using foamed concrete in circular cold-formed hollow steel columns for improving fire resistance. The hollow steel is of grade S355, section diameter of 139.7 mm, and 6 mm thickness. An experimental and numerical programs were carried out in this study. Nine columns were tested in fire testing furnace: three were hollow steel columns and six were hollow steel columns filled with foamed concrete (FCFHS). An ISO-834 standard temperature-time curve was used for the fire resistance test. All the columns were tested without any external fire protection, and were subjected to concentrically applied load under fixed-fixed end conditions. Fire resistance time, temperature development, and axial displacements on the column were the parameters recorded from the fire test. The experimental result showed that filling hollow steel column with foamed concrete improves its fire resistance time. Maximum fire resistance periods of 36 minutes and 43 minutes were achieved for hollow steel column filled with 1800 kg/m3 and 1500 kg/m3 foamed concrete density, respectively, at 15% load level. Comparison between experimental and Eurocode 4 design axial buckling load revealed that the columns filled with 1500kg/m3 foamed concrete density can be predicted accurately using Eurocode 4 general design method. Failure mode observed in hollow steel columns was global and local buckling (inward and outward). However, only global and outward local buckling were observed on FCFHS columns, as concrete filling prevents the inward local buckling. A three-dimensional non-linear numerical simulation was performed on FCFHS columns using ABAQUS software. Sequentially coupled thermal stress analysis was used for the thermo-mechanical analysis of the columns. The model developed was validated by comparing the predicted numerical fire resistance time and maximum axial displacements with the experimental results. Parametric studies reveal the influence of column diameter, length, load level, steel tube thickness, and concrete strength on the FCFHS columns. A simplified design equation was proposed using multi linear regression analysis for calculating the fire resistance of FCFHS columns under fire. The proposed equation can accurately estimate the fire resistance time of FCFHS columns without using standard codes or performing fire resistance test. Finally, foamed concrete enhances the fire resistance of steel hollow columns. FCFHS columns can be used for structures that require moderate fire resistance time.