Fire Behaviour and Design of Square Steel Tube Confined Reinforced Concrete Columns: Isolated Columns to Framed Columns

The steel tube confined reinforced concrete (STCRC) column, is an innovate type of steel-concrete composite column comprising an outer thin-walled steel tube that does not bear axial load directly and inner reinforced concrete core. STCRC columns possess several advantages, such as high bearing capacity, good ductility, excellent seismic performance and ease of connection to RC beams. STCRC columns are gaining increasing usage in high-rise buildings and large-span stadiums. Structural fire safety is a crucial issue in the design of STCRC structures, as the steel tube may be directly exposed to fire. Due to its unique configuration and working mechanism, STCRC columns behave differently than traditional concrete-filled steel tubular (CFST) columns. Currently, fire resistance design theories and methods for STCRC columns are lacking. In this study, the performance of square STCRC columns exposed to ISO 834 standard fire was thoroughly investigated through experimental, numerical, and theoretical analyses. The deformation behaviour, load redistribution, failure mode and fire resistance of STCRC columns under various end conditions, subject to concentric and eccentric loading, with and without fire protection, were symmetrically analysed. The effects of critical parameters, end restraints as well as the frame integrity on the fire behaviour of square STCRC columns were examined. Finally, structural fire performance design methods were developed for STCRC columns at different levels, i.e. free-restrained, constant/temperature-dependent restrained, and integrated within a frame, which remain consistent with the equations give in ambient-temperature design code.