Fire performance of end-restrained circular steel tube confined reinforced concrete columns

Structural fire safety is an essential concern in the design of steel tube confined reinforced concrete (STCRC) column, an innovative type of steel-concrete composite structural member. While extensive research has been conducted and design methods are available for the fire behaviour of circular STCRC columns with pinned conditions, the fire performance of these columns with end restraints, particularly considering varying end restraints with time increasing (hereafter referred to as time-dependent restraints), remains underexplored. This study, for the first time, investigates the fire performance of circular STCRC columns in non-sway frames with end restraints via finite element analysis (FEA). The investigation covers various scenarios, including axial and rotational restraints (individually and in combination), and constant and time-dependent conditions. The effects of axial restraint ratio, rotational restraint ratio, load ratio, cross-sectional diameter, and slenderness ratio on the fire resistance of end-restrained circular STCRC columns were examined. It was observed that an increase in restraint stiffness, whether axial or rotational, markedly enhances the column's fire resistance, with improvements reaching up to 830 %. Quantitative analyses were conducted to compare the fire behaviour of columns with time-dependent restraints resulting from the stiffness degradation of adjacent beams to those with constant restraints. The cumulative negative effect of time-dependent restraints on fire resistance is generally negligible. Based on existing methods for pin-ended STCRC columns, this study develops fire resistance design recommendations for end-restrained circular STCRC columns, accounting for the beneficial effects and time-dependent degradation of end restraints on the failure axial force and buckling length.