Fire behaviour of rectangular steel tubed-reinforced-concrete columns with end restraints

The steel tubed-reinforced-concrete (STRC) column is a new form of composite column, in which the steel tube primarily serves to provide confinement to concrete core rather than directly carrying axial force. The restraining effects at the heated column ends should be incorporated into the fire performance design of STRC columns, while current research on rectangular STRC columns is still inadequate. This study presents the numerical study on the fire behaviour of end-restrained rectangular STRC columns. The effects of axial restraint and rotational restraint on the overall deformations, internal forces, fire resistance, axial force and buckling length at fire limit state, of rectangular STRC columns with various cross-sectional sizes, aspect ratios, slenderness ratios and load ratios were thoroughly analyzed. The end-restrained rectangular STRC columns fail in fire mainly due to runaway of axial contraction, and the failure mode may shift from dynamic to static as the axial restraint increases. The fire resistance of rectangular STRC columns subjected to bending around the major axis is generally higher than in the case of minor-axis bending. The axial force inside the column decreases nearly linearly while the fire resistance experiences an almost linear increase with increasing the axial restraint ratio. The beneficial effect of rotational restraint, i.e. decreasing the buckling length and enhancing the fire resistance is prominent, especially for columns under relatively low restraining levels. Finally, simplified equations were given for designing the structural fire performance of rectangular STRC columns with considering the end restraining effects.