Prediction formulas of buckling and critical temperatures of restrained box-section steel columns in fire

As important parameters reflecting the fire resistance of restrained steel columns, buckling and critical temperatures have been extensively studied. However, the existing simplified design methods for buckling or critical temperature are based on column models with either pin-ended or with large rotational restraints, their applications on steel columns with limited rotational restraint stiffness ratios, i.e., rotational restraint stiffness ratios smaller than the critical rotational restraint stiffness ratio, is questionable. In this paper, parametric analyses were performed on a validated numerical column model with rotational restraint stiffness ratio spanning from 0.0001 to 10.0. Additionally, the influences of slenderness ratio, load ratio, and axial restraint stiffness ratio were also considered. Results reveal significant errors when using the existing simplified design methods to predict the buckling and critical temperatures of steel columns with limited rotational restraint stiffness ratios. Based on the parametric analyses results, new prediction formulas for buckling and critical temperatures of box-section steel columns were proposed. Results of the proposed formulas are in close alignment with those of the parametric analyses across the full range of considered parameters. Moreover, the effectiveness of the proposed formulas was further validated by test results in literatures.