EFFECT OF EXTERNAL BENDING MOMENT ON THE RESPONSE OF BOUNDARY-RESTRAINED STEEL COLUMN IN FIRE

摘要

Traditionally, fire resistance of a steel column is determined by the standard fire resistance tesl conducted on a pin-roller column subjected to ISO fire curve. Under such conditions, the external bending moments at the column ends were found to significantly decrease the column critical temperature. However, a steel column within a frame is axially and rotationally restrained. To investigate the influence of bending moments on the column behaviour in fire, in this study, columns of slenderness ratio λ = 100 are analysed with visco-elasto-plastic finite element program FEMFAN. All columns are subjected to constant external loads and monotonically increasing temperature. Some dominant parameters are considered, namely, external load utilisation factor, axial and flexurai restraint ratios. Extensive analyses show that (1) the column end moments substantially affect both the structural responses of columns with and without flexurai restraints during the heating; (2) the end moments significantly reduce the critical temperature of a column without any flexurai restraints, while they have limited effects on one with flexurai restraints. This is due to the restoring effect of flexurai restraints towards the end of heating. Based on this study, it shows that the load utilisation factor, which is used to predict the column critical temperature as proposed by EC3 Pt.1.2, will overestimate the influence of external bending moment if a flexually-restrained column is concerned.