Behaviour of Frame Columns in Localised Fires

摘要

A static/dynamic version of the software Vulcan has recently been developed, in which the numerical singularity of a static analysis, induced by a local instability of a structure, for instance the buckling of a column, can be covered by switching to the explicit dynamic procedure. This version of Vulcan allows the post-buckling behaviour of a member to be traced, finding a re-stabilized state if it exists. In this paper a study of the behaviour of steel columns in localised fires is presented. A simplified model is developed, taking into consideration the axial restraint of the column in fire. An axial elastic stiffness is used to represent restraint from the superstructure, and the full post-buckling behaviour is covered. Results obtained from the simplified model have been validated against previous numerical studies. A full-frame model has been created for comparison. The simplified model has been extended to investigate the effect of beam yielding on the column's restraint conditions. This is modelled by giving a bilinear force-displacement relationship to the spring element. The influence of the stiffness and strength ratios of beam to column on the behaviour of the column in a localised fire are investigated. This study indicates that, although the member interaction is complex in a frame in a localised fire, it may be possible to find a simple way to deal with this behaviour, and to determine not only the initial buckling temperature but also the possibilities of re-stabilization and progressive collapse.