The collapse behaviour of braced steel frames exposed to fire

Progressive collapse mechanisms of braced two-dimensional steel-framed structures, subjected to fire heating, are investigated using a robust static–dynamic procedure developed by the authors. 20 cases have been analysed to provide a comprehensive view of the mechanisms of progressive collapse for these frames, with different bracing systems under different fire conditions. The influences of stiffness and strength of the bracing systems are also analysed. The results indicate that the pull-in of columns is one of the main factors which generate progressive collapse. Horizontal “hat truss” bracing systems have limited capacity to avoid pull-in of columns supporting the heated floor, although they can directly redistribute the vertical load lost by buckling columns to adjacent columns. On the other hand, vertical bracing systems have the effect, not only of increasing the lateral restraint of the frame, which reduces the pull-in of the columns, but also of effectively preventing the collapse progressing from local to global. Stronger vertical bracing systems can redistribute load from a buckled column to its surrounding structural members. Frames with a combined hat and vertical bracing system can be designed to enhance the capability of the frame as much as possible to prevent progressive collapse when a heated column buckles.

► The collapse of braced frames is studied using a static/dynamic procedure.
► “Hat trusses” and vertical bracing have different effects on collapse prevention.
► Influence of strength and stiffness of bracing systems has been investigated.
► Combined bracing can effectively enhance the robustness of frames in fire.