Composite beams in large buildings under fire — numerical modelling and structural behaviour

A good engineering assessment of the fire safety of a building structure should be based on a sound understanding of the mechanics of its behaviour under fire. Existing standards and methods of design for fire assume that the structural behaviour is effectively the same as that at ambient temperature, allowing for the reduced material properties. This simple assumption is valid for statically determinate structures, but is in serious error for highly redundant structures, and may be unconservative in certain cases. In particular, the effect of thermal expansion is generally ignored, even though it may swamp the effects of all other phenomena in a large highly redundant building under a local fire. This paper presents some of the results of an extensive investigation (Usmani et al., DETR-PIT project, final report (draft), March 2000) in which the structural action in a two-way slab and composite beam structure subjected to a compartment fire has been explored. These results show that thermal expansion dominates the response of highly redundant structures under local fires, and that local yielding and large deflections can be beneficial in reducing damage to the complete structure. However, it is now clear that explicit cognisance should be taken of thermal expansions in design calculations, but this can only be done when a thorough understanding of the behaviour, appropriately generalised, is in place. This is the main motivation behind the results presented in this paper.