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

A good engineering assessment of the fire safety of a building structure sh ould be based on a sound understanding of the mechanics of its behaviour un der fire. Existing standards and methods of design for lire assume that the structural behaviour is effectively the same as that at ambient temperatur e, allowing for the reduced material properties. This simple assumption is valid for statically determinate structures, but is in serious error for hi ghly redundant structures, and may be unconservative in certain cases. In p articular, the effect of thermal expansion is generally ignored, even thoug h it may swamp the effects of all other phenomena in a large highly redunda nt 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 co mposite beam structure subjected to a compartment fire has been explored. T hese results show that thermal expansion dominates the response of highly r edundant structures under local fires, and that local yielding and large de flections can be beneficial in reducing damage to the complete structure. H owever, it is now clear that explicit cognisance should be taken of thermal expansions in design calculations, but this can only be done when a thorou gh understanding of the behaviour, appropriately generalised, is in place. This is the main motivation behind the results presented in this paper. (C) 2000 Elsevier Science Ltd. All rights reserved.