Fundamental principles of structural behaviour under thermal effects

This paper presents theoretical descriptions of the key phenomena that gove rn the behaviour of composite framed structures in fire. These descriptions have been developed in parallel with large scale computational work undert aken as a part of a research project (The DETR-PIT Project, Behaviour of st eel framed structures under fire conditions) to model the full-scale fire t ests on a composite steel framed structure at Cardington (UK). Behaviour of composite structures in fire has long been understood to be dominated by t he effects of strength loss caused by thermal degradation, and that large d eflections and runaway resulting from the action of imposed loading on a 'w eakened' structure. Thus 'strength' and 'loads' are quite generally believe d to be the key factors determining structural response (fundamentally no d ifferent from ambient behaviour). The new understanding produced from the a forementioned project is that, composite framed structures of the type test ed at Cardington possess enormous reserves of strength through adopting lar ge displacement configurations. Furthermore, it is the thermally induced fo rces and displacements, and not material degradation that govern the struct ural response in fire. Degradation (such as steel yielding and buckling) ca n even be helpful in developing the large displacement load carrying modes safely. This, of course, is only true until just before failure when materi al degradation and loads begin to dominate the behaviour once again. Howeve r, because no clear failures of composite structures such as the Cardington frame have been seen, it is not clear how far these structures are from fa ilure in a given fire. This paper attempts to lay down some of the most imp ortant and fundamental principles that govern the behaviour of composite fr ame structures in fire in a simple and comprehensible manner. This is based upon the analysis of the response of single structural elements under a co mbination of thermal actions and end restraints representing the surroundin g structure. (C) 2001 Elsevier Science Ltd. All rights reserved.