A. M. Sanad

Fundamental principles of structural behaviour under thermal effects

Abstract This paper presents theoretical descriptions of the key phenomena that govern the behaviour of composite framed structures in fire. These descriptions have been developed in parallel with large scale computational work undertaken as a part of a research project (The DETR-PIT Project, Behaviour of steel framed structures under fire conditions) to model the full-scale fire tests on a composite steel framed structure at Cardington (UK). Behaviour of composite structures in fire has long been understood to be dominated by the effects of strength loss caused by thermal degradation, and that large deflections and runaway resulting from the action of imposed loading on a ‘weakened’ structure. Thus ‘strength’ and ‘loads’ are quite generally believed to be the key factors determining structural response (fundamentally no different from ambient behaviour). The new understanding produced from the aforementioned project is that, composite framed structures of the type tested at Cardington possess enormous reserves of strength through adopting large displacement configurations. Furthermore, it is the thermally induced forces and displacements, and not material degradation that govern the structural response in fire. Degradation (such as steel yielding and buckling) can even be helpful in developing the large displacement load carrying modes safely. This, of course, is only true until just before failure when material degradation and loads begin to dominate the behaviour once again. However, 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 failure in a given fire. This paper attempts to lay down some of the most important and fundamental principles that govern the behaviour of composite frame structures in fire in a simple and comprehensible manner. This is based upon the analysis of the response of single structural elements under a combination of thermal actions and end restraints representing the surrounding structure.

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.

Structural behaviour in fire compartment under different heating regimes — Part 1 (slab thermal gradients)

Modelling the full-scale Fire Tests at Cardington has led to new understanding of the behaviour of structures under fire conditions. Much of this understanding has come from parametric explorations using models verified against the tests. The structural phenomena observed in highly redundant, composite structures, during a compartment fire are dominated by restrained thermal expansion. The large deflections experienced in the structural elements in the region of the fire are almost entirely attributable to thermally induced strains. The mechanisms responsible for producing these large deflections are restrained thermal expansion and thermal bowing. Material degradation and loading are secondary influences. A clear understanding of the response of the structure to an average temperature increase and through depth temperature gradients is essential. This paper discusses the structural response when subjected to different heating regimes obtained by changing the mean temperature and temperature gradient applied in the concrete slab of the composite floor slab system to a computer model of the British Steel restrained beam test.

Structural behaviour in fire compartment under different heating regimes - Part 2 (slab mean temperatures)

The effect of varying the thermal regime in a highly restrained composite beam in a steel frame structure is studied using a finite element model. The variation of through depth thermal gradients in both directions of the orthotropic slab was studied in the first part of this paper. In this part the effect of varying the mean temperature increase in the slab is investigated using the same model of the British Steel restrained beam test.