Keywords: fire, buildings, finite element, non-linear, structural response.

Considerable attention has been directed in recent years towards investigating the performance of structural systems under fire conditions. Following close examination of the behaviour of large steel-framed structures subjected to major fires, it was observed that most buildings were significantly over-designed and/or over-protected. Consequently, there has been increasing awareness of the benefits of using more rational approaches which are based on true behaviour rather than on idealised representations of isolated elements.

Due to the complex interactions that take place between various members in a frame, extensive redistribution of loads occurs during fire. These interactions are being examined through a number of experimental and analytical investigations in Europe and elsewhere. As part of a concerted effort to study the structural fire behaviour of buildings, a large experimental programme has recently been completed on a full-scale eight storey building in the UK. Several major tests were undertaken in the building in order to examine the response under fire conditions. The tests varied from the heating of one internal member to cases in which the influence of large compartment fires was investigated. The results of these tests enable verification and calibration of computational programs which can then be used for further parametric studies.

In this paper, computational numerical models constructed to simulate the response of composite steel/concrete buildings are described, and the results are compared with findings from the tests undertaken on full-scale structures. The numerical simulations described in this paper are undertaken using the nonlinear analysis program ADAPTIC, which is particularly suitable for modelling the large displacement structural response. Careful attention was given throughout the stages of development of the program to combining a high level of numerical accuracy and stability with optimum computational efficiency. The program allows for geometric and material nonlinearities and includes facilities for static and dynamic analysis in three dimensions. It includes elements accounting for thermal effects and temperature-dependent nonlinear cyclic constitutive models for steel and concrete, and enables the representation of non-uniform temperature variations across the section and along the length of members.

The approaches used to represent the various structural details are discussed, and the procedure employed for incorporating the experimentally-measured temperature profiles and histories is described in the paper. A grillage representation of the building floor is used in which slab, beam and column members are represented by temperature-sensitive elasto-plastic beam-column elements. Link elements, with user-defined end conditions, are utilised to connect the various grillage layers. These include links between steel beam elements and slab elements, beams and columns, and primary beams and secondary beams. Within the constituent elements, temperature variations are introduced through the cross-section and along the length. A comparison between the numerical results and experimental measurements for three of the fire tests illustrate the reliability of the modelling tool and approaches. The analytical results are shown to be in agreement with the test data, particularly in terms of the magnitude of vertical deformations induced in the floors at elevated temperatures. The analysis is capable of predicting the same behavioural trends observed in the fire tests. Moreover, the extent of the structural models may be considerably reduced provided that representative boundary conditions are utilised, leading to improved computational efficiency.

Close examination of the numerical results provides an insight into the complex interactions that occur in a structure at elevated temperatures. Most significantly, the influence of the restraint to thermal expansion of the heated floor area, which is provided by the surrounding parts of the structure, is shown to be of paramount importance. For a heated floor area, enclosed by a fire compartment, the surrounding structure can provide a high level of in-plane resistance. This typically causes an early buckling of the floor system at relatively low temperature due to significant compression leading to a rapid increase in the out-of-plane deformation. The dominance of thermal expansion effects in the response of the structure is largely contributed to by the typically low initial imposed load. This relatively low load level is a consequence of typical design idealisations in which the load ratio is estimated based on isolated behaviour of an axially-unrestrained and simply- supported composite beam.

In order to carry out design-related investigations, it is necessary to employ suitable structural failure criteria. Based on the results of experimental and analytical investigations, it seems appropriate to relate these criteria to the floor slab, particularly in terms of limiting mechanical strains in the reinforcement. Using numerical models, such as those described in this paper, which are verified against the results of full-scale tests, in conjunction with realistic limiting criteria, more rational and cost-effective design procedures may be developed for the design of structures under fire conditions.

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