Simulation of the structural behaviour of steel-framed buildings in fire
A three-dimensional finite element computer program has been developed which can predict the behaviour of steel-framed buildings, including the supported floor system, in any specified fire scenario. The developed software provides a more realistic prediction of structural behaviour at elevated temperatures, than is possible with models which are limited to frame analysis, since the building structure is considered as a more complete entity. Confidence in the software is strengthened by comparison with test results to the extent that different structural and fire scenarios may in future be investigated very cheaply. This could lead to a better understanding of the behaviour of steel, framed structures during fires and to more rational approaches to specification of fire protection requirements, which are currently rather expensive in terms of material and fixing costs. The steel beam-column members are represented by one-dimensional two-noded elements which incorporate both material and geometrical non-linearities. These can model three-dimensional steel member behaviour including lateral-torsional buckling. Temperature gradients can be specified through the steel cross-section and also along its length. Spring elements, of zero length, have been introduced to represent semi-rigid joints. These degrade in stiffness and strength with rise in temperature and are represented by any specified moment-rotation-temperature relationship. Unloading from an inelastic state has been modelled for both the steel members and connections, allowing the behaviour of the frame during the cooling phase of a fire to be investigated. This will enable the repairability of the frame to be assessed after a fire has occurred. The flooring system is represented by shell finite elements, which are linear elastic and include thermal strains, although the temperature distribution through the slab's thickness must be assumed uniform. A simplified method of representing cracking in the concrete has been introduced by placing a limit on its maximum bending stress. The node position of the steel one-dimensional finite elements can be displaced to allow connection to the two-dimensional shell elements at a common point. This allows composite action between the beam and supported slab to be modelled. Comparison has been made between computer simulations and fire tests on the full-scale test frame at Cardington. It has been shown that modelling isolated members is highly unrealistic. However models which incorporate a significant amount of the structure surrounding the heated zone, including the membrane action of the flooring system, perform far better when compared to actual tests. These comparisons indicate that the future development of design methods for fire safety of structures needs to be steered away from its traditional emphasis on isolated member behaviour, and towards considering the interaction of the whole building structure with the aim of avoiding disproportionate collapse.