Abstract
The paper describes the principles of a three-dimensional frame analysis 3DFIRE which has been developed for the purpose of modelling the behaviour of skeletal frames under fire conditions. It is based on an existing two-dimensional program for nonlinear ‘spread of yield’ analysis of rigid steel frameworks at ambient temperatures. This has been developed to a three-dimensional capability covering both geometric and material nonlinearities, including the changes to material properties as temperatures increase. Temperature distributions across members can be nonuniform, causing differential thermal expansion and a spread of elastic and inelastic properties across the section. The model has been validated against a range of previous analyses of large-deflection elastic, inelastic and fire problems. It is initially being used mainly to investigate the behaviour in fire of column-subframes extracted from orthogonal rigid and simple frames. An example is shown of the predicted behaviour of such a subframe under the influence of a fire which is restricted to a single compartment, and which may cause either uniform or variable temperature distribution within the column cross-section. The former case is shown to correspond well with the current British design code's prediction of failure temperature.
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