Abstract When modelling material mechanical behaviour, an analytical description is required of the relationship between stresses and strains. A computer oriented mechanical behaviour model for steel is described. The model is based on the fact that the deformation process at transient high temperature conditions can be described by three strain components which are separately found in different steady state tests. It is shown that a behaviour model based on steady state data satisfactorily predicts behaviour in transient tests under any given fire process, load and strain history. The test method used for determining strength properties is of great importance for the obtained result. By using the model a procedure is shown how to couple steady state and transient state results and make them comparable for design. Sometimes the behaviour model for design purposes is simplified by using constructed stress-strain curves based on transient state results. This means that creep from the transient test is included in an approximate way in the stress-strain curves. The consequence of this simplification in a computation is illustrated for steel structures approaching creep failure under fire attack.