It is clarified that the ultimate temperatures of steel beams are in good agreement with the solution based on the simple plastic theory considering solely the reduction in material yield strength according to temperature increase. The solution so predicted is herein called a primary ultimate temperature which is a simple structural characteristic quantity independent of thermal stresses. In this study, simple beams and steel beam-with-concrete slabs are analyzed by a refined three dimensional finite element analysis which takes both material and geometrical nonlinearities as well as thermal expansion into account. Comparison between both primary temperatures derived from simple plastic theory and temperatures analyzed by numerical analysis suggests the practical applicability of the simple plastic theory for the prediction of the ultimate strength of steel beams subjected to fire.
