Abstract
A study is made on theoretical clarity as well as practical applicability of the simple plastic theory for the prediction of the ultimate states of steel frames subject to fire. Ultimate temperatures of simple steel frames were estimated by two different means ; i.e., by a refined finite element analysis which takes material and geometrical nonlinearities as well as thermal expansion into account and on the basis of the simple plastic theory which considers solely reduction in material yield strength according to temperature increase. The latter solution is called herein a primary ultimate temperature which is a simple structural characteristic quantity independent of thermal stresses. Comparison of both solutions leads to the results that the primary ultimate temperature is not only a first order ultimate temperature approximation but it also plays the part of reference quantity for the evaluation of stability or other second order effects on the ultimate temperatures of steel frames.
