This paper deals with simple portal or gable steel frames with varying moment of inertia. Critical load for such frames is calculated by means of a very simple and approximate method through which the variation of moment of inertia for the members for the members is considered by a quadratic function and then the equilibrium and continuity conditions have been used. The degree of precision of this method has been checked by a computer method in a numerical example. The method is applicable only for one bay steel frames.

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