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This work provides a derivation of an expression, usually found in fire steel structures Standards, like NBR 14323:1999 [2] and Eurocode 3 [5]. It allows calculating the temperature of thermally uninsulated steel members. Within these standards, the limits of the utilization of the expression are not clearly stated. The main contribution of this work is to clarify this subject. The adopted hypotheses are detailed in order to understand its use. With the aid of the SuperTempCalc computer software, the situation of a steel plate protected by a concrete slab over one face is thermally analyzed. Considerations are made on the determination of the section factor in other situations that, despite being usually found in civil construction, do not follow the hypotheses of the regarded expression.