Two corresponding fire resistance assessments, specified for the same simple steel beam but obtained after applying the alternative computational approaches, have been compiled and quantitatively compared with each other. The conventional standard algorithm according to which the representative values of the load effect and the beam resistance are identified respectively, was the first considered procedure. The solution found on this basis was then juxtaposed with an analogous result, determined however after using a fully probabilistic analysis. The evaluation criteria specific to an accidental design situation were taken into account in these considerations. The critical temperature of steel from which the considered beam was made of was accepted here as the authoritative safety measure. This temperature value was associated in the paper with the fire resistance limit state calibrated for the maximum acceptable level of failure probability. When forecasting such the probability, not only the risk of a potential fire being initiated but also not being effectively extinguished was included in the calculation.