Nowadays, the teaching in civil engineering courses related to the analysis and design of reinforced concrete beams subjected to pure bending is based on models that use simplifications to facilitate calculations. When analyzing the plastic capacity of a rectangular section, its yield moment is usually calculated assuming that the stress-strain relationship can be idealized using a straight line. However, the validity of this relationship is only verified when the levels of compressive stress are less than 45% of the compressive strength of concrete f´c. For this reason, it is convenient to make a rigorous review over the convenience of using this simplification at medium and to high stresses close to failure. This work studies how a reinforced concrete section behaves according to the simplified model and compares its results to those obtained when using the function proposed by Hognestad in 1955 (still vigent). Interestingly, as a general conclusion, it can be said that the results predicted by the simplified model fit well respect to those obtained using the greatest mathematical rigor.