In many cases of practical interest steel structures represent the best choice to fulfil the requirements of resistance, ductility and extreme adaptability to the needs related to the use of the buildings. The actual standards incite to design steel structures taking advantage as much as possible of the ductility features of the steel considering the behaviour of the structural elements beyond the elastic limit. In the most common cases, related to frame structures, it is common to adopt I-shaped cross-sections and their limit behaviour are referred to plane stress states due to the combination of axial and shear forces and to bending moment. In literature and in actual national and international standards different formulas of the corresponding domains defining either the elastic or the plastic limit behaviour can be found. In literature many formulations to calculate both the elastic limit and the plastic one, considering the simultaneous action of the three generalized stresses, can be found but always referring to approximate formulations. Therefore, in the present paper the authors propose a further in-depth step to define the plastic behaviour of steel I-shaped cross-sections considering in a more accurate way their corresponding strength limit allowing to obtain more reliable domains. The material behaviour is assumed to be elastic-perfectly plastic, reference is made to Von Mises criterion and the elastic stress fields are obtained following the Euler-Bernoulli beam theory. The obtained results will allow to define new yield domains for the more common cross-sections, showing the ease of extending to any other steel section. The comparison with the usually adopted domains will give a crucial information to evaluate the affordability of the safety factors to be adopted.