This paper deals with the analytical proof of the equivalence between the relative biaxial bending resistance of a rectangular solid reinforced concrete section and the biaxial bending resistance of the transformed solid square section of unit-length side. Similar proofs are also derived for rectangular hollow sections. The results of the analytical proof show that the relative biaxial bending resistance of a rectangular solid section is identical to the biaxial bending resistance of the transformed square solid section of unit-length side. These results only occur when the concrete fiber and reinforcing bar coordinates in the transformed section are in conformity with the transformation that maps the rectangular section into a square cross section of unit-length side.
