The hypothesis of Navier Bernoulli consists in assuming that the sections normal to the average fiber remain flat during the deformation of the beam to the plates.- The principle of Navier Bernoulli amounts to neglecting shear and warping of the cross sections in the study of displacement and deformation of a beam element to plate.Similarly, when we study the torsion, we see that a non-circular section, with two symmetric axes, holds under the effect of a torsional torque a radial warp.It can therefore only be applied to very thin structures.