A beam provided with at least one first part that has a main body with an overall U-shaped straight cross-section provided with lateral flanges protruding outwards from the sides of the U, the main body further comprising a bottom part as well as two wings forming the sides of the U.The central portion of the main body defines cells working under tension and under compression, respectively, on either side of a neutral main bending axis formed by two transition zones, which are each located between the U-shaped element and a lateral flange.The bottom part of the main body comprises at least one transition formed by a curvature discontinuity. And at least one wing comprises at least one transition formed by a curvature discontinuity.