Beams with U-shaped cross-section

A beam comprising a first portion, a second portion, and a third portion, the first, second and third portions having a U-shaped cross-section, wherein the U-shaped cross section comprises a bottom and a first and a second side wall extending substantially perpendicular to the bottom, and wherein

• in the first and third portion, the first and/or the second side wall comprises a first substantially straight portion, a second substantially straight portion and a side transition zone between the first and the second substantially straight portions of the side wall, wherein
• the side transition zone comprises a substantially straight portion and wherein the first and second substantially straight portions of the side wall are substantially parallel to each other and the straight portion of the side transition zone is not parallel to the first and second substantially straight portions, wherein
• in the second portion, the first and second side walls do not have a side transition zone, and wherein
• in the first portion the side transition zone is arranged between 50-90% of a height of the side wall, and the straight portion of the side transition zone is inclined with respect to the bottom of the U-shaped cross section.