light metallic material in particular aluminium or magnesium or some sort of special alloy made of these or other metals having a low specific weight in correspondence of the area of engagement with plate 1 crossbar 2 comprises a longitudinal rib 4 apt to tightly fit groove 3 of plate 1 at the bottom of rib 4 crossbar 2 has suitable flutes 5 apt to allow a perfect abutment between the inner face 1 a of plate 1 and the corresponding resting surface 2 a of crossbar 2 when these two elements are brought into contact by introducing rib 4 in groove 3 at the top of rib 4 a recess 6 is instead provided apt to ease the plastic strain of rib 4 during the operations of introduction and upsetting of said rib into groove 3 in order to accomplish the coupling between plate 1 and crossbar 2 said elements are firstly joined introducing rib 4 into groove 3 and they are then exposed to pressure in a mounting press during this operation rib 4 which for this purpose has a height slightly greater than the depth of groove 3 undergoes a plastic strain which allows it to adapt perfectly to the inner form of groove 3 hence remaining tightly and steadily anchored to the same due to the undercut with which such groove is shaped the crossbar obtained by the above described fastening in addition to allowing a perfectly stable and slack free coupling over time between crossbar 2 and plate.