of proceeding to the disassembly of the metallic barrier the support arm 5 is provided with a horizontal elongated hole 25 at its top face 28 placed below the attachment hole 24 also horizontal the object of the which is to leave free the passage of the bottom wing of separator 3 thus preventing the interference between the same and the support arm the support arm 5 is provided on its top face 28 with a vertical rib 26 or vertical grooved surface that extends from the top edge or borders up to the proximities of the fold between the top face 28 and the consecutive one 29 and presents a preferred and appreciably semicircular section though it can also be triangular or trapezoidal the vertical rib 26 is situated very near the vertical edge of the top face 28 of the side opposite to the apertures of the horizontal elongated holes 24 and 25 the object of the vertical rib 26 is to stiffen the top face 28 of the support arm 5 at the zone comprised between the bottom horizontal hole 25 and the edge thus preventing that the support arm 5 bends at this zone during impact the bottom face 32 of the support arm 5 presents a vertical elongated hole 21 not necessarily centred on the face that permits the union between the support arm 5 and screen 4 by means of a screw washer nut assembly 20 that successively penetrates the central horizontal elongated hole 9 of each one of the two consecutive screens 4 at the overlap between both and the vertical.