35 a but that portion does not follow the inclined shape of portion 35 a so that the width of the plates increases toward apex 35 b and the corresponding bottom apex 36 b in the vertical section the width remains substantially constant except that the curvature at the section 36 d is of a larger radius so is curves more slowly thus increasing the width of the plate between the portion 35 d and 36 d so as to provide increase strength in this area the bottom edge includes a further horizontal section 36 e generally following the path of the top edge 35 e extending again to the bend line 39 in the section in front of the bend line the bottom edge 36 g generally follows the top edge 35 g to the coupling 13 but is inclined at a slightly greater angle downwardly so that the width of the plate slightly increases as it approaches the coupling 13 this again provides increased strength at the forward area the inner plate 30 follows substantially exactly the shape of the outer plate 31 with two exceptions firstly the top edge 30 e in the horizontal section is lower than the top edge 35 e so that the top edge 35 e forms an upstanding flange portion 40 above the top plate 32 and the tope edge 30 e this flange portion is visible on the outside and covers edges of the decking extending across the cover plate 32 and the rails 22 the flange 40 commences at the portion 35 d so that the curvature is slightly less than the curvature of the inner plate 30 thus causing the edge to rise to a height greater than the height of.