Method of forming a box-shaped structure from a foldable metal sheet

A method of fabricating a box-like steel metal structure from a channel-shaped metal sheet (12) having a flat rectangular bottom section (13) with four lower corners, a lower corner being defined at each corner of said rectangular bottom section (13), longitudinal side edges extending along said rectangular bottom section (13) between corners, and a pair of side sections (15) rising upright from opposite edges of the bottom section (13) said method comprising the steps of:

• (a) forming four vertical folds (36) each fold extending upwards from one of four inner points (38) each inner point being located on one of the edges spaced from one of the corners, the distances from each corner to the nearest of said inner points along the edges being substantially equal, said vertical folds (36) defining two rectangular end portions in each of said pair of side sections (15), each rectangular end portion having an outer corner point diagonally opposite the inner point in its rectangular end portion;
• (b) forming a diagonal crease (39) between said inner point and said corner point in each of said rectangular end portions, said diagonal crease (39) defining in each of said rectangular end portions an intermediate and an outer triangular section (15A,15B);
• (c) inwardly pressing each of said intermediate triangular sections (15A) so as to fold said intermediate triangular section and said outer triangular section in the same end portion towards each other along the vertical fold (36) and the diagonal crease (39) in the end section;
• and,
• (d) bending opposite end portions of said channel-shaped metal sheet upwards substantially at right angles to said bottom section (13) to form upright end wall sections with inner surfaces on said formed end wall sections so as to create a lapping wedge-shaped portion (3) consisting of the overlapped intermediate and outer triangular sections (15A, 15B) on said inner surfaces of said formed end wall section (41).