To widen a range of application to beams having a rectangular cross- section formed of a light metal alloy extrusion formed sections and, at the same time, to make it possible to manufacture various kinds of structural units by building up the beams each other.

Each of the beams is formed in the shape of a hollow formed section 10 having a rectangular cross-section with different sectional length (a) and sectional width (b). At least one undercut extending on a parallel with the longitudinal axis of the extrusion formed section is provided to the outside of the beam for the purpose of accepting an element for connecting with another hollow formed section. The sectional length (a) of one hollow formed section is equivalent to the sectional width (b) of the other related hollow formed section.