Connection means connect a transverse girder (4a or 4b) to longitudinal girder means (3a or 3b). One end of the transverse girder (4a or 4b) is provided with a pin (6) extending at right-angles to the axis of the transverse girder, and the longitudinal girder means (3a or 3b) is provided with a plate (8) formed with upwardly-open slot (8a). The pin (6) is engageable within the slot (8a), and the pin and the slotted plate constitute the connection means.