A beam comprises a pair of aluminium extrusions (10, 12) each having a web (14, 16) and a pair of spaced apart abutments (18, 20, 22, 24) extending substantially over the entire length of the web. The two webs (14, 16) are maintained in spaced parallel relationship by a plurality of sets of spacer plates (34, 36) and connecting bolts (38); rotation of a bolt (38) between an associated pair of spacer plates (34, 36) causing the plates to move away from one another into clamping engagement with corresponding facing abutments (18, 20, 22, 24) on the webs (14, 16) and apply tensile loading to those parts of the webs intermediate the abutments.