A joint assembly comprises a hollow extrusion member of an asymmetrical hexagonal section and a flange casting comprising a base flange and several webs including a top web and upper side webs which have joint faces presented to corresponding joint areas on the outside of the extrusion while a bottom web and lower side webs have joint faces which are presented to corresponding joint areas on the inside of the extrusion. To assemble the joint assembly an adhesive is applied to the joint faces and the extrusion aligned with the flange casting but offset vertically. The extrusion is then moved longitudinally towards the flange casting to the full extent permitted by abutment of an extrusion end face with steps to bring the joint faces into overlapping alignment with the corresponding joint areas on the extrusion, there being a substantial clearance due to the vertical offset so that the adhesive on the joint faces does not contact the joint areas on the extrusion at this stage. Then the extrusion is moved transversely relative to the flange casting so that each joint face complete with the adhesive is brought into contact with its corresponding joint area on the extrusion. Holes are then pierced and blind rivets inserted to ensure full contact of the adhesive with the joint areas. The assembly of extrusion and flange casting is then placed in an oven to cure the adhesive.