Abstract: A hollow tube constitutes a structural member having an opening in a wall thereof which surrounds a short hollow sleeve closely fitting within the tubular member, the sleeve having diametrically opposed openings one of which is of the size and shape of the opening in the tube wall. The margin of one of the openings in the sleeve is shouldered inwardly. The sleeve is so positioned within the tube that the openings are aligned, the shouldered opening of the sleeve being diametrically opposed to the aligned openings in the tube and sleeve. A hollow tubular bracing cross-member is provided of such dimension as to pass through, with minimum clearance, the aligned openings in the tube and sleeve, the free end of the cross member being received within the diametrically-located opening in the sleeve and resting upon the shoulder thereof. Means are provided to lock the bracing cross member removably within and to the tube and sleeve.