A supporting frame includes a truss composed of an upper straight frame member and a parallel lower straight frame member supported at their ends in spaced relationship relative to each other. At least two spans of divergent inclined brace member extend between them with the ends of the brace member spans each being securable to the upper or lower straight frame member by means of a fastener. Limited relative longitudinal movement of the upper and lower straight frame members is permitted before a fastener is secured to fix the positions at which the ends of the brace member spans are attached to the upper and lower straight frame members. In one application two trusses meet at a corner with a pylon being urged in the plane of the supporting frame into a corner between two frame members of each pair of trusses to extend at right angles to the frame.