and each lower chord portion 18 39 is at its two ends provided with connecting flanges 22 23 alternately engaging into each other as can be seen in fig9 so that each connecting flange 22 contacts a respective connecting flange 23 the individual frame portions 16 39 are bolted together at the connecting flanges 22 23 as can be seen in fig5 the connecting flanges 22 are angularly arranged to the longitudinal axis of the upper or lower chord portions 17 39 18 39 each connecting flange 23 includes two separated parts one each on each side of the upper and lower chords 17 18 each connecting flange 22 23 is provided with holes 24 25 through which connecting bolts extend a number of the holes being slots 25 lying on a circular arc about a circular fastening hole 24 it is obvious that by removing the bolts connecting plural frame portions 16 39 one or more portions can be removed and the angles at the corners of the polygonal ring can be adjusted to form a frame 16 of a smaller diameter it is equally possible to open the frame 16 at a joint between two frame portions 16 39 and to insert new frame portions 16 39 to increase the diameter of the frame as can.