fig2 and 3 there is formed laterally in each case a vertical bore 60 the inside diameter of which is less than the width of the bottom surface 38 of the depression 36 and less than the distance between the reinforcing ribs 44 46 in the upper frame part 6 there can be provided instead of the bores 60 lateral recesses 90 92 and 94 as in fig1 in the wall 48 and the bottom surface 38 as in the lower frame 8 the width of the recesses is likewise less than the width of the bottom surface 38 and less than the distance between the reinforcing ribs 44 and 46 the middle frame parts 10 12 as in fig1 and 7 are made as box shaped hollow profiles of aluminum or plastic with inside transversely and longitudinally running reinforcing walls 62 64 in the outer walls 66 68 parallel to the reinforcing walls 62 64 there are formed circular openings 70 72 lying opposite one another for the pipes 4 the radius of curvature of the openings 70 72 corresponds to the radius of the pipes to be received and the maximum depth of which is less than the distance between the outer walls 66 68 and in each case the reinforcing walls 62 64 adjacent to the walls 66.