US 201514676463 A description fig1 shows the first step in one embodiment where a plurality in this case seven individual tetrahedral units 1 7 are selected and oriented such that in the plane of the paper vertical edges 8 and 9 are aligned and horizontal edges into and out of the paper 10 and 11 are aligned each tetrahedral unit has four faces and six edges which can be pre formed by bending and welding sheet metal cutouts as shown in fig2 the vertically abutting edges 8 9 of the first and second units 1 2 are tacked together as indicated at 12 and the abutting horizontal edges 10 11 of the second and third units 2 3 are tacked together as indicated at 13 similarly the vertically abutting edge 14 of the third unit 3 is tacked at 16 to the vertical edge 15 of the adjacent fourth unit 4 and the sequence is repeated to form a segment 17 of connected units with a center line passing through a multiplicity of vertically oriented tacked edges and a multiplicity of horizontally oriented tacked edges it should be understood that in this context u201c vertical u201d and u201c horizontal u201d are proxy terms for mutually perpendicular edges such as 8 and 10 without regard to orientation relative to the horizon and that the u201c centerline u201d of the u201c straight.