(Well, 6.7m diameter actually (22 feet))
The geometry of the dome is based on an icosahedron, by adding new vertices at the midpoints of each edge, and then pushing these out to the circumscribing sphere. More edges are then added to create a mesh of triangles, with 4 for each original face of the icosahedron. The full polyhedron would have 42 vertices, 80 faces and 120 edges.
There are 2 different edge lengths, 2.06m and 1.82m (about 6'9" and 6'), which in the full "sphere" would be present in equal numbers (60 of each). This structure is only a 1/2 sphere, and lacks the (long) edges around the equator, and hence has 25 long poles, 30 short poles.
The entire sphere would have 12 5-way joints and 30 6-way joints. Breaking at the equator means that this dome has 6 5-way joints (one at the central apex), 10 6-way joints and 10 2-way anchor points. For the structure to be stable these anchor points need staking down.
In the completed dome, so long as forces are only applied to the joints, the structure exhibits great rigidity and strength, since the poles or joints do not experience torque (bending) forces. (This is not, however, the case during construction!)
The total length of aluminium tubing used is 103m or 343'. The total area of the triangles is 64.8 sq metres (~700 sq feet). The total mass is 43kg (96lb), of which the tubes comprise 85%. The central apex joint is capable of taking the weight of a person (80kg) swinging on the end of a rope, and is 11' above the ground. The usable floor area for a 6' individual is approximately 20 sq m, 220 sq ft. Total floor area is 32.6 sq m (~350 sq ft, or half the area of the dome, which would be an exact relationship for a true sphere)
The structure was raised for the first time on the afternoon of Sat 28 Sept 1996, and this took about 4 hours. It was taken down the following day in about 1 hour. Part of the time to put it up was due to the need to drill larger holes to give the bolts (sorry, panel-head machine screws) more clearance. 220 nuts/bolts is quite enough to make you sick of them! 2 nuts and no bolts were lost during this process.
My next project is to create a covering for the current framework-only design. The joints each have auxiliary holes to allow the bolting on of fixing points, exterior or interior.
Last updated by markt@chaos.org.uk
Fri 4 October 1996