Curiosity and the pleasure of finding things out motivated this project.
In order to understand the geometry of geodesic domes and to get a good feel for its structural behaviour, one must build one!
There are two major design problems: connector and cladding.
Looking into the published images on connectors for geodesic domes, I could not find a really simple joint suitable for DIY.
Starting with the knowledge that in a triangulated frame the connector can be a ‘pin-joint’ (hinge) as it is primarily subjected to axial forces (compression or tension) – no bending! The Eureka moment came while I was playing in the workshop joining bits of wood and metal. The universal joint (see Figure 2) takes advantage of the bendability, strength and durability of metal.. With this connector there is no need to calculate the axial angles of the struts and ties. By tightening the nut of a long bolt the metal strips, and the ends of the members, will bend to the correct axial angle.
Struts and Ties Lengths
I wanted my dome to be a hemisphere and without dissecting the triangles at its equator. The lengths of the struts and ties should not exceed 1 metre. A 4-frequency icosahedron met this brief. The chord factors (length of members) were simply worked out using the appropriate table (Dome Book 2, published by Pacific Domes, 1971). For this 6 metre diameter geodesic dome I used birch dowels 18mm dia.
- 93.50 cm. 30 members
- 90.00 cm. 70 members
- 85.90 cm. 30 members
- 80.00 cm. 30 members
- 84.75 cm. 60 members
- 72.85 cm. 30 members
Note the very high 52/1 slenderness ratio (length divided by diameter) of the struts.
To clad a doubly curved surface with prefabricated materials and to make it watertight is a challenge.
For my experimental, lightweight, demountable shelter I chose a parachute (price £4.50, 1971). It is dome shaped and fits loosely inside the 6 metre diameter dome. Curtain rods (springs) fixed the parachute to the joints of the dome, thus prestressing the fabric.
For the location of my experimental shelter I chose a camping site in the hills near Roquebrune-Cap-Martin on the easterly end of the Cote d’Azur (not far from the ‘hostel’ where Le Corbusier spent the summers and where he dies of a heart attack while swimming in 1965). A slightly sloping spot in a wooded area was ideal.
The assembly of my prefabricated ultra-lightweight geodesic dome resembled the ‘growth’ of cells, expanding in a centrifugal nature. The ends of the struts and ties had a colour code.
I began by assembling a triangulated pentagon on the ground, next adding 5 triangulated hexagons each sharing 1 edge with the former. A shallow dome began to form. I supported this structure on a stool in order to lift it off the ground. Adding 5 more triangulated hexagons, each sharing 2 edges with the above. Next adding 5 triangulated half-hexagons, each sharing 3 edges with the above, thus completing the top part ½ of the 4-frequency icosahedron dome.
The 5-fold symmetry of this structure dictated where the rest of the various lengths of struts and ties should go, making sure, at every stage, that the colour coded ends of the members did match. It took me about 2 hours to erect the dome single-handed.
Fixing the parachute to the space frame was easy and quick, like hanging a curtain! It was exciting to experience how this spherical double layer prestressed system increased the rigidity of the whole structure – synergy.
This project was first published in AD (Architectural Design) 02.1973.
Lighter than Air!
Having read that the geodesic domes (Biomes) at the Eden Project in Cornwall weigh less than the volume of air they enclose, I thought to check whether this is also true with my small geodesic dome?
1.2 kg (density of air per cubic metre) x 56.5 (volume of air enclosed by hemispherical dome, 6 metre diameter) = 67.8 kg.
Weight of the structure + cladding: 38 kg (struts and ties) + 3 kg (parachute) = 41 kg!