Hexagonal Geodesic Domes - Dimpled Domes

Deviations from a Sphere

It appears that dual-layer geodesic domes are not "naturally" spherical.

In particular, if the truss material forms shapes which are close to tetrahedra (which it seems that they should for the sake of stability), then the thickeness of the dome will inevitably come to vary over its surface.

Usually the regions where pentagons appear in the surface of domes will have the shortest struts. In a conventional (icosohedral) geodesic dome, this results in these being the strongest (and heaviest) areas of the dome - due to an increased density of structural material.

In a dual-layer dome - by a miracle of geometry - the reduced strut length is matched almost exactly by a reduced thickness of the dome - resulting in strength and weight being more evenly distributed over the dome's surface.

This variation in thickness means that it's not possible for both the inner and outer layers of nodes in such domes to be spherical - unless the trusses deviate significantly from being tetrahedra.


Because strut size decreases linearly (first degree approximation) as the pentagons are approached - where as the surface of the sphere at that point is flat (again, a first degree approximation) that means that in high- frequency domes - if the inner surface forms a conventional geodesic dome, the external surface must form a series of "dimples".

Similarly, if the external surface of such a high-frequency dome forms a sphere, the inner surface will form a series of "pimples".


These deviations from a convex shape may have some practical consequences.

In particular if a dimple is present on the very top of a dome, it may form a rain-water reservoir, or interfere with the ventalation shaft that is naturally placed at the dome's highest point.

Dimples or pimples?

Are dimples to be preferred over pimples - or the other way around?

Or perhaps such domes should have both dimples and pimples - with each surface being deformed inversely with the density of material in each layer?

The point seems to be rather open to debate - and could even be decided on aesthetic grounds.

Tim Tyler | Contact | http://hexdome.com/