In one of the workshops I led at
Mathcamp 2008,
we made this two-layer geodesic sphere.
It is modeled after Fuller's Montreal's Expo67 dome.
[Photo above by Sam
Javanrouh]
The actual Montreal dome has frequency sixteen, while our model
has
frequency four.
A single-layer dome of high frequency buckles easily under local loads,
but
thickening it to two layers makes it a truss, which adds greatly to its
rigidity.
In this design, there are triangles for the outer layer and hexagons
for the inner layer.
These two tessellations mesh well because {6,3} and {3,6} are dual to
each other,
i.e., the vertices of each tessellation will lie in the centers of the
faces of the other
tessellation and the edges of each cross the edges of the other at
right angles.
The two layers are connected by a middle layer of struts which join the
vertices of each layer to nearby
vertices of the other layer. Notice the volumes created: a hexagonal
pyramid above each hexagon,
a triangular pyramid (tetrahedron) below each triangle, and a
tetrahedron for each pair of crossing edges.
To make a dome, substitute a triangular chunk of the structure for each
face of an icosahedron,
then project the vertices to two concentric spheres (one for the outer
triangular layer and
another slightly smaller sphere for the hexagonal layer). One
difficulty in such a design is
that after projecting, there are many strut lengths to calculate and
keep track of.
I wrote
a Mathematica program to
calculate all the strut lengths. All the different sizes are shown
The unlabeled edge lengths are the same as others already labeled,
using symmetries of the triangle.
We built the model using
TubeSpace
parts, starting by assembling small units.
Thank you Jim Hausman for providing the parts.
We joined the small units into double-layer modules corresponding to a
face of an icosahedron.
We started connecting the modules together to make chunks of an
icosahedron.
There are many small details that must be attended to.
(It can't hurt to give it a quick acoustic check.)
Near the end, there were many small connections still to be made and
adjusted.
With some tweaking it could be made more spherical, but we stopped as
it was getting late.
The material glows cooly in the dark, but it is hard to get a good
photo of it.
Thank you Zhumeng for this last image.