This
page shows a relatively quick and easy method for making simple but fascinating
tensegrity structures. The only materials needed are plastic soda straws,
small paper clips, and rubber bands. The assembled tensegrity structures
are polyhedral forms which vividly illustrate the separate forces of tension
and compression. They are robust enough to squeeze and jiggle. Rubber bands
provide tension and plastic soda straws provide compression. The instructions
below are for making a tensegrity dodecahedron,
but the underlying technique is suitable for a wide range of polyhedra.
Being fast and inexpensive, this is a good classroom project. Based on my kids, I would suggest this as a project for middle school or high school students after they have seen and made paper models of the Platonic solids.
The photo above shows a complete tensegrity dodecahedron. There is one strut (made from half a straw) for each of the 30 edges of a dodecahedron. The struts are rotated slightly compared to a dodecahedron edge, so rather than three struts meeting at a vertex, they make a small triangle.

You can slide the straws along the rubber bands to make them meet at a point other than the 1/3 and 2/3 points. If you slide them to meet as close to the endpoints as possible, the little triangles shrink to nothing and you get a dodecahedron. If you slide them to meet above the midpoints of the straws, you get a kind of icosidodecahedron in which the straws form six big pentagons, like equators, which weave in and out through each other.
Note how the tensegrity structure is chiral, meaning that it has a left handedness or right handedness. You can construct its mirror image, or just look at its reflection in a mirror, which has the opposite handedness. Visualize how if you could turn yours inside out, it would switch handedness. In contrast, a plain dodecahedron is the same as its mirror image, and so is not chiral.
Observe how no two struts in a tensegrity structure touch each other,
yet the diameter of the overall the structure is roughly twice the length
of a stick. There is nothing else like this: with an overall size larger
than the size of its rigid components, yet the components are not rigidly
tied to each other. The compression lies in disconnected pieces. Experiment
with other shapes than just spheres.
Although
the soda straw method can be applied to other polyhedra, very complex tensegrity
structures will be too flexible unless the rubber band is very tense. But
a taut rubber band will cause the plastic straw to buckle, which is the
ultimate limitation of this technique. For more complex polyhedra, stiffer
struts --- wood or metal --- are essential. This photo shows a part of
one, based on the truncated icosahedron,
which I made from wooden coffee stirrers and dacron thread. (Another
photo.)
You will have noticed that rubber bands are rather stretchy, so your structure bounces and deforms easily. To make a more rigid structure, you can use a piece of string instead. This requires careful measurement so that each string is exactly the correct length. String solves another problem also: after about a year, a rubber-band tensegrity structure will self-destruct as the rubber becomes brittle with age.
Tensegrity
structures need not be spherical or based on a polyhedron. The concept
of tensegrity was first realized by the sculptor Kenneth Snelson, who began
constructing structures in 1948 and showed them to Buckminster Fuller.
The term "tensegrity" was coined from the phrase "tensional integrity"
by Fuller, who proposed that the method could be applied to large architectural
domes. However, the flexibility of the result is not suitable for most
space-enclosing purposes, and has found much more interesting application
in sculpture. At right is Needle Tower by Kenneth Snelson, an 18
meter tall sculpture at the Hirshorn Museum and Sculpture Garden of the
Smithsonian Institution, Washington, D.C. The lower components are larger
in size, which gives the viewer standing at the bottom an unusual sensation
that the artist has captured infinity.
For more information on tensegrity structures, see the books by Pugh,
Kenner, Gabriel, and Grip, listed in the references.
An advanced mathematical analysis of tensegrity, and pointers to other
references in the literature can be found in the articles "Rigidity" and
"Mathematics and Tensegrity" by Robert Connelly.