Floating compression, also known as tensegrity, is structural magic.
You have struts and tendons, tied together in such a way that they hold each other up. The struts undergo compression and the tendons undergo tension. In other words, you have sticks which push and strings which pull. Also, you get bonus points if you build a structure such that none of the sticks or strings are touching each other, the parts should be suspended in space. It should hold itself up by itself.
These two complimentary components can be combined into such astounding forms, it is little wonder that so much mysticism surrounds the subject. There's the dualism of yin and yang, of push and pull, there's mutual support, everything is in equilibrium, there's uncommon symmetries which are boldly three dimensional. There is a special joy in imagining and contemplating and building and playing with floating compression.
- For the aspiring sculptor of floating compression, Kenneth Snelson offers a great place to start in his lovely self-published booklet, Tensegrity and Weaving.
- For a technical treatment, Hugh Kenner reveals the mechanics of these special structures in his book, Geodesic Math and How to Use It.
- For a handy, weekend introduction, George Hart provides a tutorial on building a spherical model with straws, paper clips and rubber bands.
- Finally, there's the Tensegrity wikispace, which is perhaps the most thorough compendium of tensegrity knowledge available.
One of the biggest challenges in construction is getting the tendons to have the right length and the right tension. Too short and you can't connect the final piece. Too long and the structure is flimsy or can't hold itself up.
Rubber bands are very forgiving, because they're like loose springs, but this makes them poor at holding a structure up. Twine, string, or wire is tight and strong, because it has comparatively little stretch, but this means the length has to be just right.
The ideal is to have a built in mechanism of tension adjustment, like the knobs on a violin, or a rigging turnbuckle. But such gizmos can distract from a bare and pure aesthetic. Kenneth Snelson uses a threaded rod inside hollow struts to "suck" the wires inside. But for small models made with the likes of dowels, this isn't really feasible.
If you can't "tune" your tensegrity post-assembly, you've got to have all the tendons made out to the right lengths at the outset. For a regular form this likely means all tendons of the same length. What can help a lot is a knot-tying jig, such as nails in a board. When dealing with something low stretch like twine, the required precision in length may be as small as 1%!
You may fabricate a batch of pre-tied tendons and go to assemble and find that the effective length isn't quite what you set out for. Maybe the knot itself tightens thus consuming less cord, and maybe also the twine "settles" into a longer length after the initial stretch. To preserve regularity in the final model you would then have to adjust the jig by a hair and fabricate a whole new batch.
Given these hurdles, my first real breakthrough came from tensegrity builder Jim Leftwich. Jim discovered that the end of a piece of nylon line can be melted to make a stopper. Brilliant! Applying a little heat to the end is faster, easier and perhaps more precise than tying a knot. For a spherical tensegrity, he takes this one step further, and runs the twine through pre-notched dowels and melts it in place, for an instant, and highly consistent result.
So, I adopted Jim's brilliant idea of using nylon thread and melting the ends, and I was off to building with many fewer headaches. Melting in place, with the twine already in the notch is the technique used in my piece Bubble with ninety sticks.
Notches are good when the pull direction is downwards relative to the strut (i.e. attached on one end, being pulled towards the other). But if the pull direction is upward relative to the strut the line would pull out. So, another technique I've devised is using the melted end as a stopper for an overhand knot. I'll fabricate a bunch of pieces of nylon twine and pre-melt the ends at the desired length. Then for assembly I tie an overhand knot to hitch it to a post, such as a nail or an eye. I use this overhand knot technique in my pieces Eight level dowel tower, and Calling ball.
Later on, I realized an upward pull doesn't necessarily have to result in a pull-out, if you layer an upward tendon underneath another downward tendon. In other words, the tendon first in the notch wants to pull out but another tendon on top, pulling into the notch traps it and prevents escape.
For structures requiring both pulling directions, this is a handy trick. It also applies to tendons which loop over a headless post such as a finish nail at the end of a strut. A loop would pull off the post with an upward pull, unless it was held down underneath another loop with a downward pull.
With adjustability comes much more freedom. You don't have to achieve the perfect length at the fabrication stage. You can "tune" the structure as desired. Over the lifetime of a tensegrity, if it settles or deviates, you can correct it.
Turnbuckles really distract from the look of a tensegrity. From what I can tell, Ken Snelson uses a custom-built turnbuckle mechanism hidden inside the ends of the tubes. I experimented with my own amateur version of such a system, which can be seen in my piece "Nine tight lines". But I was ultimately disappointed with the complexity and overly mechanical feel. I wanted less machine craft, more hand craft.
At the same time, Jean-Pierre encouraged me to explore different materials for tensegrity, more natural, more raw. The clear choice being bamboo, nature's ultimate struts.
So, I went for tension-adjustable knots. No screws, wrenches, gears or cams required. No high precision on the fabrication length or attachment point required. Also, knots are beautiful, they have no reason to hide.
The trucker's hitch is a good candidate, but it it comes with a lot of knot baggage and a free tail end which I find visually unappealing. My preferred technique is to use a pair of prusik hitches mutually attached to each other. Which I've done for the pieces, "Narcissus" and "Aiolos" which combine bamboo and thin braided nylon cord. Braided cord works better than twisted or laid cord with prusiks because there's no tendency to twist when adjusting the prusik.