Pulling towards the middle
The Tension Triangle
These structures, like the prefab tension one I made a few weeks ago are made up of triangles of tension.
But this time we’re going to look at doing the tension differently. All that really has to happen is that the three connections being pushed apart by the compression must be held together somehow. Let’s try doing it with radial tension instead:
The shape will be the same, but it will be interesting to see how it behaves when under stress.
I happened upon a useful raw material for this kind of project when I was browsing the Metal Store in the south of Rotterdam. My eyes automatically skip over everything rectangular and square (yuck!) when I’m glancing around, because I only register triangles, and hexagons.
So when I was near the metal-cutting machine, I saw a big collection of left-overs, but the perforated aluminum caught my eye. It was triangular, it was hexagonal, it could have been made by a hive of aluminum bees!
Left-overs are of course cheap so I got this strip for very little.
Aluminum is, as I found out, the ideal material for this kind of construction. It’s quite soft and malleable so you can cut it easily, and then file it smooth without much effort, and even bend it. Yet, as you’ll see, it is more than strong enough to hold the tension we need here.
I cut groups of three holes, both in the triangular shape, and with three holes in a row.
Then I filed them until they had the right shape and no sharp edges. After a while you can’t tell that it originally came from a perforated aluminum sheet, because it looks like a very nice connector, just right for the purpose.
There’s just one more essential ingredient, the tie-wrap. Tie wraps easily fit through the holes and can then be tightened very precisely, basically click-by-click if you want to count the clicks you make, so that means accurate to the millimetre.
The center of the radial tension is one of these triangular-shaped aluminum connectors, and that’s where the tie wraps meet.
The three-in-a-row connectors I was able to slip over the end of a set of bars I had laying around. The middle hole went over the bar’s end and was held by the rubberish covering of these bars. I think these bars were once rollers of some kind of machine, not sure, but they have nice stainless steel bars.
Putting it all together, we once again get the six-bar tensegrity but this time with radial tension instead of triangular to hold the sets of three end points together.
The triangular tension model, when stressed, exhibits a rotating action which Bucky Fuller talked about at length. Every triangle rotates together or none of them rotate.
Not this one!
So, as expected, with radial tension the bars are suspended in space the same way as with triangular tension, but the behavior is very different!
When stressed, this version of the 6-strut tensegrity can be easily flattened. The omni-rotation that you get used to with these 6-strut tensegrities is completely absent!.
Sometimes we might think of replacing the tension triangles with something, perhaps with a fabric triangle. This experiment makes me realize that a fabric triangle would behave very differently, because it would be a middle ground between the original and this one.