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Having looked at the drawings, the next step is to build a model from skewers and elastic bands, to better understand how the structure works and to get precise measurements. We will estimate dimensions and use the model to adjust our measurements, then scale them up to build the full size tower.
Before you can start building, you will need some skewers and some elastic bands cut to make long strips of elastic. You will also need superglue (cyanoacrylate adhesive)- this is the fastest and easiest way to glue the elastic bands to the skewers. Set aside an hour to do this from start to finish the first time you do it. Once you have practiced, it should take about half an hour to build a model like this.
The idea with this type of tensegrity is to always make the structure lying flat or unrolled like the drawing at left, and then to attach all the pieces together to raise it into shape at the end.
This photograph shows two levels of the structure, attached together and ready to be 'rolled up' into shape as a tower. There are two things to notice about this structure: the two levels are sloping in opposite directions, and the two levels are joined without the skewers touching. The upper deck's skewers all touch the middle layer of elastics halfway between the bottom deck's skewers. One last thing here is the proportion of the structure: if the elastic is 1 unit long, then the skewer is roughly 1.5 (actually 1.414...-the square root of two) units long. I chose this dimension for the sake of simplicity, and it seems to work fairly well. You might be thinking "how is 1.414 simple?" If you look at the image, you will see that the elastic bands lie roughly at right angles to each other. That is because of the proportions of 1: 1.414ish. The elastics were all cut to the exact same lenght as each other, and the skewers were then marked with a permanent marker at the 2 points where the elastics were to be attached. 1 drop of superglue on the end of the elastic, place the skewer on, and in a few seconds the glue has cured (as always with cyanoacrylate, be careful about sticking your fingers to the model).
Once you are finished with the grid and are ready to roll your model up, seperate out the elastics that are going to run horizontally (ie. the blue,turquoise or green ones from this drawing) from the diagonal (or red) ones. Glue the horizontal elastics first, starting at the bottom and working your way up to the top. When you are finished with this, you will bave a very dis-ordered looking bundle of elastic and skewers. The most confusing step is coming. You need to work methodically from the bottom up, attaching the diagonal elastics. The thing to watch out for here is that you work within one level all the time- don't attach an elastic from the first level to a skewer on the second level. Diagonal elastics should only be joining the bottom of a skewer to the top of a skewer on the same level.
The completed model,squashed between two cupboards. The handy thing about using elastic bands is that they will just stretch to adjust the structure to the best shape. So instead of a tower, this is more of a bridge, and we can simply neasure off with a ruler and scale the dimensions up for a larger structure.
A final shot, looking down into the model, showing how the structure is arranged. The next post will look at the actual full-scale structure, built with bamboo and nylon, which was set afloat on the pond in front of the architecture department at Wits university for 2 weeks.