My first goal in building a full scale prototype was to see if the basic engineering for the Sky Yurt tension structure would work. The PVC pipe was 2″ in diameter, dirt cheap (I did have to wash it all) and under tension I could get it to bend. I built two plywood hubs, about 2 feet in diameter with slots for the PVC beams made out of 2×4’s sandwiched between the top and bottom plywood circles. On the ends of the PVC beams I added end caps with eye bolts to catch the stainless steel cable. I had a primitive system to tension the outside cable. The structure went up with a minimum of hassle. It nestied inside the 14 foot diameter trampoline frame Andy and I bolted together.
Checking out the forces on the structure
As I increased the tension on the Sky Yurt structure the PVC beams wanted to move closer together. They began to bend in a sweet curve vertically, but they also wanted to begin to bend horizontally. Also they wanted to rotate the hubs so that the beams could begin to “pass” each other. Another problem – the lower beams were resting on the trampoline frame; they were carrying the weight of the whole Sky Yurt structure, and were beginning to bend upward. I experimented with adding struts – 1/1/2″ PVC pipe – between the upper and lower beams. It was fun using the chop saw that Andy lent me to cut PVC fittings at the angles I needed to connect the struts with the beams.
Structural lesson learned
The struts, by tying together the upper and and lower beams, added lots of rigidity to the beams, but I wanted a structure that could be tensioned, so it needed to be strong, but flexible. I didn’t want to make the lower beams beefier than the upper beams. I would be screwing around with the symmetry, making the structure more complicated, and even more difficult for me to get my head around. One thing that was clear was that I would need beams that had a rectangular cross-section or a built-in vertical curve to minimize sideways bending under tension. I was also going to need some cross bracing between the beams to provide support for the cover and to stiffen up the beams. I did end up with a pretty elegant set of solutions to these issues that were incorporated in the second prototype.