Prototype hub – PVC
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
Prototype 1 – struts
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.
Strong but not heavy
Sky Yurt – model #3
To hold the Sky Yurt up in the air, and to attach the awning for the workspace I was going to need a strong core structure. But the Sky Yurt itself could be made of much lighter structural materials. It only had to hold itself together, like a pop-up camping tent. I was driving into Willsboro one afternoon and saw a backyard trampoline with a kids low play tent stretched over tramp. I knew about trampolines from my trimaran sailing. A few people jumping on a trampoline generate a strong downward force that needs to be resisted by the frame. So trampoline frames are built to be very strong. .But they are also portable and are pretty light for the stresses they have to deal with.
Using the trampoline frame to hold up the Sky Yurt
For the next model I went hunting for circular rings. Found what I was looking for in a craft shop – those hardwood rings that are used for needlepoint work. I made a new balsa wood and plastic straw model, and I really liked the look and feel of it. I was ready to build my first prototype. Now I was getting into the real building project. My scare-level increased to the point where I thought I might need to start wearing a metaphorical diaper. I asked my friend Andy Wekin – he was an artist who had gone back to school to be an engineer and he came with his partners Otis and Ezra (more on this team later) – to be the consulting engineer.
Beginning to build the first prototype
Sky Yurt – PVC prototype
I found a trampoline frame on Craig’s List and hauled it home. They are really available, sort of like old hot tubs and spas. They are in those class of things that at the time seem like a good idea, but they stop being used (you don’t want your kid to break her neck) and they take up space a lot of space in the outdoors. My friend Robin runs a preschool at a farm. They had a lot of PVC pipe that had been used in an irrigation project, and for a small donation I had my beams. I made hubs out of plywood and scrounged for PVC connectors that I could cut up and experiment with. I had SS wire from some of my old sailing rigs. Mostly I wanted to learn from this first structure -see if the basic engineering would work, where I would need extra support and how the Sky Yurt structure would interface with the trampoline frame. And it started out looking pretty promising.