“I’ll huff, and I’ll puff….”
As I was working on my build/design project with the Sky Yurt, one big issue kept churning around in my mind – the power of the wind. I knew from sailing just how powerful the wind can be. On a sail, as the speed of the wind increases, the force on the sail increases exponentially.That’s why almost all boats have sails can be reefed and the sails have lines (sheets) so they can be depowered and lines (halyards) so they can be dropped completely. I began to look on the web for tent structures similar to the Sky Yurt – I wanted to see how professionally engineered structures dealt with wind forces on a large expanse of fabric. There are some huge fabric structures – most of these are tied to steel masts set in concrete and looked beefy enough to dock the Von Hindenburg zepplin. Way out of my league.
Gone with the Wind
One big concern was the fact that I really couldn’t find any-tent like structures that were set up like the Sky Yurt – none at all. I found tree houses well-secured to trees and structures built on slopes with some space underneath, but nothing that was trying to combine a large umbrella like work space with a living quarters above. It just might be that if similar structures were built, they were impossible to secure against strong winds, and blew way or were quickly abandoned. Working on dealing with wind forces was going to be a major challenge – and maybe would scuttle the whole project.
Crash and burn
I also knew that tent structures could be hot. Fabric can trap solar energy and hold on to it. We had an old army tent as kids and it could be stifling in the summer. I had to make sure the whole structure wouldn’t blow away, but also make sure that the air could escape from the workspace without congregating in the upper yurt. I also wanted decent airflow in the yurt itself. I had a sense that if I could design the shelter to be secure in strong winds, that solutions to the ventilation issues would suggest themselves. I started to look on-line for earth anchors and tie down straps, the kind they use to keep small planes from blowing away when parked on the ground. The project was getting more complex, more challenging and more exciting.
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.