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Aug. 04, 2011

New Heights: Tree Houses and Engineering

by April Garbuz

Click to enlarge images

By April Garbuz, Wilton High School

I learned from Tom Chudleigh, Canadian carpenter and founder of Free Spirit Spheres, how engineering and physics can be used to just have fun! Using science to build floating spheres, Tom revolutionized tree houses and has made a living doing so. Talk about a dream job!

Do you have a background in engineering?

I have a background in biology and engineering. I studied biology at the University of Calgary for 4 years in the 1970’s and 80’s, then I got into power engineering. I worked about 15 years as a power engineer and then started building boats and sailing. I made the money to fund my boat building and sphere building by teaching power engineering and as a technical writer, writing training manuals for process operators.

When making the houses, how do you account for the laws of gravity?

The law of gravity is easy to contend with. It basically sucks everything down toward the earth. All you have to do is tie ropes up into the trees and pull the sphere up harder than gravity is trying to suck it down. Each of my larger spheres weighs about 850 kg. That weight is evenly distributed over 3 trees. The 3 tethers that hold the sphere up are each capable of holding up 4500 kg. You just have to make sure the trees and the ropes are all capable of holding up a lot more weight than they actually hold up. You want a lot of “Reserve Capacity” or a big “Safety Factor” to ensure nothing breaks. We aim for a safety factor of 10.

How does the suspension concept work?

The sphere is designed to fit into the center of a triangle of trees. The sphere is a ball and if you look down on it from above there are 3 attachments spaced at 120 degree intervals. The attachments points on the sphere are very near the equator. Each of the attachments connects the sphere to one of the 3 trees.

You want to find a triangle of trees where the trees are just slightly further apart than the diameter of the sphere, so there is room for the sphere to exist and move. That way the suspension ropes are very close to vertical and the load on the tree is straight down and there is not much of a horizontal component. Any horizontal load on the trees can be counteracted by installing stays on the trees. A stay is a rope that is installed opposite the sphere suspension rope and goes to the base of another tree further away from the sphere out in the forest.

Geometrically speaking, how is the structure of a sphere significant?

A sphere is unique in that all points of the skin are equidistant from the center. The skin of the sphere continuously curves in all directions. This curvature gives the skin strength. In the case of a small sphere like a ping pong ball the skin can be amazingly strong and light. As the radius of the sphere becomes larger the curvature is reduced and the strengthening effect of the curvature is also reduced. One of the affects of the curvature is to spread the force of an impact further away from the point of impact. In a way it distributes the force around the skin. It makes the sphere much more resistant to puncture than a flat panel would be. Think of hitting a ball with a bat as opposed to hitting a flat panel with a bat. The two surfaces react in much different ways. That is why the sphere is the shape evolution has favored for packaging seeds and nuts. Simply because of the shape, it creates a hard shell that protects the contents. Using that shape for a treehouse is “bio-mimicry”. We take a natural solution for a problem by mimicking nature.

How do you calculate what size the sphere needs to be?

What size the sphere needs to be depends on the intended use. An empty sphere works fine for a ping pong ball. To house a couple of people you must have a much larger sphere. A planetary sized sphere is needed to establish a planet.

Our objective is always to keep the spheres we build as small as possible. This is for transportation purposes; ease of construction and to keep the load on the trees as low as possible. Also, as the diameter of the sphere increases, it tends to act less like a ping pong ball and more like a flat panel.

What is bio-mimicry and how is that applied to your tree houses?

Bio-mimicry is when you design something that looks and/or acts like something biological. Our spheres are shaped like a nut shell or seed pod. They also hang from a web of rope the acts to suspend the sphere and to stabilize the trees and the forest around the sphere. The web of rope is strong and stretchy so it allows the trees to continue to move and also lets the sphere move around. That way if something impacts the sphere, the shape dissipates the force. Also, the movement of the sphere absorbs the energy over a much longer period of time so the peak load is not as high and won’t fracture the shell.

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April Garbuz is a TalkingScience summer intern and a junior at Wilton High School. She loves science, debating, acting, and swimming. Ultimately, she'd like to be a research scientist.

 

About April Garbuz

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