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This article contains practical tips based on my own experience with domes and Natural Fiber Reinforced Concrete (NFRC), and as I gain more experience, Iíll report it to you here. (Update 4-29-12.)
First of all with domes. The calculated strut lengths are quite important. For a 32-foot dome, the A struts (the center of the pentagons,) should be 8 foot-8 15/16 inches long, there are 30 of them.
And the 35 B struts should be 9-foot-10 7/16 inches long. Those distances are very precise. Any deviation will prevent the dome from assembling properly. If youíre building a complex shape, such as a froggy, you actually measure the final distances, as I show in my article on building a froggy, and push the chicken wire armature into its proper place.
The dome on-the-other-hand, is built from struts laid out separately, and they must be precise. If you intend to use wooden struts, as shown, they may be 1x2s, however 2x4s are much stronger. Place the largest width toward the center of the dome, not flat with the skin.
Build a cardboard scale model first so you see how it works.
I built a 48-foot geodesic dome frame of 3-inch (the small end,) wooden fence rails, cut to length, drilled, and fitted with large bent I-bolts on the ends. It took 2 people to assemble it. It was beautiful, and I sold it to the Missoula County Fair Board. But it wasnít anchored to the ground, and when the wind blew, it ďwalkedĒ to a near-by fence, and tripped. So it fell down. Oh well, live and learn.
Iíve built 5 domes. Not the ones shown.
So first of all, the struts have to be of the correct length, within 1/16 of an inch, (1 mm.) They are made of triangles for strength. Do not cut any of the struts. Itís designed to use minimum materials. And if you make doors that must cut a strut, surround it with triangles to carry the load. Solid triangles are the best.
If youíre building a plastic-covered greenhouse, itís essentially, and generally legally, a tent that does not require a building permit in most places. But it must be strong enough so that it does not fall down. Build measuring sticks, so the struts will all be of the correct length, while preventing measuring mistakes.
The plastic tent dome shown, used wooden struts, using large ďplumberís tape,Ē metal with holes, cut to length and screwed to the wooden struts, as shown with at least 2 screws into the wood. The plastic was stapled to the struts, however, except for the bottom edge, that holds the whole plastic sheeting in place, it may leak. In that case you can plug the outside with black roofing tar, or put a string in place to catch the drops, then direct the drops into your growing pots on the inside. Plants need water, and this may help, depending on where you live.
The hub connectors must not twist. I built a dome with twisting hub connections, and when I climbed on it, they twisted, and the dome went out of shape. So make them solid. Thatís why 2 screws are used in the wood.
Another method may be to use ordinary, smaller plumberís tape for the tensile forces, and press concrete in the hub to take up the compression. I havenít used the pipe style, but you could fill the pipes with concrete, or just use strong pipes.
The flat plastic should be folded, not cut, to fit the domeís shape tightly.
On Yap, I built a 40-foot dome of bamboo struts about 3-inches in diameter. They were supposed to be soaked for a week in seawater to kill the bugs inside, but the bundles floated above the water in their centers, and so the ants that were left caused sawdust to rain down inside the dome from one strut.
I put nice windows on it too, but my health wouldnít allow me to use concrete as planned, so I switched to corrugated sheet metal. It was a beautiful dome, and I loved it. But a typhoon came along, and tore the window covers off, leaving the whole dome at risk for the next storm. So itís gone now. Live and learn. Itís amazing what 200 mph winds will do.
Next time, being at typhoon risk, Iíll make the windows so they can be closed tightly to fit the natural dome shape, during a storm.
In your project, there are 2 kinds of dome areas. The solid, NFRC, and the plastic windows. Ether way, it should be laid like shingles with the upper edge underneath the one above, and the lower edge, over the top of the one below, so it will shed rainwater properly, whether the triangles are cut, folded or NFRC.
If it rains a lot where you live, you can direct the rainwater into a holding tank, or cistern.
I built a 30-foot dome with rolled roofing paper covering it. It didnít leak, but it sure was ugly!
Have a sturdy latter. Once I was on top of a wooden dome, and slipped down, going faster and faster at each triangle, until I had to jump to the ground. I just missed a propane tank on the ground. The dome was only 24-foot, but it can be dangerous too, and surprisingly so. Be careful!
If youíre using NFRC, especially for the struts, practice with your test pieces first to make sure your technique is correct. In a strut, the fibrous material should be laid out to take the tensile forces correctly. That is, they should run parallel with the strut. You can include a small #3 rebar in the struts if you wish. Heat them with a torch to bend the curve, at the correct angle too, on the ends. Rebar only bends once, so if you donít heat them for the bend, they just might break.
The hub connecters should be bent at an 18į angle from the strut. So they can bolt together properly with a 72į bolts angle to the domeís center. (See Domebook II for further details.)
The NFRC is there to keep the rebar from bending, certainly itís strong enough to carry both the tensile and compression forces, or the NFRC, if you are good enough to use it right.
Our underground dome had struts around the windows 8x8 because it had to hold a million and a half pounds of dirt, but your little project shouldnít need anything bigger than a 1x2, if you form it right.
Take pictures and send them with your comments to me
. More later.
Thanks for your interest.