How to Build A 24 foot Geodesic Dome |
Well I learned a lot from my first geodesic dome. It was time to build another one, better, stronger, faster - or something. I decided it was mission critical to be able to hang up a hammock or two, because it was scorching last year. I considered a 2 frequency 20 foot diameter dome, or a 24 foot diameter dome like some friends had last year. These domes are strong enough to survive the desert, and are pretty simple to build.
If you do the math, you will see that for a 20 foot dome, there are 35 struts at 6.18 feet and 30 struts at 5.47 feet. Since EMT conduit comes in 10 foot sections, this is pretty wasteful. A 24 foot dome is even worse with 7.42 and 6.56 feet struts required. Another problem is that the 24 foot dome struts are too long to fit in my pickup truck bed.
So I decided to go with a 3 frequency 5/8 sphere, 24 foot diameter dome. This involves 165 struts, none longer than 4.95 feet. Using the the dome calculator, and specifying a radius of 12, yields A=4.183, B=4.842, C=4.948. Actually my dome is about a two inches smaller than 24 feet. The good news is very very little conduit is wasted. The bad news is there are lots and lots of struts to make, and the dome is pretty heavy. It should be a bit stronger than a 2 frequency dome, not that this is a big deal, as the 2 frequency domes are strong enough. Also hanging hammocks should be pretty easy. I bought 90 pieces of 3/4 inch EMT conduit, which isn't much bigger than the 65 pieces that would be required for a large 2 frequency dome. Another feature of a 5/8 sphere dome is you can use some of the pieces and make a 3/8 sphere dome, if you don't need lots of headroom.
The material of choice for domes is EMT tubing. It is reasonably cheap and readily available. In addition it is reasonably easy to flatten the ends of the tubing, which is needed in order to bolt the dome together. It is also coated with zinc in order to minimize rusting. I have considered using chrome-moly steel tubing, but I don't have a good source of it, and I am sure it would be quite difficult to flatten the ends of the tubing, and it would certainly cost much more than conduit. Some folks use PCV or wood. PVC certainly isn't as strong as steel, and I wouldn't consider hanging hammocks from a PVC dome. Steel is also recyclable, so I don't feel guilty in using it. Wood would cost quite a bit more, weigh more, be bulkier, and be harder to work with. So EMT conduit is the clear winner.
EMT conduit comes in different diameters. The bigger stuff is obviously stronger, as well as being heavier, more expensive, and harder to flatten. I bought a piece of 1 inch condiut as a test, and it was at least twice as hard to flatten as 3/4 inch conduit. 1/2 inch conduit was trivial to flatten. I bet 1 1/4 inch conduit would be quite hard to flatten without power tools or a monster press. I have seen several domes made of 1/2 inch conduit. They are strong enough to stand up to high winds. I don't feel comfortable hanging hammocks from them, and I certainly wouldn't climb up on one. When you hang a hammock it is best to attach it to the intersections of the dome, as that is the strongest part. When you climb, it is best to only touch the conduit near the intersections.
I decided to use 3/4 inch conduit for my domes, and it is reasonably cheap, strong, light, and easy to work with. I have hung many hammocks from my domes without any problems. If you are careful, you can climb a 3/4 inch conduit dome. A friend built a 12 foot 3 frequency dome out of 3/4 inch awning tubing which is slightly stronger than conduit. Because each piece is pretty short, all parts of the conduit are reasonably close to an intersection. Here is a picture of 4 adults, each weighing roughly 180lbs sitting on top of the dome. That is clearly a quite strong dome. For Thunderdome, where lots of people will be climbing on the dome, I would use 1 inch, or perhaps 1 1/4 inch conduit. If I was building a big dome with really long sections (not the 5 foot or shorter sections of my 24 foot dome), I would consider using 1 inch conduit. I have seen a 24 foot 2 frequency dome which uses 7.5 foot long pieces of conduit, and it was strong enough with 3/4 inch conduit. It had several hammocks attached to its intersections. I think that is the longest section I would make out of 3/4 inch conduit.
For my first dome, the real mastermind of dome construction was my friend, who was unavailable to help. So I had to be the mastermind. Unfortunately I didn't have a camera to capture the construction process. I bought a 1 ton arbor press, a small shop drill press, a reciprocating saw, a drill bit sharpener and some other random stuff from Harbor Freight, a supplier of inexpensive tools. One very useful tool was a deburring bit which I got from Home Depot.
My friend and I used a Sawzall to cut the conduit for the first dome. It was pretty easy. It was more difficult to cut the conduit for my second dome. Perhaps it was me not being familiar with the Harbor Freight saw, or perhaps it is a junky saw that doesn't dampen vibration. In any event, it took a while to cut the conduit.
The next step was to flatten the ends. I used a vice for the first dome, which worked well, but was slow, and gave me quite a work out. A friend Howard had build a 3 frequency 20 foot dome, see Home Dome for the desert 2000, as well as Desert Nose, and he used a 1 ton arbor press for everything. I decided if it was robust enough for 320 nose struts, it was robust for me. He also has helped folks build a few other domes with it...
For the first dome, I crushed 2" at the end of each strut. It was a lot of work, and overkill. This time I decided to crush 1.5" instead. I didn't permanently mount the arbor press, instead I made a temporary mount. I mounted the arbor press to a 2x6, and then clamped the 2x6 to a stable platform. It crushed the ends quickly. I had to use a 1/4 inch thick plate for the bottom of the press, as well as a block of wood as a backstop in order to make sure the flattened part 1.5" long. One thing I learned was to crush the tube at the 1.5 inch mark first, because if I crushed the end first, it was very tough to crush the tube inboard of the first crush. I also managed to split a few tubes. It is critical to mark the seam of the tube on the outside, and place it around 45 degrees so it won't split. The vice was much more forgiving, and I didn't split any ends using it. If I screwed up with the arbor press, I was usually able to clean up the flattened end with my big vice.
Next I had to drill all the ends. I had gone through a few bits for my 2 frequency dome, so I bought extra bits as well as a drill bit sharpener. I made a jig to locate the hole center 3/4" from the end of the conduit. It was not nearly as nice as the one for the previous dome, but I didn't have a table saw to cut a V groove in a piece of wood. Instead I used some C clamps on either side of a 1x4. I had to run the drill at the slowest speed, and drill slowly or the motor would stall. I also got a 15 inch fan and directed it on the drill press motor to help cool it. I really should get a real drill press if I do more of this stuff, but the press I got was cheap, and will take care of most of my needs. I drilled the holes with no pauses. I think pausing for 5 minutes every 15 minutes would help cool the drill motor.
Next, I drilled a hole in the 1x4 at the appropriate spot for each strut, and drilled the second hole. I only sharpened my drill bit once. I was surprised at that the drill bit stayed so sharp, unlike my previous dome construction. I am sure that the titanium-nitrite coating on the drill bit helped quite a bit, as well as the slow drill speed. For the first dome, I had to sharpen the bit several times. For the second dome, with about 3 times as many holes, I only sharpened the drill once. So the bit lasted roughly 5 times as long between sharpenings. Titanium-nitride drill bit sets are really inexpensive at Harbor-Freight.
Next I needed to bend the ends of the struts. The exact angle is bend A's 10 degrees, B's and C's 12 degrees on each end (which can be found at dome tips). I decided that 10 degrees was close enough. My friend figured out that I could use my vice and the edge of the door (once the vice was positioned in the right spot) to put in a 10 degree bend. I used a hand drill and the countersink / deburring bit to clean the ends of the holes. It was much, much faster than cleaning the holes with a file and using a 1/2 inch drill bit and twisting it by hand.
In order to make it easy to assemble, I considered using electrical tape like I did for my 16 foot dome. I decided to use a permanent marker (Sharpie) instead. I labeled each intersection with a letter starting with 'A'. I also labeled each intersection with the level starting with '1'. So the first strut said 1A on one end and 1B the other end. For the second level I used '2' and the same letters. This way, all you do is match the number and letter to put each end together. I marked the symbols on the inside as well as outside of each tube, so it was obvious from inside or outside the dome, how it went together.
Here is the second layer is completed, and 2/3 of the third layer is completed.
Here the third layer is completed, and the fourth layer is just starting.
Here the dome is almost completed, and one of the 9 foot high rectangular bottom tarps is being installed.
To attach the turbine fan to the top of the dome, I bought 10 U bolts, with a spacing of 3/4 inch inside the U. I centered the fan on the top 5 pieces of conduit, and attached the 5 perimeter pieces of conduit, so each piece of conduit was exactly positioned. I marked on the sheet metal flat plate of the turbine fan where the conduit went, and drilled holes near the center and edge of the flat plate for two U bolts. That is 4 holes per pole, a total of 20 holes. For the final dome assembly, I bolted the turbine fan to the 5 central poles, and tightened down the 20 nuts.
Here is the final bolt tightening. The center 5 struts were connected together along with the turbine fan, and were lifted up as a whole. The tarp with a precut hole was already in place. The center five bolts were already attached and tightened down. When the center assembly was installed, we just placed a second washer and nut after the main washer and nut. Note the 16 foot ladder, bent in the form of an L, with the main ladder being 12 feet high, and the short arm of the L being 4 feet long.
Here is the completed 24 foot 5/8 sphere dome in the desert 2001. The top of one of the rectangular bottom tarps was lowered to form a vent to let air in. On the other side (out of the wind), is a larger opening which was used both as a door and as an air vent. The rectangular tarps could be easily moved to completely seal the dome, or be vented at the top or bottom. It was dustier than I expected. I had planned to have about 2 feet of air gap around the entire dome bottom, which would have provided excellent venting. As it was, it was sometimes a bit warmer inside the dome than outside, but it was always much less dusty.
The turbine fan did a great job of sucking air out of the top of the dome. I chose it because it was reasonably weather-tight, and would encourage ventilation. It was almost always spinning, even when the air seemed still to me. When it got really windy, the turbine fan was undamaged.
Also visible are some of the rebar stakes I used to secure the dome. I used 5/8 rebar, not for strength, but for ease of hammering. I covered the ends with commercial rebar covers, which look like orange mushrooms. See securing a dome below.
Well, lots of folks found me and thanked me for this web page
helping them build their dream dome. People kept on walking up to
me, and recognizing me, my dome, and my truck. I learned a few
useful things in the desert this year.
If you have comments or suggestions, Email me at turbo-www@weasel.com
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