Greenhouses forum: Paul's geodesic dome, part 2: Frame construction

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Imagepsa
Feb 9, 2010 1:41 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
Part 1: Design and Preparation
Part 2: Frame construction
Part 3: Plastic covering
Part 4: Door, Ventilation, and more
_________

Here's the first hexagon that I put together. I originally planned to build from the bottom up, but now I'm thinking it might make more sense to build from the top down. Fastening the strut ends is difficult to do high in the air.

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Imagepsa
Feb 9, 2010 1:45 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
Here's the bottom layer of struts plus a few of the next level. I'm going with a hybrid assembly with the top pieces preconstructed and the rest built from the bottom up.

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Imagepsa
Feb 9, 2010 1:48 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
My DW took an interest in the assembly and has been diagramming our progress. It should be noted that assembling this structure absolutely requires more than one person, since someone has to help hold and brace the pieces while they are being put in place.

I'm constructing small pieces of the dome a few struts at a time on the ground and then lifting them into place to limit the amount of work that needs to be done in the air. The outside of all of the straps is attached before the pieces go up so that they only need to be screwed from the inside.

This open hexagon is where the door will be eventually.

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Imagepsa
Feb 9, 2010 1:50 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
We lifted one of the large hexagons up on top with the help of my father-in-law (and my indispensable DW, of course). Then we added struts in around it to hold it securely in place.

It got a bit dark along the way...

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Imagepsa
Feb 9, 2010 1:53 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
From the other side of the dome you can see the 2x4s that we used to prop up the pieces as we connected them in. At this point none of these are technically required anymore, since the dome can hold itself up, but we left them in place for greater stability (especially if we encounter very much wind) until we can finish the other side.

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Imagepsa
Feb 9, 2010 1:57 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
We had sustained winds to 40mph and gusts to 55mph the next day. The structure didn't move at all. Although I have faith in the strength of the final structure I was worried when I woke up this morning. Nice to see that even a portion like this is strong.

It's time to lift the other patch of the top of the dome up and fasten it in place.

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Imagepsa
Feb 9, 2010 2:00 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
First I added all of the struts to support the piece and everything on the sides which didn't directly connect to it. At this point the constructed portion was basically symetrical, except for the door and one dangling strut above it.

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Imagepsa
Feb 9, 2010 2:05 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
Two of us lifted the double hexagon into place on ladders while two others braced it with long 2x4s in the hubs. We then fastened the bottom ends as we were able to line them up. Finally, we added two struts at the top of the dome to connect it to the other side. At this point the only things left to add were the connecting struts on either side of the piece we had just added and the 5 central struts of the pentagon on top.

We had to push and pull the parts of the structure to make each connection line up properly. Sometimes this required bracing various points with 2x4s and sometimes it simply meant that someone heaved on it while it was fastened. Gravity made some of the portions sag, while the difficulty of maintaining perfect positioning of the base was responsible for other misalignments. We managed to get everything into place though, and each section we added gave further rigidity to the structure.

It got dark while we were putting these in place, and started to sprinkle a little bit.

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Imagepsa
Feb 9, 2010 2:08 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
We next went back and added the rest of the side struts, leaving only the open pentagon on top.

Here's a closeup of one of the hub connections, showing how everything is attached. The first screw is attached closer to the hub, at an angle. I actually insert the screw at a steep angle and then pry it back to tension the strap a bit. When the screw goes all the way in, it tensions the strap even more, taking all of the slack out of it and pulling the joint tightly together. The second screw is then placed further out to provide greater strength. This being cedar, the wood is quite soft, so two connections is a must. Also the second screw prevents the connection from rotating, adding more stability. As I mentioned above, these are 22 gauge straps, so they're a bit stiff to work with, but strong. My DW actually climbed up onto the first tier at one point when we were pushing the connections into place, and I can attest that the joints are better able to support weight than the struts themselves.

She's the one that said that I should include this closeup photo, but she didn't know that I was going to mention her climbing up on the structure. Smiling

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Imagepsa
Feb 9, 2010 2:13 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
Here's a picture of the first screw being put in. Note that the strap itself is bent out from the wood, so that the point of entry of the screw into the wood is farther away from the center than the hole in the strap itself. This is what pulls the strap tighter when the screw goes in.

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Imagepsa
Feb 9, 2010 2:22 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
We assembled the five spokes of the pentagon on the ground and then I carried it up the ladder and poked it through the hole to rest a bit off center on top. It took a bit of work to get all of the spokes butting up against all of the right hubs, but once they were in place everything fit perfectly and I was able to screw them all in without pushing or pulling on anything. This was a good thing because by this time the rain was picking up and bringing a bit of wind along with it.

We were all so wet and tired by the time we were done that we took everything inside and turned out the lights without even getting a picture of the finished dome. Here I am putting in the last screws.

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Imagepsa
Feb 9, 2010 2:27 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
So here's the frame the next day, complete except for the door.

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Imagepsa
Feb 9, 2010 2:34 AM CST
Name: Paul
Eastern WA
DG: tropicalaria
Glazing is next in Part 3: Plastic covering
http://cubits.org/midcolumbiagardens/thread/view/3098/

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