Dome
Prototype: Phase 2

**Project begins: Decem****ber 2019**

My idea for the next project is a partial dome, sort of like an overturned satellite dish. I needed something for the garden to diffuse sunlight that was burning the vegetables during summer. After Project Sunburn failed to do this, I knew that I needed something far sturdier for a shade structure. Here is a rendering that I put together with Blender with what I had in mind.

The all wooden construction of the prototype turned out to be much too fragile to support the weight of the dome and survive the weather. This was likely due to the use of 1x1/2 strips with glue and screws, which evidently is woefully inadequate for the structural support that domes require. The weakest points were the hubs where panel corners connected. Therefore, I designed a hub jig from which to fabricate steel joints, then connect the hubs with 2x3 struts. The* *horizontal angles are
all 120º to allow formation of hexagonal shapes. The
vertical angle is 15º to allow for a 2V frequency. This
allows fewer hub joints but forms a blockier-appearing
dome.

This is the first hub joint fabricated. Turned out to be extremely sturdy, allowing me to stand on the top of the joint without collapsing.

There were several problems with this design. The first and most fatal to the design was the 120º angles. I mistakenly assumed that 120º made sense for what appeared to be interlocking hexagons. The truth is that this dome design consists of interlocking pentagons, which didn't appear obvious until I completed several hubs and nothing fit correctly. Therefore, I needed to change the hub horizontal angles to 108º with two 126º angles. This meant cutting all of the 120º hubs apart to reuse in the updated angle design. The updated hub hub jig allowed me to use the pieces salvaged from the previous design to use the correct angles.

The second problem was the 2V frequency.

In dome design, a 2V frequency is great for smaller domes and requires fewer hub joints. However, it wasn't going to work out for a 25-foot diameter cover for the garden. A 3V frequency with a 10º vertical angle will require more hub joints but will produce a flatter, more rounded look.

Therefore, I had to create a new hub jig with a 10º vertical angle to accommodate a 3V frequency.

[image]

The third problem was the gauge of the metal that I was using the fabricate. The thinnest that I could find in 2-inch with bar at my local steel supply was 3/16 inches thick. While it made for a very sturdy hub joint, it was probably overkill and was definitely harder to work with. This is a shot of my drill press setup where I had to drill the bolt holes.

I solved this issue by finding a steel supply place online that had bar steel as thin as 1/16 inch. I ended up going with 1/8 and ordered about 60 feet. I also got the three-inch width to strengthen strut connections with two bolts instead of one.

Throughout this process, there was also the issue of figuring out the length of the struts to achieve a desired dome diameter. I solved this problem by measuring the distance between the center of the pentagon and the edge of a connected hexagon, using an arbitrary strut length. After that, it was just a matter of calculating ratios.

[calculation]

My idea for the next project is a partial dome, sort of like an overturned satellite dish. I needed something for the garden to diffuse sunlight that was burning the vegetables during summer. After Project Sunburn failed to do this, I knew that I needed something far sturdier for a shade structure. Here is a rendering that I put together with Blender with what I had in mind.

The all wooden construction of the prototype turned out to be much too fragile to support the weight of the dome and survive the weather. This was likely due to the use of 1x1/2 strips with glue and screws, which evidently is woefully inadequate for the structural support that domes require. The weakest points were the hubs where panel corners connected. Therefore, I designed a hub jig from which to fabricate steel joints, then connect the hubs with 2x3 struts. The

This is the first hub joint fabricated. Turned out to be extremely sturdy, allowing me to stand on the top of the joint without collapsing.

There were several problems with this design. The first and most fatal to the design was the 120º angles. I mistakenly assumed that 120º made sense for what appeared to be interlocking hexagons. The truth is that this dome design consists of interlocking pentagons, which didn't appear obvious until I completed several hubs and nothing fit correctly. Therefore, I needed to change the hub horizontal angles to 108º with two 126º angles. This meant cutting all of the 120º hubs apart to reuse in the updated angle design. The updated hub hub jig allowed me to use the pieces salvaged from the previous design to use the correct angles.

The second problem was the 2V frequency.

- The mockup that I built in the garden turned out to be far too unwieldy for a 2V design
- The 15º vertical angle of a 2V frequency makes a shape more like a salad bowl than an inverted satellite dish
- The hexagonal sections were too big to make trellis inserts for

In dome design, a 2V frequency is great for smaller domes and requires fewer hub joints. However, it wasn't going to work out for a 25-foot diameter cover for the garden. A 3V frequency with a 10º vertical angle will require more hub joints but will produce a flatter, more rounded look.

Therefore, I had to create a new hub jig with a 10º vertical angle to accommodate a 3V frequency.

[image]

The third problem was the gauge of the metal that I was using the fabricate. The thinnest that I could find in 2-inch with bar at my local steel supply was 3/16 inches thick. While it made for a very sturdy hub joint, it was probably overkill and was definitely harder to work with. This is a shot of my drill press setup where I had to drill the bolt holes.

I solved this issue by finding a steel supply place online that had bar steel as thin as 1/16 inch. I ended up going with 1/8 and ordered about 60 feet. I also got the three-inch width to strengthen strut connections with two bolts instead of one.

Throughout this process, there was also the issue of figuring out the length of the struts to achieve a desired dome diameter. I solved this problem by measuring the distance between the center of the pentagon and the edge of a connected hexagon, using an arbitrary strut length. After that, it was just a matter of calculating ratios.

[calculation]