Dome Prototype: Phase 2

Project begins: December 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.

dome penta
                      center

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 120 to allow formation of hexagonal shapes. The vertical angle is 15 to allow for a 2V frequency. This allows fewer hub joints but forms a blockier-appearing dome.

Dome hub first attempt

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.

home hub completed prototype

There were several problems with this design. The first and most fatal to the design was the 120 angles. I mistakenly assumed that 120 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 108 with two 126 angles. This meant cutting all of the
120 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.

updated hub angles

The second problem was the 2V frequency.

garden pentacenter mockup

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 10 vertical angle will require more hub joints but will produce a flatter, more rounded look.

2V frequency
3V frequency

Therefore, I had to create a new hub jig with a
10 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.

drill press setup 

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]