Designing a Dome Structure

When we first started planning this project we really wanted to build a geodesic dome to maximize the uninterrupted space, to take advantage of the snow shedding effect of a round structure, and because geodesic domes are pretty awesome.

We researched a few techniques for constructing the dome hubs. We had several factors to consider when deciding on a building technique such as snow load, which would be best for a permanent structure, ease of construction, and cost. See below for our breakdown results.

Jointed

We originally considered this frame type when our design was a lot smaller and less permanent. My brother and I are fairly competent wood workers and we thought this would look pretty.

Pros:

  • Fairly fast to put together if you can handle creating the angles
  • Looks nice.
  • Able to pre-fit and modify during installation.

Cons:

  • The joint is not as strong as other methods due to needing to insert screws or pegs at odd angles.
  • Requires compound angles in each strut.
  • Triangle faces are not flat.

Our experience:

We did not actually try this out because we planned on a larger dome than I was comfortable creating with joints.

Frame type (good karma)

The frame type looks super complex from the rendering above, but it’s pretty straight forward when you break the process down. The idea is that you create every triangle “frame” individually, then screw them to each other at assembly. If you’re interested in this method I highly suggest purchasing plans from http://geo-dome.co.uk/. The owner vastly simplifies the process of creating frames and is quick to return emails and answer questions.

Pros:

  • Allows you to pre-fabricate the entire dome offsite.
  • Assembly is quick at around 3-4 hours.
  • The joints look nice and are much stronger than the plain jointed technique because each frame is screwed to each other, so the whole frame shares the hub load.
  • Each frame is flat which makes installing panels super easy.

Cons:

  • Frames are extremely time consuming to make.
  • Multiple angles in each strut.
  • Frames have high tolerances for getting each frame exactly the same. however the plans I mentioned above offer some great techniques to help with this.
  • Does not have a flat base.
  • You do not have any wiggle room to fit the frames at assembly if they’re a little off (this is a big one).

Our experience:

We actually built our first dome using this technique. It took us well over 100 hours to create/paint all the frames and we test fit all the triangles into their respective hexagons/pentagons as we went.

Unfortunately at assembly, we found that some of the frames had warped a fraction of a degree on a few angles and this propagated to over an inch gap in multiple spots, so we had to scrap it.

This was mostly our fault. The plan’s techniques specified using 2×2 lumber and we weren’t confident that would support a full roof plus snow load for a 20 foot dome, so we used 2×4’s that we split down the middle, which tended to increase warping. Our lumber was probably not all the exact same dimensions as well. We probably would have had better luck if we had planed all the boards before we cut our angles.

See below for a few images of myself, my wife Lindsay, and my brother Adam helping out with the frame building process and my uncle Dean who helped me with the assembly.

Piped

The piped method involves the struts coming together around a circular hub. The struts are then attached to the hub usually with a type of bracket or you can screw directly to the hub for smaller domes. This works because domes are primarily compression structures, there is very little horizontal force on each individual strut, so the brackets are there to keep everything centered and make it easier to attach the strut.

Pros:

  • No compound angles in the struts.
  • Very flexible and forgiving at assembly.
  • Easy to scale brackets according to the load required. Use anything from wood, pvc, or metal for hubs.
  • Can have a flat base.

Cons:

  • Can be more pricey depending on what you use for the hubs/brackets.
  • Triangle faces are not flat

Our experience:

This is the technique that we decided to use for our permanent structure. It provided the easiest and most forgiving construction/assembly and according to my amateur engineering figures, the more straight forward way to estimate potential load strength.

We used schedule 40 steel pipe for the base hubs where we attached it to the base building, schedule 80 pvc for the other hubs, and high tensile steel strap for our brackets.

Materials and Numbers

A common question for dome builds regardless of the construction method is what is the load capacity of the dome. This isn’t as simple as a standard truss roof where you have load and strength tables to consult. Domes are self supporting structures that have no supports like columns or beams.

***FULL DISCLOSURE*** I am not a structural engineer, this is purely my reasoning to convince myself that I over engineered the structure.

Let’s forget about lateral wind loads and assume that all weight on the dome will be evenly distributed (in most cases this will be mostly true due to the aerodynamic shape). A dome is a compression structure with some struts in tension.

The structure gets stronger as the weight is dispersed to more and more hubs with the highest point of failure being the single hub at the very top. However, this is a moot point because it is doubtful that the dome will ever experience a strong point load on the top hub.

That said, I’m comfortable with assuming some things about the load capacity by breaking down the strength of the materials used.

  • 2×4 pine stud – compression strength: 1,300 lbs
  • 4″ schedule 80 pvc pipe – crush strength: 240 psi
    • The 2×4 footprint on the pvc hub can support around 1,300 lbs
  • 4″ schedule 40 steel pipe – 30,000 psi
  • high tensile steel strap – tensile strength: 1,250 lbs
    • This is the steel strapping used to secure freight crates.

So considering these numbers the top pentagon alone should be able to support around a 6000 lb dispersed load at the high end.

Dome Plans

I did not use a kit or pre-drawn plans for the final structure. I ended up using a pretty slick online tool to configure and generate the cut list and assembly diagram specific for my project. Once I figured out how to read everything, it was a piece of cake to generate what I needed.

Using the dome generator

Most of these options are self explanatory and you can see the effects on the generated model to the right. Be aware that some of these values are for meters and others are for millimeters.

Once you have your dome set, try scrolling down the page to the diagram cut list. If you’re having trouble scrolling down, put your mouse in the bottom left corner. The ad and rendering block prevents you from scrolling.

Green C strut cut dimensions

The blue end angle is the angle that sits against the hub. If you set a 2×4 on it’s narrow edge, this diagram is looking down at the top of the board.

  • 0-1990 is the total length of the board in millimeters
  • 0-16 is the amount to cut in for the angle.
    • Measure 16mm from the end and draw a diagonal line from the bottom corner to your mark. The angle of that line should be 80 degrees.
  • 0-1974mm is your mark for the angle on the other side.
  • 82.8 degrees is the rotation of the resulting strut from the face of the assembled triangle.
    • Think of when you go to install plywood or polycarbonate on the outside, you will not have a flat surface to attach to. You’ll be screwing your sheeting into the strut at that 82.8 degree angle.
    • This number is not important for dome construction.
  • The circles with 1 and 5 on the ends is the vertex number that strut attaches to. More on this below.
Triangle face

The face diagram is going to be your diagram for cutting your plywood or polycarbonate.

Vertex diagram

This diagram specifies the angles between the various struts. Notice the undefined sections here. This tells us that this vertex is a base hub.

Assembly diagram

If you go back up to the configuration settings, you’ll see a grey button in the middle named schema. This will assign letters and numbers to your dome. I found this diagram to be the most useful at assembly rather than only using the the individual diagrams.

Wrapping it Up

There you have it. We didn’t use a kit and once you lay out all the details building a dome structure isn’t super complex, only an exercise in consistency. Feel free to leave a comment if I can clarify something a bit more or if you have any suggestions.

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