If you're looking to build a geodesic dome there's a few things to you'll need to consider. It's not as straightforward as a normal building, as the geometry is a little more complicated.
However the upshoot of which is that your material costs are driven down and it allows to build large structures with relative small costs, compared to more traditional styles of construction.
Also they can be errected in a matter of days not weeks and they are extremely robust and look damn cool!
Just a note, this is my first instructable and so feel free to ask any questions. If you feel I've glossed over anything or you need help in working anything out or need more photo's? Leave a message and I'll update it gladly. (Also I am working on a minature 3d printable version and I'll upload the stl files over the coming week or so)
Step 1: Choosing a Style of Dome
Basically to design your framework you need to have an idea of what kind of dome you're looking to build. So here's a few examples and how to work it out. (I know it's the boring theory side of things, but it's imperative that you get this right or you could run the risk of having some seriously costly problems later on in your build)
All domes start from one of the 5 platonic solids, in short equalateral 3D shapes like a cube or a dodecahedron. I've drawn a few quick examples. Most common form you will have seen is a dome based from an icosahedron. (a twenty sided shaped comprised of triangles) and there is a reason for this being the most common, I will get to later.
Now an Icosahedron albeit strong, isn't a smooth and elegant looking shape in fact it's quite chunky and ugly. So to smooth it out and give it that spherical appearance, you need to find the mid points between the triangles and divide them up equally. (as illustrated) This technique works for all of the platonic solids, and to find how far they need to protude, use the same radius accross the entire strucutre.
The more you do this, the more rounded the structure will become. However the more sides you put in the shape, the more it's going to cost you in materials, fixings and time to build. So the trick is to find a balance between looks and cost depending on your budget.
Step 2: Eliptoids
One thing to note;
In certain circumstance you may not want a perfectly circular dome, the land you're building on may not permit it or even if it's a stylistic design choice, you can alter the shape to suit your needs but make sure it's proportionate. This will matain the structural integrity.
I would only recommend doing so if you have good CAD skills though, and again be aware that this is something that can increase the costs of the build and the amount of hubs you'll need to make.
This is one thing I learned with the dome I built, it's that it's not a semi sphere.
I cut the dome off a 3/8ths of a sphere. This meant there was a lot less hubs needed to build the shape and this both saved time and money and only shrank the pitch of the building slightly.
It's still 9m tall and 20m accross but required a lot less work and money.
Step 3: Struts
I said before there's a reason for most domes you have seen, are derived from an icosahedron. The reason is that the triangles join together to form hexagons. Hexagons are both strong and use minimal materials. Not as strong as triangles but they use a lot less material. So configurations that form hexagons are usually preferable. Icosahedrons divided up do just this.
Also they wrap around a sphere, leaving few equal pentaganol voids. This is the reason most domes built are using this approach. Efficiency. It minimises the amount of connecting hubs you need.
I built my struts from wood. I used glulams that were bent to the radius of the dome and cut down to size. I notched out 5mm thick at the ends to fix a steel plate with screws. But I'll cover this in the hub section.
Using curved struts meant, using a very simple dome configuration, it still gave a very round looking structure for very little cost. It does make the fixings at the hubs a little more tricky but not impossible. Also given that my dome was to be clad in canvas, the cover could be simply cut to size and laced together, and wrapped around a curved structure.
If you wanted to build your dome with windows or clad in wood, then I would reccomend using straight struts as then you can use flat peices of glass and it makes the connecting hubs easier to fabricate.However to get the whole stucture to look more curved, you'll need to divide up your platonic solid a few more times.
I am planning to build another, but next time it's going to be clad with turf and some windows. Think turbo hobbit hole! This will make it not just warm, and large but very cheap to build. Ideal for an eco home.
Step 4: Hubs
For my hubs I laser cut the shapes from a flat sheet of mild steel. This was suprisingly cheap and they were all cut and ready to go in a few hours.
All of the fixings were held together using grub screws. (essentially using compression to hold things in place). So the struts fit into the hubs and then using a socket, tighten up the grub screws. This is why I notched out a small slither from the struts to put in a steel plate. This meant when you tighten the screws they don't bed into your struts damaging them.
All the hubs interlocked and were strip welded at the joints. this took a couple of days but as they were all cut by a laser they were accurate and I knew that they fit when assembling the overall structure.
Then I had them all dipped in galvanise to protect them from rust and degradation. One thing to note if you are using galvanise account for it. It adds a few mm onto the thickness of your steel.
So I now had all the struts cut to size and varnished, all the hubs laser cut welded and galvanised.
Step 5: Canvas/ Cladding
The dome was to be clad, in canvas and to work this out I broke the shape down into a 2d net. I used these dimensions to then cut the canvas to size and then stitch together.
In the end I broke the whole skin into 5 peices; 4 walls and a cap. This meant it was a bit more managable and to put on we used a couple of lever hoists to pull it into position.
Once they were in position, they simply laced together.
Step 6: Errecting
Once all the componenets were fabricated, it was a case of putting each piece in like a big jigsaw or lego set. This took a bit of organisation having all pieces laid out in an order for easy access when needed. But all in, it took a bit of graft to hold the beams in place whilst fixing them, but the more than went in the stronger the structure became.
(Video is currently uploading)