Most geodesic domes you see are variations of 'icosahedrons'. All the sonostar domes are derived from icsosahedrons and they all have 6 pentagons built into a design of primarily hexagons that help contribute to the geometric ‘roundness’. If you were given the problem of making a sphere out of nothing but triangles, you would find that the smaller the triangles, the more spherical your model would look. Given the same sized sphere, the smaller the triangles, the more of them there would be. That’s the basic idea around frequency. An easy way to determine the frequency of a dome is to count the poles/struts that connect the centers of the pentagons. So if you start at the top of a geodesic dome and go down toward the next pentagon, you can count the poles from center to center and that will tell you what the frequency is. As the frequency number goes higher, the complexity and number of connectors and struts in the dome also increases. So in a frequency 3 dome, there are three poles between the centers of the pentagons. In a frequency 6 dome, there are six.
The hubs are designed to fit snugly, but it will only be a matter of time before the structure starts to work its way apart. Gluing the joints does preclude taking the dome apart and rebuilding it somewhere else. Bolting it doesn’t take that much time, and it definitely won’t work it’s way apart. You could also use cotter keys, or zip ties, or any other manner of connector, but if there’s going to be people around it, don’t risk it coming apart.
Can I attach metal poles to Sonostar Geohubs?
We’re aware that many users have used steel, aluminum, wood, bamboo, and other non-pvc materials. The hubs were designed to be used with schedule 40 PVC, and they fit perfectly with it. However, we also know there’s a lot of art applications that require alternative materials, and if you use nuts & bolts, you can use anything that fits into the hub - metal, bamboo, wood, aluminum, etc.
We started with ½” hubs, and then we started getting a lot of requests for larger sizes. Several years ago, we brought out the 'Megahubs', that accommodate 1" PVC on the inside of the hub arms, and 1 ½”PVC on the outside of the hub arms. These allow for much heavier weight loads on the domes.
Is 1/2" the size of the inside or outside diameter?
Good question. As for the hubs, it's neither. PVC pipe is measured based on the inside diameter, because they use it to measure the air or water volume that is passed through. The size of the PVC pipe at your local hardware store goes by that inside diameter measurement. So when we say the hubs are ½”, it means that ½” is the inside diameter of the pipe that is used with the hubs. The outside diameter of the pipe is the critical size - and it's .84", and that's what the inside diameter of the hubs are expecting. This diameter of pipe is commonly available at hardware stores and is sold as "Schedule 40", and it's usually found in the plumbing or irrigation sections. Always try to buy the longest length they sell - ie. 20'.
All is not lost. One of the really great things about working with PVC pipe is that it's easy to cut and glue. So in this case, if the strut was too long, cut it someplace in the middle and measuring carefully, figure out how long it needs to be using a female/female 1/2" connector from the hardware store. Once you glue it, it will be just as strong as the rest of the struts, only with another joint in the middle. If you put too short a strut in place, you'll need to add length, so you may need to use two female/female connectors to splice a piece into the middle.
The first time you build a dome will be the longest it will ever take. That’s because the concepts are new, and inevitably you’re going to mess up and put the wrong strut in the wrong place, and you’ll have to go back and fix it, etc. The cutting of the PVC will take a couple of hours. You can make it very efficient by using a table saw or a cutoff saw. Once the PVC poles are cut and labeled, one person can build a frequency 2 dome in about 2 hours. A frequency 3 dome is going to take about 4 hours, and a frequency 4 dome will take about 8 hours. Truly, the more people the better, and if one of them knows what they’re doing, and can manage the others, it can cut the time in half. For instance, in the video on the 'Builds' page, it took a half dozen people 8 hours to build a 25' diameter frequency 6 dome, but some people were definitely working harder than others.
We're glad you’re thinking about this before you build your dome. There’s a lot of alternatives, based mostly on the intended use. The easiest is to use an army surplus parachute. The hardest is to individually cut triangles out of wood, polycarbonate, Styrofoam, or sheet metal. If you order a parachute, make sure it will cover all the way to the ground, or at least close enough to the ground for what you want. There are also templates for covers, where you plug in your dome diameter and it will measure out the ‘gores’ (pie-type sections) that make up your cover.
This obviously depends on the size of the dome and the frequency. PVC is strong and lightweight, but get enough of it and it adds up. A ½” 2V dome of 10’ diameter is only going to weigh about 30 lbs. A 26’ 3V 5/8 megahub dome with 1 ½” PVC struts will weigh over 400 lbs.
The 1/2" PVC is not strong enough for climbing. The megahubs are strong enough, but for insurance and liability purposes, we aren’t going to advertise that. We can't stop anyone from using their dome as a climbing dome, but we want to go on record stating that it isn't their intended use.
There’s a certain logic to this, but if you’re building a dome of 8’ radius or greater, you’re going to get to a point where you have to lift up the base to fit on the bottom rows. The torque on the joints could get extreme, and that could cause the joints to fail. The cracked hubs would have to be replaced, and if you only had the minimum quantity to begin with, you’d have to order more and wait for them to come. Just start at the bottom and work up. You won’t be sorry.
The research I’ve seen suggests the optimum temperature range of PVC pipe is 0 degrees F to 140F. Above 140 F, you start to see deformation based on softening, and below 0 F it becomes less impact resistant. Since Schedule 40 PVC was developed to be used under high pressure, that’s what the research was investigating. On a practical level, we’ve sold
domes in Iceland, where it gets really cold, and I haven’t heard of any issues with the PVC getting brittle or cracking at low temperatures.
By the straight math, the angles don’t work. However, depending on the size of the 1V dome you’re building, you could get away with it because of the inherent flex PVC provides.
The 1 ½” goes around the hub like a sleeve, and the 1” goes inside the hub arm. They’re dual purpose. The hubs are designed for Schedule 40 specifications.
The domes are scalable. We have some standard sizes listed with every configuration, but within reason, you can make the domes any diameter you want. The height and floor area will be dependent on the diameter.
We use pan head bolts, because they’re tapered and they fit right into the molded holes in the hubs.
The mathematical angles actually vary according to the frequency and according to whether it is a 4, 5, or 6-star hub.
The tops of the hubs are flat, but each arm tapers off at 12 degrees.
Using the 3V dome as an example, there are 15 hubs around the base, forming a circle.
15 hubs X 2 arms X 12 degrees per arm = 360 degrees. Exactly what you’d expect for a circle.
But there’s fewer degrees on a 2V and many more degrees on a 4V, 5V or 6V. How does that work?
The answer lies in the flexibility of the PVC. If you make a dome out of rigid steel, there is no tolerance to play with. When you use PVC pipe, there is a great deal of tolerance, and the angles provided in the hubs provides plenty of curvature to start the turns. 12 degrees is actually just a mid-angle number that works well in the broadest range of applications.
The hubs were designed to be used with Schedule 40 PVC pipe, but we know that lots of people have used different materials for struts. The important consideration is the inside/outside diameters. Schedule 40 pipe is measured by its inside diameter, so that users can calculate air or fluid volume through them. So ½” Schedule 40 pipe actually has an outside diameter of .84”. Similarly, 1” pipe and 1 ½” pipe have those corresponding inside diameters. Our mega hubs are designed to accommodate an outside diameter of 1.33” on the inside of the hub arms. So you need to get the specs on your EMT and find out if you’ll be using the inside or outside diameter of the hubs.
Another consideration is the lack of flexibility of the EMT. Because of the flexibility of PVC, we can get away with all 12 degree angles, even though the pure math says the angles should range from 9 degrees to 14 degrees. If your dome is big – like 20’ diameter, it won’t be as much of an issue. If your dome is smaller than that, you’re going to have a challenge getting the pipe into the hubs.
For the Mega Domes, we are aware of many people using polycarbonate panels, screwed right into the struts and/or hubs. Glass would be a little tricky, unless you were able to mount a track. Wood panels or other sheet materials would be easy to cut and affix.
The life expectancy for these covers are about 5 years.