Geodesic Dome & Hub Connector FAQ
What do you mean when you say ‘Frequency 4’ or ‘Frequency 6’ dome? What does the ‘frequency’ refer to?
Most geodesic domes you see are variations of 'icosahedrons'. All the sonostar domes are icsosahedrons and they all have 6 pentagons built into a design of primarily hexagons that help contribute to the geometric ‘roundness’. 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. For a more graphic description of frequency, go to http://geo-dome.co.uk/article.asp?uname=domefreq .
Wouldn’t it be smarter to start at the top and work down?
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 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.
Do I really need to glue the joints together? What if I want to take it apart and put it back together somewhere else?
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. An alternative would be to drill through each leg and hub and then use a threaded screw with a nut and washer to hold the pipes in place to the joints. That’s a lot of drilling and screwing, but if time isn’t an issue, it would solve the reusability issue. You could also use cotter keys, but you'd have to make sure you drilled all the holes in exactly the same place.
Can I attach metal poles to the Sonostar geohubs?
It's not recommended. The hubs were designed to be used with schedule 40 PVC, and the glue may not hold if you use metal conduit or metal pipe. Of course, if you use nuts & bolts, you can use anything that fits into the hub - metal, bamboo, wood, aluminum, etc.
Do the hardcore hubs come in any size other than ½”?
Yes. Last year we brought out the 'Megahubs', that accomodate 1" PVC on the inside of the hub arms, and 1 1/2" 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. It's neither. PVC pipe is measured based on the inside diameter, because that's how air or water volume is calculated. So 1/2" 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", or "Schedule 80" pipe, and it's usually found in the plumbing or irrigation sections. Always try to buy the longest length they sell - ie. 20'.
I messed up and glued the wrong size strut into a geohub and now I can't get it out. What should I do?
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.
How long does it take to build a dome, using the hardcore geohubs?
The cutting of the PVC will take longer than the assembly of the dome. Once the PVC poles are cut and labeled, one person can build a frequency 4 dome in less than 4 hours. It took a half dozen people 5 hours to build a 25' diameter frequency 6 dome, but some were working harder than others. Here's a timelapse record of that project:
Is there a recommended cover for the dome?
I’m 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.
What does a dome weigh when it’s done?
The hubs alone (for a Frequency 6 dome) weigh 74 pounds. The PVC poles’ weight will vary with length, but for a 12.5’ radius dome, it’s over 212 pounds.
Can my kids use the dome as a jungle gym type climber?
The 1/2" PVC is not strong enough for climbing. The megahubs are strong enough, but for insurance and liability purposes, we can't advertise that. We can't stop anyone from using their dome as a climbing dome, but we are very careful to state that it isn't their intended use.
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