Ask Blair the Geodesic expert
You have question about geodesic domes, homes, or something related? Ask Our resident expert, Blair Wolfram.
Jonah asks: I’m submitting a grant proposal for burning man this year and I want to build a geodesic dome that will have a throne on top. With a heavyset person sitting in the throne, I can’t imagine there will be more than 600 lbs of pressure at the very top. I’m wondering if there is any issue of structural integrity on a 12’ diameter 3V dome, whether using aluminum, EMT or wood struts.
Blair: A 12′ diameter 3v dome is about 7′ tall at the center point. If you are going to use the inside of the dome you might consider a larger diameter so you will have more headroom.
A 12′ 3v dome will be very strong, but 3/4″ conduit will probably bend with 600 pounds concentrated at the apex. Use 1 3/8″ 16 Gauge top rail for chain link fence and you should be okay.
Marv L. asks: I want to build a forty foot diameter dome, using 2×4′s. What is the best hub to use to make it simple and strong?
Blair: I don’t know of any good hubs for 2″x4″ lumber.
Domerama: 2″ X 4″ lumber is almost never used in large domes (30 feet and more). For example, Blair’s line of hubs is made to accept 2″ X 8″ lumber. There are hub connectors that work with smaller dimensions of lumber, but they are not recommended for the 40-foot size you specified.
It just does not make sense to use small timber struts for the dome you wish to build; it would have to be a 6v, meaning 555 struts, and that is a lot of lumber to cut at both ends! However, there can be various hub solutions. See our connector hubs page.
Yona A. asks:
I have a question question that I haven’t been able to find an answer for on the internet. I am intending to build a 5v 40′ dome for a Burning Man art project. I’m trying to decide what piping to use, and there isn’t a lot of information out there about it.
I need the dome to be pretty strong. It will be supporting a significant amount of weight attached to the vertices (maybe 500-1000lbs total). In addition, I would like to put an Aerial rig from the top 5 vertices.
My plan (unless otherwise convinced) is to use 1″ EMT piping. Its cheap and seems fairly strong.
What would you recommend? It would be really cool if someone had a nice chart about what the maximum recommended struct length for each material is (EMT, steel pipe, aluminum).
Blair:
I recommend a couple different options from your ideas. From an engineering perspective, I’ve researched the lightest and least expensive steel tube dome frame to find the ultimate combination of cost and strength for domes. 1″ conduit isn’t structurally rated. It is designed as a protective sheathing for electrical wires. That being said, by far the most common dome frame at Burning Man is 3/4″ conduit made into higher than typical frequency domes. The higher frequency slightly offsets the wimpy frame material, but not really.
You won’t find an engineer to sign off on any conduit frame unless maybe the strut lengths are an impractical 24″ or less. The smallest diameter tube I have been able to have engineer stamped is 1 3/8″ 16 gauge galvanized steel tube, and if the spans are 52″ or less I can use readily available top rail for chain link fence. Otherwise structural steel tubing or mechanical tube is required.
A typical five frequency dome doesn’t have a horizontal or flat base. The configuration of the base is like a wave, rolling up and down, in and out of plane. I recommend a 4 frequency or a 6 frequency dome.
With stronger material you can use fewer struts, which saves time and money both in manufacturing and in on site assembly time. A 4 frequency half sphere has 250 pieces, and a 6 frequency sphere has 555 frame pieces.
Domerama: it is possible to get geodesic dome designs stamped by an engineer using tubing under 1 3/8″, but this depends on the use and time of year when the dome will be erected. Blair is a professional geodesic home builder of permanent structures, unlike event domes which require different specifications, for example, if used in winter regions with snow accumulation or in heavy wind areas.
Quin asks: I am attempting to build a geodesic dome model for school. I would like to make to make one with straws and have found helpful instructions on this website. The measurements for each straw are as follows:
30 X A struts: 8.25cm (or 3 1/4 inches) long
35 X B struts : 9.6cm (3 7/8) long
80 X C struts : 10.5cm (or 4 1/8 inches) long
20 X D struts : 11cm (or 4 3/8 inches) long
I would like to make mine smaller, if I were to subtract two centimeters from each measurement, would I still have a successful geodesic dome?
Blair: Taking the same 2 cm length off each strut will not make a successful dome. You should reduce the struts by an equal percentage. For example reducing each strut 25% will make a good dome (assuming the lengths you start with are good).
I’ll check your numbers if you tell me the chord factors you are using.
Andy M. asks: Where do they sell the hubs for a 30ft to 44ft dome? I live in Mexico so I could only bring a certain amount of hubs at a time. These would be for 2×6 3v or 4v depending the size. How much would they cost and about how many would I need?
Blair: Geodesic dome hub connectors are available through Dome Incorporated on this page:
http://domeincorporated.com/concrete-dome-kits-prices.html
Scroll down to ‘Hubs and Wood Frame Dome Prices’. The smallest lumber suitable for these hub sets measures 2″x8″
Julian C asks: I’m considering making a bucky dome with 6’ or 8’ poles? It will get covered with fabric. It’s not meant to be climbed on. To have a 40’ diameter bottom, how many poles would be needed? What structure would you recommend?
Blair: A 40′ diameter dome with a maximum strut length of 8′ will require 190 struts if less than a half sphere. A dome exactly half a sphere requires 250 struts. The easiest dome for you to manufacture yourself would be to use steel tubes.
OldStone50 asks: There seem to be lots of calculators for figuring strut length. Advice on the other strut dimensions seems in very short supply. Assuming wintry snow loads, say ~50 lbs/ft^2, and a 4V, 32 ft dia. half-dome, what’s enough if made from ordinary eastern soft wood (spruce/fir)? Or if using structural tubing? I’m not asking for a full, certified structural analysis, merely an empirical rule of thumb. Or is so little said because one wild guess is as good as another?
Blair: The only certified lumber I know of for structural loads must be either Southern Yellow Pine or Douglas Fir/ Larch. A 32′ diameter 4v is quite strong because the strut lengths are comparatively short. The skin needs to be 3/4″ plywood unless intermediate supports are used.
Kiegan L. asks: I am studying relationships of the bulk modulus of geodesic domes and spheres. I would like to ask you if you knew how many sides a 5V 3/8 Geodesic Dome has.
Also, for a 5V 5/8 dome, the volume is 5/8 of the volume of a sphere with the same radius. For the 6V domes, what is this ratio, is it still 5/8? Because it only says 6V.
Blair: One way to identify a dome is called the profile, or percentage of sphere. This is expressed as a fraction. The denominator or bottom half of the fraction is equal to the frequency times three.
A two frequency (2v) dome is expressed in 6th’s.A 3 frequency dome is expressed in 9th’s. 4 frequency is in 12th’s and a five frequency dome is identified in 15th’s.
You cannot have an improper fraction, so if you need exactly half a sphere, you need an even numbered frequency. A 4 frequency half dome is a 6/12 dome. If a 4 frequency dome was a complete sphere, it would be a 12/12th’s sphere, or 1 complete sphere. A 5v dome less than half sphere is a 7/15 and more than a half sphere is 8/15; complete sphere 15/15th’s.
Domerama: regarding Kiegan’s question about how many faces does a 5/8 dome have, see the chord charts page.
Bill O. asks: The notation under perimeter says it is the total of the lengths of the base struts but adding 10 x strut C plus 5 x strut B leaves me 4-5/9” different than what is shown for 18’ diameter.
Using diameter * pi gives the perimeter shown but why is that number greater than adding the lengths which I would think would be more accurate as the dome curves inward past 1/2 of the sphere.
Reason for asking: Trying to calculate and plan for concrete slab with short riser under where dome sits so that it is about 3” above rest of slab.
Blair: only two points of a foundation section actually lie on a circle or on the circumference around the foundation. The foundation consists of chords which cut a small slice off the circle. The total length of struts will be less than the total length of the circumference.
The red area identifies the space between a dome at its base and a circle
Parker O. asks: Blair, Has the classic way to connect geodesic struts together as described on this website, acceptable to commercial building code? If not which strut connectors would be required for a 20′ diameter geodesic greenhouse dome?
Blair: for generic size smaller domes, if your goal is to build a dome that meets a specific load requirement identified in a building code, you should have the bolt hole 1.25″ on center from the finished end of the strut.
Conduit doesn’t meet any structural load test. It is made differently than mechanical tubing, and will split at the seam when loaded to a minimum amount.
If you’re using structural or mechanical tubing, you can slightly reduce the diameter of the tubing if you increase the number of struts. 16 gauge is a minimum wall thickness. A common tube diameter is 1.375″ o.d. Typically a 20 foot dome has 95 or 135 struts. You can use up to 310 struts and maybe reduce the o.d.
Domerama note:buildings under a certain size, usually in the range of 100 square are excluded from having to meet building codes (you need to verify this for your area). An example of a hub which has been approved in construction of geodesic structures is the Dome Incorporated hub system. Other hub systems are also available. Please see our geodesic hub connector page.
Larry P. asks: I often hear that geodesic dome roofs leak when it’s raining. Is this because of the shape of a geodesic home? And is elastomeric paint a good choice for a geodesic roof?
Blair: domes don’t leak. Improperly roofed domes leak. There is nothing about a dome shape that causes leaks. If you have leaks in your building, there is a 99 percent chance it is caused by improper flashing at an opening in the roof, such as around a chimney, a vent pipe, a skylight, an extension where two different roof lines meet, or a dormer. Flashing these openings on a dome is no different that flashing these on a square house, and if they leak on your dome, they will leak on a square house if shingled and flashed the same.
How many conventional square houses have you seen with elastomeric roofing? My guess is none. Elastomeric roofing has a long way to go before it is a proven waterproofing for a home, and I’m concerned there will be a backlash of condensation problems caused from trapping moisture in non-ventilated insulation spaces. The main reason elastomeric roofing seems an attractive option to people is because they hope it will be an easy and cheap roof. A proper elastomeric roof takes a 5 step application process, and a resurface is required every 10 years.Follow instructions on how to flash a chimney, and shingle your dome.
