Building a geodesic dome greenhouse

I have been looking at a number of ways to do this for quite some time now, and I think I'm going to go ahead and put up a simple hub-strut wooden structure. Most of my calculations derive from the groundwork at http://www.tekstaronline.com/monkeyhouse/ -- though the calculator and tips at http://desertdomes.com/ are quite valuable as well. Feel free to skip to later posts as I hope to post progress and pictures as I go along, but the planning stages require a bit of math that I realize may only be of interest to a select few.

I'm building a 20' frequency 3 dome, 3/5 (or 4/7) sphere using cedar 2x4s. Since eight foot lengths are the most economical, I've adjusted my strut lengths to fit this with two struts per eight foot section. 3 frequency domes have three different strut lengths, each with their own vertex angle. Since I will be connecting these together with hubs for simplicity, I also have to factor the hub size into the strut lengths.

The angles are
A - 10.038 degrees from a right angle
B - 11.641 degrees
C - 11.9 degrees

For practicality with my miter saw, I will simplify these to 10, 11.5, and 12 and expect my hub attachment to give enough to make up the difference.
I will use 3" sch 80 pvc pipe cut to 3" lengths for the hubs, attached with perforated steel strips. This adds three inches total to the effective lengths of all struts. This is actually a simplification, since the hub widths are not measured at the same angle as the struts will be, but it's close enough. I can't afford to produce materials at greater tolerances, anyway.

3.5" tan 12 degrees = .75", which is the effective overlap obtained when I cut two C struts out of the same 8' 2x4. So, measuring all strut lengths from the outside (longest) edge, I can cut struts up to 8' 3/4" / 2 = 4' 3/8"

Add 3" for the hub, and the effective C strut length will be 4' 3 3/8". I can now work backwards from the reverse strut calculator at http://desertdomes.com/rev3calc.html to find the appropriate lengths for the other two, shorter, strut types. I calculated with decimals for the math:

Laid out as Effective Decimal, Actual Decimal, and Actual Inches:

C - 4.281 - 4.031 - 4' 3/8"

B - 4.189 - 3.939 - 3' 11 1/4"

A - 3.619 - 3.369 - 3' 4 3/8"

So these are the measurements to which I will cut the lumber on the longer edge, mitering at the angles listed above.

There are some fine graphics of the layout of these struts in the sites listed above, so I won't attempt to illustrate it here. I will need numbers of struts as follows:

A 30
B 55
C 80

In reality, I'm warping the bottom layer of struts slightly, both to give a more even bottom edge and to normalize the height of the first tier of triangles on the dome. The bottom of a 3 frequency dome isn't quite flat; it bulges and pulls up an inch or so every couple of vertices. In practice we can expect this small bit of warping to cause few problems. Additionally, the edge isn't flat on the ground; it contacts at an angle, since the sphere it projects would be expected to continue at an angle under the ground.

Internal angles of a regular polygon are given by 180 - 360/n, where n is the number of sides. In this dome, there are 15 sides, so the angle is 180 - 24, or 24 degrees from being flat. Split in two for the contribution from the miter on each strut, gives 12 degrees, which fits nicely with the calculated 11.9 degrees on the C strut which makes up most of the middle circles. The difference comes from the insertion of an occasional B strut and the contribution of the extra-planar tilt which I want to correct for. In this case, all I'm going to do is not angle the bottom of the vertical struts at the bottom and maintain this 12 degree angle between the ground struts in order to keep it flat. I'm not going to bother reprojecting the vertices in order to get strut lengths for a flat bottom since the variance is in fractions of an inch and I don't think it will make a difference.

I also need a door in this greenhouse, and after much examination I'm going to put it in one of the hexagons, removing the six internal C struts. I will drop two vertical posts between the top and bottom of the hexagon with joist hangers and use half-sized C struts to brace on either side to the midline vertices. This will connect at a 12 degree angle with the bottom strut of the hexagon. I'll then add horizontal extensions from the side posts to hold a vertical door.

The final bill of cut pieces:
30 A
45 B
10 B flat end
54 C
20 C flat end
2 B half struts
2 vertical door posts

This will be cut as
2@
1 vertical door post each

27@
2 C struts each

20@
1 B strut, one C flat end each

10@
1 B strut, one B flat end each

1@
1 B strut, two 1/2 B struts

7@
2 B struts each

15@
2 A struts each

horizontal door braces will come from scraps from A brace cuts

This adds up to 82 8' 2x4s. Cedar is not the strongest material, but I don't think it will have to be in this structure, and it should hold up to the damp better than fir. It runs $6 at Lowes, for a total of $492, plus tax, not counting any extras I have to purchase for mistakes and the occasional bad section. Redwood is much more expensive here. Pressure treated isn't much cheaper, requires more expensive hardware to resist corrosion, and introduces chemicals that I really don't want. I don't really want to paint this whole thing, but I am still a bit worried about the UV exposure.

The plan is to glaze with greenhouse film inside and outside (two layers separated by 3.5") the first year and consider things like polycarbonate down the road. I want to insulate the north wall, probably with a standard fill backed internally with a radiant barrier.

Ground contact is still a bit up in the air. Since the dome itself is a rigid structure, ground motion is less of an issue than with other structures. I will probably set sub-frost line anchor piers at each of the 15 vertices and rest the ground struts on something like compacted gravel. I'd like to put raised beds around the inside edge of the dome with radiant heating through them, but I'm not sure what to line the inner edge of the dome with for contact with the soil. I may also decide to use ground contact rated pressure treated lumber for the ground struts.

More to come. Any input is more than welcome.

I feel good now - I'm not the only techno-geek on DG (LOL - certainly no offense intended)

I think if your lumber is relatively knot free you won't have to worry about the cedar being strong enough. I used pine lumber simply stained with something I can't remember (27 years ago) but I recall it had some kind of preservative in it. It still seems ok with film outside and in.

You may have some challenges with the film. Since you have a continuously curved surface, a single piece of film would have many pleats in it. Also you need to insure you eliminate any sharp things such as bolt heads on the exterior surface. Inside glazing is, of course, very desireable. But I can attest it poses installation challenges, as gravity is working against you.

I agree that you probably want pressure treated timbers at ground level.

Your door poses a problem. A house a few blocks from me had a dome on it's top. Apparently the access was from below. One option for you would be to add a circular footing/wall or excavate down a few feet and make use of the earth's thermal capacity. If the dome goes all the way to the ground, the outer edges are less useful for planting since there is not much vertical space.

The links you included show that you have done your homework. Where you and I would part is that I wouldn't even think about trying to construct it. More power to you! Keep us informed.
Dave

> I feel good now - I'm not the only techno-geek on DG
> (LOL - certainly no offense intended)

I always figured that since Dave himself built the site, I couldn't be the only one! I greatly appreciate your feedback.

> I think if your lumber is relatively knot free you won't have
> to worry about the cedar being strong enough. I used pine
> lumber simply stained with something I can't remember
> (27 years ago) but I recall it had some kind of preservative
> in it. It still seems ok with film outside and in.

So what kind of structure did you build? Have you reapplied anything to the wood since then? What kind of foundation did you provide?

> You may have some challenges with the film. Since you
> have a continuously curved surface, a single piece of film
> would have many pleats in it. Also you need to insure you
> eliminate any sharp things such as bolt heads on the
> exterior surface. Inside glazing is, of course, very
> desireable. But I can attest it poses installation challenges,
> as gravity is working against you.

Yes, I've posted questions here in the past, without response, on that very subject. Since then I've come up with several solutions for the problem, and I'll be posting more details on them once I get past the actual frame. I intend to keep the hub connection hardware as smooth as possible, though I've also considered improvising hub caps, partly to protect the pvc hubs from UV.

> I agree that you probably want pressure treated timbers
> at ground level.

Yes, I'm starting to lean more that way, though I still haven't fully figured out the wood/ground interface.

> Your door poses a problem. A house a few blocks from
> me had a dome on it's top. Apparently the access was
> from below. One option for you would be to add a
> circular footing/wall or excavate down a few feet and
> make use of the earth's thermal capacity. If the dome
> goes all the way to the ground, the outer edges are less
> useful for planting since there is not much vertical space.

The door frame timbers will be the only members receiving forces other than tensile and compression. In addition the tensile force on the top and bottom struts will be greater than normal. For this reason I've considered making these pieces out of something more sturdy--either 2x6, which could be problematic for the 3" hub connections, or pressure treated fir which I'm still not fond of, but would be much stronger. It's worth noting that the geodesic dome structure is very impressive in its distribution of load, and the whole thing could easily be sturdy with much thinner members.

Although this wasn't the source of my decisions on the door, it is remarkably similar to both the dome and the door that I plan to build:
http://www.gardendome.com/gd3_door/gd3_door1.html

I feel more like I know what I'm doing when I find others with similar plans online to what I've come up with, but I suppose it is a tenuous validation at best, since there's no telling whether theirs is a *good* solution.

Note also in the illustrations that this kind of dome has a nearly vertical wall for the first level of triangles, extending 3.7' for the strut calculation given in my first post. The second set of triangles is at a 12 degree angle for another 3.5-3.7', giving a height of seven feet at less than a foot out from the wall. (These are the same calculations used to compute the door placement and framing).

I hope you're not thinking that I'm the real "Dave." He's from Bryan, Texas.

My greenhouse is a lean-to with a solid back built completely from selected pine 2x4s. The back is 16' high and the slanted part is 24'. ("standard" sized lumber). I believe the 24 footers are #1 grade. I have done nothing to it since the original construction. The only part that really needs some attention is the outside of the back which is covered with 4x8' exterior paneling.

The foundation is cinder block. The 2x4 footers fastened directly to the tops of the blocks. I've attached a picture showing the shell during a film recovering a few years ago.

The vertical wall for the first level of triangles will provide more usable interior space. The top of it will, however, be subjected to a considerable radial (outward) force transferred from the upper levels. (You just have to take this into consideration.) A tensioned steel wire connecting all the tops of the vertical level and anchored into the door frame could work. Of course the door frame would have to be strengthened or reinforced with a type of gusset. Just a thought.
Dave

Not sure why I can't upload pictures tonite - the one I was going to show is in this thread...
http://davesgarden.com/forums/t/644362/
Dave

Oh - it completely slipped my mind when responding to you that your name was Dave as well, and that my comment might seem confusing. I'm well aware that "dave" goes by a different nick. :)

72 8'x2"x4" cedar
11 8'x2"x4" structural pressure-treated

Doesn't look like much in this photo, but it felt like a lot when loading and unloading it (and it felt like even more when paying for it!).

Most of the pieces are 3/16" longer than 8'. Cutting at a diagonal of 12 degrees turns 1/16" into sawdust and yields 8' 1/8" on the longer side and 7' 11 3/8" on the shorter side -- a difference of 3/4", as predicted. Apparently I can still do trig after all!

This is good, as I did all my calculations based on overlapping 3/4" on the struts. Also, the extra length in original pieces allows for loss from the sawblade, which I hadn't really thought through.

Here's the first 15 struts: 10 C and 5 B all out of the pressure-treated lumber. You can see the chemical penetration on the cut ends. This consitutes everything that will be on the ground. Besides these, the doorway will be made out of pressure treated lumber, consisting of top, sides, and side braces. This stuff must have a stain included because it is orange, rather than the sickly green I associate with ground-contact pressure-treated, especially in the newer high copper formulations.

Since I took this picture I have moved on to cedar, cutting all 20 of the flat-ended C struts, plus 20 more regular C struts. Once I cut the 10 flat-ended B struts, plus 20 regular B struts, I'll be ready to construct the half and whole hexagons that will ring the bottom of the structure.

It's taking about an hour to do 30 struts, mostly because I'm being meticulous about the measurements and marking all of the struts on both ends to ensure I can tell them all apart. I've got about 3 1/2 hours of cutting to go at that pace.

Tropicalaria, please continue to post, we will be interested to see the project as you progress along.

Are you worried about shadowing by the 2x4 struts?

Yes. :)

But I'm trying not to be.

I'm thinking of using this: http://www.northerngreenhouse.com/products/polys/clear.htm for the outer layer of plastic. It has good light transmission characteristics (and strong wind, collision, and UV resistance, which I need), but as a woven material will strongly scatter the light. Even having a second layer of plastic on the inside of the struts will provide some degree of light scattering, though, preventing strong point shadowing from the struts. There will be reflective radiant insulation on the north wall as well, which should brighten things up a bit further by limiting pass-through and providing light from the other side of the structure. The dome shape helps reflect horizontal winter sun down onto the plants from all facets, anyway. The dome is about 13' high, too, and the upper structure captures and bends that light back down.

As far as general constraints on permissivity due to strut coverage, a worst-case back of the envelope calculation would give

Side:
1/4'x4' per strut
x
~2/5 dome surface exposure to side radiance
60 struts blocking
=60 out of 718 sq ft, or 8% coverage

Top:
1/4'x4' per strut
x
~2/3 dome surface exposure to top radiance
100 struts blocking
=100 out of 1257 sq ft, or 8% coverage

so, 8% is my worst-case blockage due to struts. The actual blockage will be less because this inflates the size of the struts a bit and assumes that none of the radiance on the sides of the struts will make it back into the dome. This will need to be added to shading from the glazing to find final permissivity numbers. Hopefully top dome light gathering and north wall reflection will make up for a bit of this as well.

Good, you are OK on shadowing. 8% doesn't sound like that much to me.

Does the film you are considering come with an IR additive? I didn't see that in the link. Polyethylene has a high rate of thermal IR transmittance. The link I recently added to the sticky at the top suggests that an IR additive in the poly film could save you 20% in energy costs in the winter.

What about condensation/drips? Two issues here...first, the nearly horizontal surfaces may shed the condensation in unpredictable ways, so you might get drips where you don't want them. Second is the issue of solar/thermal IR transmittance...this could work two ways for you...condensation that doesn't drip off of the horizontal surfaces would reduce the transmittance of solar radiation, limiting how much you warmed up during the day. On the other hand, the same condensation would help with the transparency of the poly film to thermal IR at night, since water has low rates of thermal IR transmittance.

So what I am envisioning is condensation on these horizontal polyethylene surfaces, which might keep you from warming up as much during the day (due to reduction in transmittance of solar radiation) and prevent you from cooling off as much at night (due to the condensate's opacity to thermal IR and the corresponding reduction in nighttime heat loss). Am I making sense?

I'm not trying to throw any killer balls here...you asked for input! I really like your project! Keep us posted.

SB

Have you seen sources for this poly with an IR additive? Sounds like I would get less solar heating during the day, which might impact me more here where we have a lot of sun throughout the cold winters. I definitely would not want the poly absorbing this daytime IR itself, so I would guess that we're talking about a reflective coating?

I haven't decided what kind of poly to use on the inside, but I was considering a standard greenhouse film with UV and condensation inhibitors. Perhaps this is where the IR additive could be used as well?

Inside film is going to be harder to put up because it will require additional lathing on almost every strut. Condensation between the two layers may become an issue, as well.

Here's one, the first one I found, from Hummert: http://www.hummert.com/catalog.asp?C=65&SC=24&P=13753 The IR films start about 3/4 of the way down the page. There are 4-5 others referenced in the sticky link, attached below.

The chart in that link shows single layer poly to have PAR transmittance of 87%, double layer poly 78%, and double layer IR 78%. But the thermal IR transmittance is 50, 50, and less than 20 respectively. I suspect that these films are just relatively opaque to LIR, the wavelength responsible for nighttime heat loss...if sufficiently transparent to PAR and other IR wavelengths, it should work.

http://www.agra-tech.com/thermal_glazings.htm

http://www.hort.wisc.edu/mastergardener/Spec%20Training/fruits%20vegetables%2005/Greenhouse%20EC%20-%20Master%20Gardener-%20April%202005.ppt#325,9,Glazing Material Comparison

Over the past week I have cut more struts, gotten some of the materials for the hubs, and started preparing the site for the structure.

The only material for screws that I could find which was approved by the manufacturer for use with pressure-treated wood and cedar is stainless steel. Many sites on the web say hot-dipped galvanized would work as well, but every package I looked at said specifically not to use it in this way. Stainless steel is expensive.

I looked at the box, poked through it, and then dumped everything out looking for the included bit to screw these in. Reminds me of those "I Spy" books.

I called my DW over to help.

I only started looking through the screws when I determined that the bit had come loose and was no longer attached to the inside of the lid. Yet when my DW came over, she had no trouble locating the bit.

Still scratching my head over this one.

So here's the plan for the hubs. Schedule 80 PVC pipe, 3" diameter, cut to 1/4" less than the width of the struts. Perforated hanger strap, 22 gauge, held with two screws on each side. The one farthest out is to hold the strap onto the strut and the one closer to the edge is to control pivoting. For the most part, these joints will take compression, rather than tension, so the straps maintain integrity but don't carry much load. Distortion forces should be transmitted through the structure as well, with all parts reinforcing each other. Nonetheless, each strut should be good for up to 320lbs hanging load individually, based on the ratings of the strapping and the pipe. This is important since I want to be able to hang things off of the inside of the frame.

3" sch 80 PVC is very cool stuff--solid, but expensive. Good thing I don't need much (~16').

Been pretty busy with other stuff lately, so not progressing as fast as I'd like. The most sensitive tropicals are in the cold frame to protect them until this structure gets built, and the container planted veggies, etc. are under frost protectors.

Here's a set of hubs and straps. I've precut and bent the strapping because it's too stiff to bend acutely without a tool.

Here's the first hexagon that I put together. I originally planned to build from the bottom up, but now I'm thinking it might make more sense to build from the top down. Fastening the strut ends is difficult to do high in the air.

Wow ! That is wild , can hardly wait to see more.


.·´¨¨)) -:¦:-
¸.·´ .·´¨¨))
((¸¸.·´ ..·´ Shirley -:¦:-
-:¦:- ((¸¸.·´-:¦:-

Here's the bottom layer of struts from last week and the first ones that we added this week. I'm going with a hybrid assembly with the top pieces preconstructed and the rest built from the bottom up.

My DW took an interest in the assembly and has been diagramming our progress. It should be noted that assembling this structure absolutely requires more than one person, since someone has to help hold and brace the pieces while they are being put in place.

I'm constructing small pieces of the dome a few struts at a time on the ground and then lifting them into place to limit the amount of work that needs to be done in the air. The outside of all of the straps is attached before the pieces go up so that they only need to be screwed from the inside.

This open hexagon is where the door will be eventually.

We lifted one of the large hexagons up on top with the help of my father-in-law (and my indispensable DW, of course). Then we added struts in around it to hold it securely in place.

It got a bit dark along the way...

From the other side of the dome you can see the 2x4s that we used to prop up the pieces as we connected them in. At this point none of these are technically required anymore, since the dome can hold itself up, but we left them in place for greater stability (especially if we encounter very much wind) until we can finish the other side.

We added a bit more to the right side after this picture was taken, but I need to go get some more metal strapping before we can finish. It's going to take a few more people to get the double hexagon lifted up on the open side, I think.

Return to standard time is about to rob all of my sunlight in the evenings, I'm afraid. Good thing I have the 1000 watt halogen. :)

I've got to say that this is just about the coolest thing I've seen on a GH forum in a long time.

Ahyup. That's WAY cool! If it were me, I think I'd make a nice, slotted door in the roof, for a telescope to peek through. But, then you'd have to figure out a nice way to put the dome on some sort of roller so you could spin it on the base. =)

WAAAAAAAAAAAAY Kewl ~~~~~

We had sustained winds to 40mph and gusts to 55mph today. The structure didn't move at all. Although I have faith in the strength of the final structure I was worried when I woke up this morning. Nice to see that even a portion like this is strong.

The banana leaves are completely shredded. Since they are forecasting temperatures of 26F tonight and 20F tomorrow, I don't think I'll have to worry about the banana leaves much longer. All I've got up right now for plant protection is the cold frame, so I'm really racing the weather with this new structure.

Great looking greenhouse! Perhaps the following link will be of some interest to others:

http://www.orednet.org/~jgarlitz/geodome.htm

It is for a smaller, simpler dome with all the calculations already done and with complete plans. It's intended to be a small observatory, but could be adapted to use as a greenhouse.

Looks like that one's also a 5/8, 3 Frequency dome, about half the size of mine. It uses panel, rather than hub and strut construction, which I would recommend if it's an option for you given the surfacing material.

After our record low temperatures last week, today was quite warm (60-70F), though overcast. Today we needed to lift the other patch of the top of the dome up and fasten it in place.

First I added all of the struts to support the piece and everything on the sides which didn't directly connect to it. At this point the constructed portion was basically symetrical, except for the door and one dangling strut above it.

Two of us lifted it into place while two others braced it with long 2x4s. We then fastened the bottom ends as we were able to line them up. Finally, we added two struts at the top of the dome to connect it to the other side. At this point the only things left were the connecting struts on either side of the piece we had just added and the 5 central struts of the pentagon on top.

We had to push and pull the parts of the structure to make each connection line up properly. Sometimes this required bracing various points with 2x4s and sometimes is simply meant that someone heaved on it while it was fastened. Gravity made some of the portions sag, while the difficulty of maintaining perfect positioning of the base was responsible for other misalignments. We managed to get everything into place though, and each section we added gave further rigidity to the structure.

It got dark while we were putting these in place, and started to sprinkle a little bit.

We next went back and added the rest of the side struts, leaving only the open pentagon on top.

Here's a closeup of one of the hub connections, showing how everything is attached. The first screw is attached closer to the hub, at an angle. I actually insert the screw at a steep angle and then pry it back to tension the strap a bit. When the screw goes all the way in, it tensions the strap even more, taking all of the slack out of it and pulling the joint tightly together. The second screw is then placed further out to provide greater strength. This being cedar, the wood is quite soft, so two connections is a must. Also the second screw prevents the connection from rotating, adding more stability. As I mentioned above, these are 22 gauge straps, so they're a bit stiff to work with, but strong. My DW actually climbed up onto the first tier at one point when we were pushing the connections into place, and I can attest that the joints are better able to support weight than the struts themselves.

She's the one that said that I should include this closeup photo, but she didn't know that I was going to mention her climbing up on the structure. :)

Here's a picture of the first screw being put in. Note that the strap itself is bent out from the wood, so that the point of entry of the screw into the wood is farther away from the center than the hole in the strap itself. This is what pulls the strap tighter when the screw goes in.

We assembled the five spokes of the pentagon on the ground and then I carried it up the ladder and poked it through the hole to rest a bit off center on top. It took a bit of work to get all of the spokes butting up against all of the right hubs, but once they were in place everything fit perfectly and I was able to screw them all in without pushing or pulling on anything. This was a good thing because by this time the rain was picking up and bringing a bit of wind along with it.

We were all so wet and tired by the time we were done that we took everything inside and turned out the lights without even getting a picture of the finished dome. Here I am putting in the last screws.

Great! I'm interested in how you will handle the film. SB

Really, thats what I am wondering about. Will you have to do a lot of pleating and folding to cover the grhouse with one piece of plastic? I would love to see pictures of that as you cover the grhouse. Love the metal straps that connect the pvc to the wood. Easy, cheap and effective.

Brilliant !

Well, I have some cutting to do. This is another one of those things that I've spent a lot of time considering without coming to a clear, obvious answer. Covering with a single sheet is not going to happen, because the size of the sheet would necessarily be enormous (better than 80' square) and the waste would be tremendous around the sides. If this were a shelter or something other than a greenhouse I might consider covering with five pieces around the dome (it is 5-way radially symmetric) and refolding wherever needed as I worked my way down the dome. Folding involves greater shading though, I need to the plastic tight for a number of reasons, including standing up to the wind and shedding snow and rain.

At this point I plan to cut the plastic into the shape of the individual icosahedral patches. If this were a sphere, that would require 20 patches to cover, but it's considerably less for my dome, and should guarantee that everything fits nicely on the structure.

I'm using the woven plastic material in 10' width, which is just wide enough to accomodate the height of the triangular patches that I'm cutting out. I'm going to use cheap construction plastic to trace a template directly on the dome itself, and then lay it over the real plastic to mark each section. I will leave a few inches of overlap plastic around the pieces to accomodate the seams.

The actual attachment to the dome will be done with plastic lathing on the overlapped seam, secured by staples and wide top nails at strategic points. The trick, of course, will be accessing the top of the dome to put these in...