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:
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:
10 B flat end
20 C flat end
2 B half struts
2 vertical door posts
This will be cut as
1 vertical door post each
2 C struts each
1 B strut, one C flat end each
1 B strut, one B flat end each
1 B strut, two 1/2 B struts
2 B struts each
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.
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.
> 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.
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.
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.
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
1/4'x4' per strut
~2/5 dome surface exposure to side radiance
60 struts blocking
=60 out of 718 sq ft, or 8% coverage
1/4'x4' per strut
~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.
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.
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.
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 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.
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.
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.
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. :)
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. =)
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.
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.
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.
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...
"The trick, of course, will be accessing the top of the dome to put these in..."
Everything else you have done so far seems to be pretty well thought out in advance. Besides Stressbaby's suggested rather expensive rental of a cherry-picker, do you have some alternative plans in mind for attaching the membrane? You probably don't want to rent a cherry-picker every time a windstorm damages the membrane.
Perhaps a "tinker toy" moveable cantilever and/or bridge scaffolding could save the day. Or devise a "poor man's" cherry-picker rig.
I notice you joined Dave's in 2003 and, apparently by coincidence, all development at the TekStar website stopped in 2003. Is TekStar dead? Has TekCad2 been extincted by some superior competitive software?
Since the cherry-picker may not be a feasible approach (although who wouldn't want their very own cherry-picker?), I think you need to devise a way to mount the covering from the inside. If you do mount the cover on the inside, the framework "skeleton" will be exposed to the weather. That might look kind of cool, but the skeleton should be painted to increase its weather resistance. After painting the framework, you could pick from a couple of inside-mount alternatives:
(1) Simply use your stapled plastic lathing method to mount the woven plastic material to the inside side of the 2x4s, only use a lot more staples and use the longest ones your stapler can shoot.
(2) Beef up that idea with rigid lathing with screw-on attachment and incorporate a rubber strip for added water proofing to deal with the pooling that will occur in the outside "cells" after rains.
There are advantages to having the cover on the outside. The outside cover doesn't leave an exposed skeleton that will retain water after a rain and retard snow slide-off in the winter. Also, with an outside cover in place, you have the option of adding inside insulation in the form of bubble wrap and/or an inside-mounted second plastic membrane. So in order to get those benefits, devise a way of mounting the outside cover from the inside. A couple of approaches to achieve that come to mind.
(1) Take advantage of the holes in your hubs, and bring your lathing through those and secure the end-pairs on the inside to tensioning hardware that will let you conveniently increase the tension on the lathing by turning a screw thread. That can put a lot of tension on your lathing, so you might want to choose a stronger material for it.
(2) Secure the outside lathing with "special fasteners" that will pass through small holes drilled in the 2x4s. These "special fasteners" could be simple braided wire loops or strong nylon or dacron cords. The technique for mounting these from the inside will involve pushing long "feeder wires" through the holes from the inside, and then attaching the special fasteners to the feeder wires on the outside within reach of ground level. Then pull the feeder wires from the inside to bring the lathing membrane assembly up into place and pull the "special fasteners" in through the holes in the 2x4s for tensioning and securing inside. That will, in effect, "sew" your outside cover onto the framework, while working from the inside and within reach of ground level outside.
I have considered purchasing some commercial scaffolding for some maintenance projects around here. It's expensive stuff made of demountable steel framework, and mostly suitable for conventional type structures. But steel scaffolding can be used in situations where ladders simply aren't up to the job, even the fancy multi-configuration ladders.
However, Tropicalaria's dome is about 20 feet in diameter and about 13 feet high, so a custom-built scaffold, even a movable one, that would give him outside access to all of that framework, in my opinion would be a structure comparable in design difficulty, construction difficulty, and materials requirements to the dome itself.
I plan to build a small lean-to greenhouse from a kit or from scratch (none of the attached/lean-to kits that I have seen have 10-mil triple wall polycarbonate, and it gets cold here in Maine). A dome greenhouse wouldn't look architecturally correct attached to this conventional two-story house and, for that reason alone, I doubt seriously that our town government would grant a building permit for such a structure.
But if I were to design a dome-type standalone greenhouse, based on the "experience" of this message thread, I would seriously consider a double-dome design whose exterior open dome structure would serve as a permanently attached dome-shaped scaffolding that would give the user easy access to maintain the outside cover of the inner dome greenhouse structure.
MM, I was joking about the cherry-picker. I like your suggestions, very creative. With the film on the inside, he would have to drill drainage holes in the frame so that the water would run off and wouldn't pool in the cells.
If I were doing it, I would consider a series of platforms that would allow you to climb the outside of the structure. I'm thinking of a 2x12 with a short section of 2 1/2" pipe attached at an angle on the ends, sized so that the 2 1/2" pipe sections fit into the hubs on the GH. I might build 3 or 4 of them for use at different heights on the outside of the GH. For repairs, he could pop off the "hubcaps," insert the short pipe sections of the stand into the hubs on the GH, climb up onto the stand, remove more hubs, insert another section, and so forth. All of this assumes the structure is solid enough to support the weight.
My guess is that Tropicalaria has a plan and that he has been working on it again this weekend.
Before we moved to Maine, we lived in the St. Louis area and we frequently visited the Climatron and the rest of the Missouri Botanical Gardens in Shaw park. About a year before we left, they added butterflies in the Climatron. I suspect they did that because Chesterfield (a nearby suburb where we happened to live) built a full-blown butterfly house that was loaded with a wide variety of tropical butterflies.
It was an interesting piece of architecture, but not a dome. It was a large uniquely styled greenhouse loaded with tropical plants, a few trees, and a pool. They had a hatching room to renew the supply of butterflies. We visited the Sophia Sachs butterfly house and Faust park frequently.
Your idea to use the hubs as attachment/support points for removable movable access platforms seems like a good possibility. The structure should be strong enough to support the weight of the access platform(s) and one or more persons.
I've been pretty busy lately and haven't been on the site for a while. Sorry for the long hiatus. Once the really cold weather set in, the urgency to get the structure complete went away, since it was too late to use it this season.
In the end, we just used a tall ladder on the inside and leaned over to cover adjacent triangles. I modified the the above plan only that I removed the top triangle from the patch so that I could place a single pentagon in the top of the dome, overlapping each of the five patches, rather than have the five seams culminating in a point at the top.
The first step was the installation of the five patches around the top. I created a template by pinning a piece of tarp to the side and tracing out the struts. It was tricky since there is a center cut into the middle of the piece. Then I unrolled the plastic over the top of the template and cut it out along the lines, leaving plenty of overlap.
Each piece was placed on the dome with handclamps. When the positioning was right I stapled the plastic lathing onto the center seam by bending over it from above. After I had two of them in position I secured the seam between them, reaching the top half from above and the bottom half from a ladder outside the dome. On each seam I left the top and bottom edges loose to be able to slide in the adjoining top and bottom plastic.
Next, I traced out a pentagon onto my tarp in the same way had done the previous pieces, including the center cut. In reality, no sliver is cut out of the pentagon--it is just a single cut where the plastic increasingly overlaps outward from the center, in the same way you might make a cone out of a circle with a radial cut in it.
This piece was poked up through the center and positioned around all of the edges and along the cut. Note that on all of these pieces the position of the cut on one of the struts is vital, since the overlap is very small in the middle. Also, except for this top one, all of the center cuts have to be overlapped so that they shed water downwards.
Once positioned, I started pinning down the seam of the top pentagon from the open side of the dome where I had left off the fifth side patch. Yes, I'm pretty high in the air here. Note that I'm leaning on the dome itself, and have someone below steadying my ladder.
We decided to cover the center of the pentagon, where the seam comes together in the middle of a hub on a horizontal plane (i.e. it isn't sloped to shed water) with a circular "hat". Thus, a ~14" circle was attached with small strips of lathing on each hub at the top, and the seam lathing came down over the top of that. The pentagon itself is sloped down on each face, so should shed water (though not snow!). Eventually one of more vents will be cut out from the inside around this pentagon, but that will wait for summer when we can better judge how much ventilation will be needed.
After the hat and seam were attached I went to work around the sides. Each edge was attached from the next triangle over, all around the pentagon. You can see the circular "hat" here as it appears from the inside.
Topside. I was using extra long staples to be sure that the seams would hold. The electric staple gun allowed me to easily place all of the staples at full extension, but because of the 9/16 staples they didn't fully penetrate the wood unless I really held the gun tightly to the surface. For this reason, I had a hammer up there to pound in anything that wasn't flush. I can't imagine only doing this with the hammer, though, since I generally had to keep two plastic surfaces in tension (opposite directions) under a tensioned plastic lathing strip, and I could not have done this if I needed both hands in order to place a fastener with one and use the hammer with the other. The hand clamps were in constant use and I occasionally tacked a bit of plastic in place with a single staple to keep it there while I held an overlapping piece.
You may be able to see on this picture that the plastic lathing at the ends of each strut is attached on either edge. This is to go around the metal straps that hold the struts to the hubs. Since the edge is thin and weak here, I used more staples. In fact, I used a lot of staples throughout. We've had quite a few wind storms (this area is famous for them) and nothing has loosened in the 65mph gusts. I will also mention here that the plastic quite nice to work with--much sturdier and more resistant to creasing and kinking than any other flexible poly that I've used.
When we were sure of the position, the top and middle seam were clamped and the whole side was rolled up and inserted through the middle triangle so that I could work on the top seam of the patch. Here I am temporarily tacking the top into place.
The pentagon was folded back down over the side patch and pentagon lathing on each side of the open side was finished out to the edge over the top of the plastic of the side patch. Then, the last edge of the pentagon was finally attached.
The side patch plastic was unrolled back over the side of the dome and I moved over to the next triangle to secure the center seam. At this point, the patch was not attached on either side, just at the top. After starting the seam from inside the triangle, it was finished from a ladder outside the dome.
Sorry to place these out of chronological order, but here is a picture of the first patch that was placed on the dome, attached only with hand clamps because all of the edges have to be overlapped with other pieces. Note that at each vertex all of the pieces have to be overlapped at once, so most seams were left unfinished at the ends until everything was in place.
Here are the pieces being cut out. The plastic is 10' wide X very long, so I unrolled it and restacked it accordion-style to be able to pull it off linearly. After the first couple of pieces I found it easier to use a previous piece as the template for each subsequent piece. The trapezoidal patches are rotated to fit with each other as they come off the roll.
Unfortunately, I ran out of plastic, due mostly to my failing to account for the large amounts of overlap that I ended up using on each patch. I will need to reorder to get a bit more plastic to finish off the external covering. The only pieces I'm missing are the triangles between each lower patch, but without them I can only attach the top halves of the patches, and I can't go clear to each corner. Because I don't want these flapping in the wind (currently gusting to 55mph tonight) I'll wait to do any more until I get the rest cut out.
For standing by me in the wind and through the single-digit (F) weather while I added all of the above pieces, I need to thank my Dear Wife who helped me plan and keep track of everything, my father-in-law who steadied the ladder and passed things up and down for me, and my mother-in-law who took care of the four curious children watching from the window.
All of the rest of the pieces should be doable from the ground, by a single person, but I'm sure my DW will come up with something I've forgotten which requires her assistance.
She still thinks I'm crazy for putting up a dome, but I'm not sure that has changed her opinion of me any. My father-in-law is trying to convince my mother-in-law that he needs to put up one of these for storage at his house. My kids just think it's cool.
I am agog. The design calculation, fabrication and construction is thrilling. I've been fascinated by 'Bucky's babies' for years, what a cool concept. Following your progress has been a joy, thank you.
"I notice you joined Dave's in 2003 and, apparently by coincidence, all development at the TekStar website stopped in 2003. Is TekStar dead? Has TekCad2 been extincted by some superior competitive software?"
I'm sure you were quite busy and didn't notice that question. But I've answered my own question. New activity on the TekStar website, http://www.tekcad.com/home.html has brought it "up to date." I have to admit that the TekCAD2 software and TekStar's other products look rather interesting, even though they are a bit "pricey".
I congratulate you on your dome greenhouse. Very ingenious, and interesting. Domes seem to have a fascination for everyone.
Your Greenhouse is really COOOOOOOOLLLLLLL!!!!!!!!!!!!
Thank you for detailing this project for the rest of us to see. I have always loved Domes and have yet to build one. But, you have inspired me. I am dusting of my old dome book from the seventies. Hmmm...
That Green house is Great one thing you might think about is changeing the top plastic for the new type Solar panels its thin supper strong and bends to shape that way you could heat it and light it for next to nothing paul
Not a lot to update at the moment. With the advent of spring all of the temporary plastic bottom panels are currently off. No leaks this winter, and the dome has proven rain, snow, and wind proof and I'm happy with the performance of the plastic. I'm again thinking about refinishing the north wall with something other than plastic for insulating and reflection/absorption benefits, but I'm not sure what to use. It will have to be waterproof from inside and out, and I'm just not very knowledgeable about this kind of exterior construction.
I'm also kicking around the idea of doing the lower sections in multi-wall polycarbonate, but again that's a lot of seams to seal. Preformed edges would cost more than the polycarbonate itself, and would still require taping and caulking.
I'm sorry, I did miss your question the first time around. Looks like you found your answer.
I would be concerned about loss of light. There's a lot of light diffusion and scattering up there, and I also need top light for the taller plants/trees.
I haven't been around DG much since my last post; my gardening mostly goes dormant with the plants now that I'm not propagating/selling very much. Maybe if I get the dome set up the way I want this year that will change again.
With the colder nights it's time to close the dome. The bottom is sealed with four trapezoids (basically the tessellated patch from this post: http://davesgarden.com/community/forums/p.php?pid=2884716 minus the top triangle, turned upside down) and five triangle patches (just the top four triangles from the same patch). Then there is the door and the portions around it which will need to be closed up as well.
Here is the first of the lower trapezoids clamped into place on the frame.
As in the top patches, the center seams have to be positioned, secured and lathed before the outer seams.
At the edges, the bottom patches have to be leaved under the top patches, and the triangles have to be under the trapezoids. There are only five points around the greenhouse where the corners of the patches meet, and for each of these joins there are five corners which have to be layered in the right order. All of the edge seams around one of these vertices are under the lathing before I work on the corner. I tack down the lower layers with a single staple while I pull the upper layers into place so that they can all be pinned under the same lathing.
There is 6-12" of margin on each piece which greatly simplifies positioning (each patch is up to 12' x10' but the center point and interior cut have to be positioned to with 1/2" vertically, horizontally, and rotationally) but also allows each of the corners to be secured under all of the other seams of the vertex, giving me greater confidence in the integrity of these points.
Back in the 1980s my partner and I built a 2 frequency dome out of 2x2s and hubs made from hurricane straps. They were the kind of hubs where you cut a slot in the ends of the struts and slide the arm of the hub into the slot, then bolt on. That dome lasted about two years even though it wasn't PT. It was such a cool place to hang out in; I even installed a fluorescent light at the apex for night use. With hanging plants and a pea rock floor, it was a little piece of futuristic paradise!
Now, after reading about your dome project, I'm inspired to build another one! It's been on my mind for years, but now I think it's time.
BTW, have you ever visited this link? http://www.gardendome.com/
I may get a hub kit from them for my new dome; they have so many different choices and designs, it almost makes me giddy!
We'll need pictures, of course, when you start your new dome!
I ran into the Garden Dome site when I was trying to figure out how to cover this dome. My dome is similar to their Garden Dome 3 http://www.gardendome.com/GD3.htm with the base extension. By coincidence, their proposed door opening with the base extension is in the same location as mine, too.
OK, here's a pic from the vault; it shows the finished dome with shade cloth in the background and one of my hybrid EEs (Alocasia portora) in front. The Alocasia is over 4 feet tall and the dome is about 10' to the apex (20' diameter dome overall). I did not make a special door opening, but instead made a triangular door that was hung within one of the dome triangles.
I plan for my new dome to be set on a platform built on concrete pylons so as to avoid rot. The location I have picked is the lowest part of the yard so the dome will be accessible via a small bridge and after heavy rain, will be in the middle of a pond! Can't wait to get started, but have a couple other projects ahead of it.
Dave's garden and your insight are a God send. I am in the process of doing exactly what you have done and I'm happy to see interest from so many. I have just gotten interested in gardening and have been interested in geodomes for a long time. My winter garden is all in horse watering troughs and I have built little domes over each one for the cold nights here in northern Nevada. I am in the process of planting an 1100 gallon water tank to be the center of my greenhouse, I plan on sinking it deep enough to use the top as a potting bench, leaving me room around it for veggies. The water tank will serve as emergency water storage and as a heat sink to help keep things warm at night. I don't suppose you would be interested in sending me all of your contruction details along with photos? I found Dave's Garden by accident or maybe insiration, man what a find!!! And you have been a great inspiration, Thank you!!!
Thanks for the picture! Looking forward to seeing more about your new project when you get to it.
Glad to be of some help. Pretty much all of my pictures and construction detail are in this thread, though I need to get around to posting the final pictures of the dome fully covered and the door installation. If there's some details you have questions about, I could discuss it further, but I'm afraid I've already been a bit overly-explanatory in parts.
Well, time flies and here it is nearly a year later. It looks like the http://www.tekcad.com website is no more at this time. Dave Anderson's blog indicates that the TekCAD website is being re-located, but does not say where or when. I hope to keep track of it when that happens. I am still interested in TekCAD and related "dome-ish" stuff.
Very nicely done. This is the most complete dome construction info I have seen.
Would you care to provide total cost information so comparisons may be made to other greenhouse possibilities?
I would agree that domes are too cool to worry about price, but it's the economy don't you know.