THE DETAILS ON USING MATERIALS TO BUILD DOMES
I have learned to prefer to use the breadboard foam despite the fact that it is harder to keep water out with it and its lower strength per volume, can be made up for or exceeded by its extra thickness for the same cost. It is much easier to bond to Portland cement because of its porosity, with it you only really need to simply rub the cement into it and it will bond with about the same strength as the foam. However the high density foam I have needed to coarsen its surface and then usually apply some pure cement into the coarsened surface before applying regular cement sand mix. 3 of sand one of cement by volume works for me, sometimes I get to much shrinkage with 2.5 sand to one cement. Coarsening the surface of high density SM. foam can be a real task if you have a lot to do. Usually foam cut by a hand saw will be coarse enough for a modestly good bond, for me however I haven't seen any of my bonds as strong as the foam itself made this way on SM foam. Good bonds are done with epoxy but this can be a little to expensive for a large dome. You may however consider a pourable polyurethane foam as used as flotation foam in boat building as I found it to be excellent but sticks to everything including your hands, this foam is usually much cheaper than the expanding foams you can buy in spray type cans . On one of my domes I glued strips of aluminum foil along the open area of where the triangular pieces of foam fit together, and poured the two part foam mix into the space with a small home made paper or foil funnel. Also remember to ask about other densities of pour and mix foams if you plan to use SM foam, as the more common stuff I bought was the lower density type and not as strong as SM but was as strong as the breadboard foam .
When using cement to build domes
For me I can never finish what I start at one time and sometimes I prefer to do a project in separate pieces, so in both of these circumstances always leave your reinforcement coming out of the cement where you intend to add more work later , and I have learned to if at all possible use stainless or very heavily copper coated steel that I salvaged as scrap or whatever else you can find that won't rust as when adding new cement to old I don't believe its wise to expect it to be as reliably resistant to water seepage and rusting as it is in all one pouring . I have used a lot of such wire by binding it into shapes that will act as hocks in the cement so it doesn't need to go as deep into the cement to anchor into it . It could also be a good idea if making ferrocement domes to try to always apply your cement so that if you are forced to leave it , to leave it in a way that if it leaks later it will be overlapping in a way similar to the way a shingle is laced to avoid water flowing into the dome but outwards instead . Of course epoxy is often of use to seal small cracks in cement. Usually when working with foams to build domes and other structures I will often add a folded peace of stainless wire into the still wet cement that I am using to bond the foam together with so that later on layers and attachments can be added later. Also I have decided that its usually best to just cut my foam triangles without bothering to cut them at angles to fit together without leaving a gap, when pulled together into spheres. This saves time and is less complex, that way all I have to do instead, is to just cut triangular strips of foam to fit into this space to save on cement and weight. Especially in the case of the reasonably simple 2 V Icossa alternate that I feel is most practical for most people to build. One can lay the triangles flat on a surface and hinge them together with thin fiberglass strips with one large gap between the last one, when this is turned over and pulled together it will take on part of the domes curved shape. I do only the pentagons first (these are 5 sided shapes and I'm referring to single pieces made with 5 foam triangles) here it is important to use a rather flat surface to pull them together on, as this flatness will pay off later by making sure its not twisted. Sometimes I will chose to prop up the middle or and surround with weights to help keep everything in place till things are more attached. All my dome building has always been from the top down despite that this dose sound backwards it seams to work the best for me after all foam dome parts are usually not all that heavy. Mounting the top pentagon on old drums or whatever and then balancing the other pentas on their sides with their tips touching then just add the remaining triangles into the remaining openings to make it solid results in very nicely made dome without much inaccuracy. You can lift it higher to add a raiser wall doing some on both sides and then filling in the rest later. It may be quite important if you want maximum strength to add extra reinforcement around doors and base. I usually add a rim like on a hat, but towards the inside around the base as this can also be used for weighting the structure down and providing extra area, for whatever I chose for a gasket. This light weight can be of advantage should one want the structure to be portable and that could also be a real advantage in third world countries as long as one adds the weight back or re fasten it down. You may chose to finish the inside of the top penta first as it will be harder to do when its upside down and over your head. Do remember foam will break down in sunlight so it must be painted or covered
A thin layer of Portland cement can both protect from light and lessen its flammability but some fiber reinforcement is also better still
Some on floors and basements
In October 1988 Popular Science did an article on dirt-cheap floor that you might find interesting about how to make floors with mostly dirt and only a little cement
I have chosen to go with concrete hexagon patio tiles over plastic over sand because it can be made one piece at a time and then can be taken up and moved if need be. On my main ferocement dome I have chosen to make a basement under it by just making a big upside down bowl under the building in ferrocement. It seams to serve well, but never goes to freezing, both because its about a foot away from the outer edge of the dome surrounded by dirt and because I have an outer rim of foam insulation buried under and around the dome, rather like the rim of a hat. This all helps to raise the frost line, plus I also slightly heat the place. But it is very cheap to build, as it's only about 1/2 inch thick or less. It holds the whole building up, because if the dirt came lose the whole building would sink. I may have improved its strength by molding curved hexagon patterns into it
I do plan to try this again but next time I plan to make the basement first
and make it its own curved roof, maybe foam covered later with ferrocement
but just about anything to hold plastic film in a curved surface will work.
I plan to bury this whole curved surface at least enough to make it level
for the floor and may also use a layer of foam to insure it will never get
cold enough to freeze. This will also be of advantage if you use it to store
solar heat, also to keep the dome a little up more from ground level without
needing so much fill. Its possible that the weight of the sand used to bury
it should help keep it from floating up from the ground , as this is a
possibility if its very watertight , you could consider burying anchors into
the soil to help hold it down if circumstances provide . Its also wise to
consider drainage but so that you can seal it up should you decide to store
water in it to use or for its solar heat storing advantage, and also remember
to include places for stove pipes , excess for heat exchanger pipe hookups
, excesses for low voltage electric hookups and for other possibilities that
you might think of later , not a bad idea either to have some extra drainage
pipes around the upper edges and other areas of the basement as you may decide
to use tubing to direct any water that may build up against the inside wall
to where ever as you can always plug them up if not needed , however I have
tried this and I still have a water problem with my basement because of how
later on I had decided to dig it down deeper than my drain , it appears that
water finds its way in both under where my wall ends at the floor and through
the thin cement as well .,I will continue to experiment to solve it . But
in any case you may decide to have more than one basement any ways , and
remember to decide where you want to put your portal to the surface . Also
I would like to suggest one could experiment with using bacteria to provide
heat as just about anything can be used even old tree stumps , but for that
to work quickly , treat the material with nutrients , for example many hardware
stores sell a mix to help speed up natural decay of tree stumps etc. but
don't forget if you try this to provide a source of air and moisture . Don't
forget try to decide or think up your own experiments , and improvements
to try If you plan to do this .
I plan to add some drawings and or photos as well as results
from other attempts at solving the water drainage problems .
THE DIMENSIONS AND DRAWINGS
For all your work it may well be best to do as I did and make stencils . I used aluminum door frame materials for my larger ones and square antenna tubing for my smaller stencils . It makes sense to make these durable and accurate especially because you may use them again and the accuracy you put into these is preserved for all the foam triangles you make
For 2 V Icosa Alternate , the cord factor for this is A= .61803 and B=.54653 . To use these for example a 10 ft dome on A it would = 6.1803 but as 1/2 of this to equal 3.09015 to give the length of all these struts , or you could also apply this to the edges of any triangles as would be the case with foam triangles that you assemble together in creating a dome . If measurements are in feet that would be 3.09015 feet each for a 10 ft dome . But do note I had great difficulty trying to use regular ft measure as they are divided into 1/12 and each of them into 1/16 and it was a real pain. So try if you can to get a special foot stick as used by draftsmen or do as I have done and use one that you have made yourself , that is devised into 1/10 and also into 1/100 Th of a foot and save yourself all the aggravation that I have had to deal with , otherwise use metric.
Hope I made no mistakes here but I used these measurements with no trouble
. Also you will need 10 of AAA triangles and 30 of BBA triangles , with the
BBA shorter lengths going in on the pentas with its A side going around the
outside edge . The tables below are worked out for you so you don't need
to use the cord factors unless you want different sizes .All are in feet
but you could substitute meters then an 8 meter dome would use a 2.47212
meter A edge etc.
2v Icossa in the drawing , all the dashed lines are the B struts and are
the shorter ones .
The triangles that are made up of solid lines creates only 10 triangles and
also have all 3 sides of equal length. Of course that makes them easy to
make and on my domes are often added last.
| diameter of dome | A strut | B strut |
| 8 ft 2.47212 | 2.47212 | 2.18612 |
| 10 ft | 3.09015 | 2.73265 |
| 12 ft | 3.70818 | 3.27918 |
| 14 ft | 4.32621 | 3.82571 |
| 15 ft | 4.633225 | 4.099275 |
| 16 ft | 4.94424 | 4.37224 |
| 18 ft | 5.006043 | 4.91877 |
| 20 ft | 6.1803 | 5.4653 |
| 22 ft | 6.79833 | 6.01183 |
| 24 ft | 7.41636 | 6.55836 |
Drawing here is for a simple icosahedron and its cord factors for a Icosahedron is 1.05146 and are really simple just 15 triangles with all the same length edges if you don't remove one for a door
On the hand drawing shown here for a 3v icosa 3/8 note that for a 5/8 you
need one more ring of triangles to continue the same pattern .I have tried
to show the repeating patterns of pentas and hexagons and it also shows the
3 different strut lengths with each type given its own colors . I find it
useful with these types of domes as well as with the 2vicosa , that all of
one type of strut length is always inside of a hexagon (6 sided ) or pentagon
( 5 sided) triangle assembly , ones around the outside are also all of the
same lengths. This is useful not just because its simpler to keep track of
, but because it allows you to make all of your 5 sided arrangements or 6
sided ones on a flat surface and then use these modular assemblies to assemble
your dome. Its unfortunate that the remaining hexas around the bottom must
be cut in half
Struts in pentas are shown as red and are all A s
Struts that are black are Bs
Struts shown as blue are Cs
Cord factors for a 3 V Icosa Alternate is A= .34862 B=.40355 C=.41241 and you need 75 triangles for a 3/8
These are more of a pain in that you have so many extra pieces , as a result
I'm not so sure I would recommend them unless you cant get or make larger
or longer materials.