Servant74,

I'm not a structural engineer so I can't help you with the fastener sizing. I like your desires. Since nylon has a significant amount of stretch, I wonder if you would have to pre-stretch it before placing the concrete (somewhat like pretensioned steel cables in concrete).

How are you going to transfer the loads from one 2x4 edge on the panel to another one? That is a detail that can bedevil you a lot.

Have you considered a ferrocement type of approach. Basically you are going back to steel for reinforcement, but instead of rebar you are using expanded metal lath (probably four layers would do it). The concrete is troweled into this from one side until it begins to push through to the other, then smoothed off. I believe the mix would use aggregate with a maximum size of 1/4". You could also save weight on the panels by using light-weight aggregate.

Best wishes--John M.

John,

I have a friend who is helping me with these efforts. He is a concrete additive techie type. In certain cases the short (< 1" nylon monofiliment) acts as 'springs' embedded in the concrete as he explained to me. WHEN the concrete cracks, it 'pulls' the micro-cracks back together and evidently does pretty well.

For 'significant' reinforcing, rebar is still needed, but for 'pavers' and other low stress applications, it appears that thes nylon or even steel 'slivers' can be used.

Yes, on the Ferrocement, but I want it to be theoretically 'portable'. I have even seen where folks use bamboo in the orient instead of steel in certain structures. What I would RATHER do is using the monliticdome.com approach. Blow up a 'bag', blow foam on the inside of the bag, put in re-bar, then blow in shotcreet on the INSIDE of the foam.

I have also thought of using the concrete additive that is acrylic, that makes the cement water tight. Cement is normally pretty porus without proper additives or painting. If I must make it more water tight, I guess I will paint the outside of my dome with the laytex type roofing paint that basically puts a 10 year plastic roof on top of the dome. If I do that, I might spring for some of the additive that allows it to reflect more basically adding about an R19 equivalent to the 'roof'.

... Jack

I loved many of Bucky's ideas and went out of my way to be at his lectures.

I've done a lot of structural design and I also spent a lot of time training architects on how to use computers in designing an testing.

Modern stress analysis software will identify any flaws in your design in a matter of minutes.

However, the time spent building molds, casting prismatic panels and assembling them will prove very time consuming. If I may I would like you to consider casting the entire dome over a wood form on which you have stapled chicken wire and fiberglass fly screen.

The load paths will have greater continuity reducing localized stress and it will actually be easierto build with no seams to leak.

Bucky might not recognize it but what the Heck....!

L.J.

I used to have a dome book from the 60's, the hippie commune era.

the easiest, least expensive solution evolved to a roughly 1/2 to 5/8 sphere of ferrocrete

the structure doesn't need to be specifically spherical, but can be formed over any curved mold, like a tunnel, or donut

be sure to mold your openings, cutting doors and windows is not fun.

I'm a refrigeration man so research this yourself but
I've been using a concrete product called QUIKWALL (GOOGLE-QUIKRETE PRODUCTS
to find their pdf download files)
on the outside of my old home footing foundation walls
as they claim quikwall on the outside of blocks is 3 times
stronger than a morter wall ($14 bucks for 60 pound bag Yikes)
and in their pdf files I remember some talk about chemicals
in concrete being destructive to some types of fiberglass
and or plastic materials in the mixtures of other brands

I don't know about monofilament but I know my
mistakes with concrete get corrected with a jack hammer

Good luck
JR

Hi,

iron or steel or glass reinforcement is ok.

Nylon or aramid or other low elastic-modulus material will not really do any reinforcement unless stressed considerably - (stated somewhere above that pre-stress is necessary) - if stress in non-prestressed material is to build up you will need large deformations - not possible with concrete.

If you switch the concrete to rubber or polyethylene than nylon-fibers will be good.

Mounting triangles: The modern wind-turbine masts (5 m diameter, 20cm wall thickness) come by truck in segments: half cylinders (2.5m possible on a truck, 5 m impossible or much more complicated) of 2.5 m height.

These are glued by epoxi with the help of steel rods extending some 50cm into both to be matched parts.

Total height of mast may be above 100m!

So this procedure on a smaller scale will solve your matching problem.

RHABE

artbyjoe was doing something similar. It might be worth asking him how he got on. He had at least three threads with some good suggestions and links:-

http://cr4.globalspec.com/thread/13656

http://cr4.globalspec.com/thread/15633

http://cr4.globalspec.com/thread/13435

YEA AS MENTIONED MONO IS NOT GOOD FOR CONCREETE REINFORCMENT BUT FIBERGLASS IS also two bolts may be enoufh for the bolts but localized the stress wich concrete does not handle well , instead of going larger thamn recomended size (because size will take into account the size of the beam of concrete) go for more spread out better. the point of a geo dome is the transfer of stress allong the seams so you probably need a joint at the points, mabey have a bracket fabricated, best sugestion yet was to form and pour as one(best side suggestion FORM YOUR DOORS AND WINDOWS FIRST, CUTTING IS NOT FUN OR EASY)

YOU ARE TRYING TO USE A MATERIAL FOR WICH ITS PROERTIES DO NOT MAKE IT IT THE BEST FOR YOUR DESIGN., NO (LARGE) TENSILE STRENTH, CONCRETE ONLY WORKS iN COMPRESSION WELL. yes i know other examples that prove that wrong this is just the basics of the material properties, (and tension croncrete is engeneered not back yard poured)

going concrete pour as one shell

or look into fibre-crete and see if you cant change the properties of your material to fit how this design works best in its transfer of load paths.

his casting the bolt holes into the equivalents could be for assembly...I think all seams should be filled with a significant epoxy. Filling the joints with mortar would work as long as everything was in compression but some of these joints will see moments or tension and mortar won't be enough. I say, build it out of plywood (female mould) and follow a similar construction method (spraying the slurry from the inside) as the monolithic domes I posted a link for. The guy who invented that system shares a lot of information. You could remove the plywood once your reinforced concrete inner-shell has set up and then refinish the outside to add the surface you'd like (i.e. no plywood grain marks, lines to accent the seams, etc)

My concrete savvy friend has mentioned removing the bolts (not all at once :) and string in some wire cable. Then pull the cable to generate a partially post-tensioned structure.

Iterate. Eventually the structure is basically a post tensioned structure.

The fibers in the cement tend to not be good I am told, because they fracture. And the nylon is basically a spring that pulls the concrete back together when it fractures.

This dome will be in panels, so it is not like having a single large structure, but lots of small structures that are just attached to each other. I am thinking it changes the dynamics of stress fracturs and the need for planned stress releaf joints.

To do the door, I am actually thinkin of removing 6 panels after construction and fram in a door using conventional techniques. My friend suggested cutting, but I agree, that is a mess waiting to happen!

My bud and I are trying to engineer some boards and pour them 'professionally'. We started that process last year, and having this dome or any better workshop should make it easier to work on that project later. ... He wants to cast some 5/4 boards, 6" wide to make concrete deck boards. Making them pre-stressed allows them to be cut 'easily' with a cross cut saw. ... No, it is not using a quickcreete mix, but a 'professional brew' of cement, water, sand a little agregate, and some 'chemicals', in a brased with pre-stressed filiments. Let the 'brew' cook and harden, then we have a board. ... Not the easiest but backyard projects are always supposed to take longer and cost more than anticipated (at least in my back yard :( )

Have you considered a monolithic dome instead of geodesic? They would be significantly stronger and more resistant to the elements than a geodesic. If you're after the look then you've decided...

If this is going to be a hollow dome I'd try and remove as much mass as possible...I've heard of exotic mixes that can float on water (perhaps with hollow glass bead fillers?). You can get fiberglass (instead of iron) rebar...it could help beef things up if needed.

Personally I would model all this in an FEA program and connect the panels with spring (CELAS) elements at the corners and see what kind of load is transfering from one panel to the next. Seeing the hot spots would allow more consideration for distributing the loads more evenly. Refining the panels (with your equivalents also modeled in) would help see the stresses within the panels. I'm not the person to ask about what stress amounts are allowable though...if you're making your own panels you'll have to either do a bit of testing or guestimating to find stiffness values for the material that could be plugged into the program.

That is what I would RATHER do, but I have the tools to do a geodesic dome, not the stuff for the monolithic dome. ... In my perfect world, I would call them up and say 'do it'!

Last year I went by their facility in Italy TX, really quite impressive.

If we build another house, it will be one I think (the wife is already convinced!) :)

hello kkjensen

i agree about the monolithic domes. i visit their plant twice a year.

i looked at the fiberglass rebar myself. now, i am not an engineer. but i did compare the graphs for failure mode between steel and fiberglass rebar. what i found was that at failure, steel deforms, and fiberglass shatters. the graph for the fiberglass went straight down at failure. i took this to mean that the failure mode was complete, ie. no strenth left at all.

what is supposed to happen with any rebar, is that at failure mode, the rebar keeps the structure reasonably intact, so that loss of life can be minimized.

joe

I was just throwing out ideas on how to reduce the mass of the uppermost part. The stresses and strains in the top section would be less than down at the bottom where you would be most concerned about the strength. I think more important than the material of the reinforcement is the interaction between the concrete and the material. Pretensioned concrete can be very strong but the lack of the pretension can render the reinforcement pretty much useless until the concrete has cracked and the load is transferred from the stiff yet brittle concrete to the reinforcement. Whatever method is chosen I think it should be studied and tested so something doesn't go very wrong when someone is inside...or worse, when you're latest project is inside!

Wouldn't a geodesic dome need to be designed as struts connected at nodes? If you are just connecting the sides of these castings with a couple bolts, there is no connection at the nodes. I think you need somethink like rebar running in the parimeter with a method to transfer forces across the nodes.

Have you talked to an engineer who has experience designing domes?

do Igloos have struts?

Not explicitly, but the effective 'depth' of their building blocks makes it work as struts! At least that is how I perceive them.

But thin wall concrete dome structures have been around for centuries. Many Mosques have large domes built in them without 'struts' assisting.

Each triangle in the dome basically has 3 'struts' cast into it plus a concrete 'sheet'.

I am trying to figure out whether I want to do the perimeter rebar or not. My concrete-pro friend says no., but since I will be inside (and pay for it) I will have the last word.

The only pro in domes I have talked to has no concrete experience. The other pro's I have talked to are at monolithic.com and their approach is non-geodesic.

The only reason to do geodesic, is this is more of a DIY than it takes to do a monolithic dome. If they had someone close with experience, I would have them do it. I like their domes better anyway.

One reason for posting here was to hopefully find someone that 'really know' and is a concrete structures pro that might be willing to chime in.

Thanks for your suggestions and asking.

Beware that the water absorption properties of Nylon - and its ensuing swelling are not a good match for the other materials you plan to use.

check out Allard Engineering, and structured insulated panels. I've been looking into foam made from soybean also; Inflate shape, and spray ! Thanks for DeSoto link - I intend to use for hydroponic greenhouse.

Check out monolithic.com (and tell them I sent you!), their doing kind what you are after. Only they specify expanded polyurethane blown in. They sell the 'airforms', you blow in the foam insulation, then put re-bar in place, then shotcrete inside that.

Their 'airform' dome skin lasts 10 to 25 years, and can be coated anytime in there with either an elastomer 'paint' or shotcrete. If you don't do something, the skin will break down and the insulation will break down.

Personally, I like the idea of putting shotcrete on the outside and make it look like stucco over the highly insulated dome.

To dream for another day.

Servant74

A former neighbor of mine, an architect, has done a few domes like you describe, and lives in one of them. I believe his largest was the entry atrium on the side of a hill for a university library that sits under the hill behind the atrium (and the university's maintenance shops are in the level below the library!) If you use an inflatable balloon held up by a large blower's air pressure during the shotcreting stage, you have to be careful to allow for the weight of the shotcrete to deform it a little--with a modest increase in the girth and a similar decrease in the height. VERY energy efficient. Also, very unusual.

An alternative method of construction he described to me was to form a rebar and metal lath frame, sort of like a sandwich. The center between the two layers of rebar/lath is filled with urethane foam and the shotcrete applied to both sides. There are cross-tie members to hold the layers together.

If you want further information from him, look up Roy Browne in Independence MO.

--John M.

I have looked into monolithic. Great, informative site. Especially the, plastic wrap roll, face shield = ingenious!

Like your 'shotcrete on the outside' idea also. Seems like I've seen it [some where].

Really curious to see how you project turns out. Wonder about; weight of triangles, and angles of struts.

A friend of mine in New Milford lives in a wooden geo house. Really cool acoustics = you can hear kids whisper from the second story playroom, in first floor kitchen!

You may have found the 'shotcrete on the outside' idea on the monolithic site. If a dome cover is to far gone, it is one method of keeping the insulation viable. Kind of a last ditch effort though.

Any idea the size of your friends dome? I would love to see one 'live' before building one. I am sure there are some in the area, but I don't know where. (I am near Nashville TN)

Sorry, should have typed New Milford Illinois. About an hours drive west of Chicago. Just south of Rockford. Cliff and Tammys dome must have been around 48 foot in diameter.

Worked there [Rfd.] as a metallurgical labratory technician for nearly a decade. Thought I could answer fastener question. We had a self tapping 5/16'ths bolt that held two thousand pounds ( Tensile ). Cannot recall shear strength.

Again - Allard engineering (New York) an informative site. Seems like Timberline geo-, or one of their links had a 'domes for sale' segment. Might be one near you.?

ps.. servant74, you need to put commas into your tags when you make them in the future, otherwise you get a monolithic tag.

You might want to look at American Ingenuity Domes

http://www.aidomes.com/

Their domes are 7" or 9" of foam bonded to a concrete skin. I think the concrete is about 4" thick but I don't recall exactly. The edges of the concrete are scalloped down and the reincorcing wire mesh in each panel extends beyond the edges of the panel into the scallop of the adjacent panel.

When erecting the wire mesh is wired together with wire ties (go figure). Then the seams are mudded with concrete which has a fiber admix.

We had a 48' dome designed and ready to go on a piece of ocean front property we owned in Westport, WA. A job transfer nixed the project.

Not sure if anyone mentioned, but another alternative is going monolythic concrete.

Travis

I saw a geodesic dome made out of folded sheet metal sections.

The triangles etc were simply bolted together at the flanged sides.

At some points a section was left out (pentagon, I think, but I didn't count the sides) and infill used to shape up a door.

This would seem a cheap DIY type construction. Far simpler then using thin concrete, which is not an ideal material for this sort of structure.

Another method I saw was a half cylinder slip formed using alternate lifts of concrete filled with sawdust.

For light weight concrete you can use perlite or vermiculite aggregate, but the mix must be quite stiff to prevent the aggregate separating.

Concrete mixed with detergent added and foamed with air (I think superplasticiser is also used) also gives a light weight concrete.

Another foamed concrete is made by using Al powder mixed with sharp sand (to lightly scratch the protective oxide layer). As the concrete sets, it liberates lime which reacts with the Al forming hydrogen bubbles. It forms a closed cell foam. the H2 diffuses out and is replaced with air which diffuses in. (It's not quite that simple but that is a close enough explanation).

Perhaps some of this will help, whether you decide to use sheet metal or light weight concrete.