dome shape?

[logman]

Great site! I haven't seen a trapezium dome mentioned, looks nice in that it only has 4 different sizes of bricks. I saw Dmon's geodesic build and thought about the trapezium design and maybe casting the segments. Feedback appreciated!

[logman]

Re: dome shape?

I haven't found any prohibitions about posting links, here is an example about whthe trapezium design:
Trapezium dome design

I assume you would be better off rotating each course so the joints don't line up.

[wotavidone]

Re: dome shape?

I suppose that's pretty much what I did. I intend to find out this weekend whether it will fall down when the fire is going.

http://www.fornobravo.com/forum/atta...e-800x600-.jpg

[wotavidone]

Re: dome shape?

Fascinating website you linked to. I tried out the trapezium dome calculator, and came to pretty much my only objection to the concept - the calculator gives the sizes required for a 16 segment dome, and the biggest dome you could make and still cut the sgements from 9 x 4.5 bricks is a touch under 24 inch diameter.
What is needed is a calculator for a larger number of segments.

[logman]

Re: dome shape?

That was why I paid attention to Dmun's geodesic dome - where he glued up bricks. I saw inn that thread the concept of forming these segments in a mold instead, it seems alot easier to cut the compound angles in wood and put together some forms. I'm thinking you could form two adjacent segments as one piece, so 8 pieces per row. I'm looking around western NY for some reasonable priced castable refractory material.

[nissanneill]

Re: dome shape?

logman,
what you need to decide is whether you want a challenge in ornate oven construction (which only you will see and appreciate) OR build a relatively simple and quick Pompeii which works well.

Neill

[logman]

Re: dome shape?

Hi Neill,
you probably touched on the heart of my question. I am quite proficient in wood, and have a fair amount of concrete work under my belt and I suck at bricks. The idea of making some (easy for me) wood molds and cating refractory segments seems easier than all the brick work. If the refractory is higher cost, I'm hoping not buying an HF saw would offset that. Lots of conjecture on my part at the moment, really only want to build once, so I'm open to hearing lots more input.

Wota, i've only seen the trapezium layed out with 16 sides. somebody with some geometry or CAD background could probably do what you are seeking. I had played with the calculator radius until I go to 9 inch bricks once, but they would have to be glued together like Dmun did with the geodome, so I quickly thought about casting instead...

[deejayoh]

Re: dome shape?

Very interesting concept.

Here's an article that gives a recipe for a "homebrew" castable refractory. It might be of interest to you. the hard part is probably getting the Calcium Aluminate Cement. But I am guessing it's a cheaper way to go than premixed if the cost of refractory mortar is any indicator

Keep us posted!

[logman]

Re: dome shape?

Well, I'm been off thinking and working, and I'm on plan B. I got a 30 inch stryofoam ball (in two halves) from Plasteel and cut all the trapezoids to make a 38+" outer dome shape, so I will have about a 3 inch gap between the forms to pour castable refractory into. I need to wait for spring and make the base before I put it together as I could get it thru the door in the house.
I'm debating a single pour versus just the dome, and a second pour for the front arched doorway and chimney starter.....

[shuboyje]

Re: dome shape?

Are you still planning to form trapezoid "bricks" or are you going to cast over the half sphere? I'm a bit confused.

If you plan to cast trapezoid bricks make sure to get the manufacturers data sheets prior to purchasing. It will contain info about making cold joints which will be pretty critical to your plan. Castable refractory is not brick, and you might need to take this into consideration with your design.

[logman]

Re: dome shape?

Sorry, yes I plan to pour over the foam sphere and just use the trapedome as an outer shell. they don't make a foam ball big enough for the outer shell.....The wood forms should let me vibrate the pour.

[david s]

Re: dome shape?

If you plan on using a castable refractory you will need to vibrate it or you'll end up with voids. How thick do you plan on making the cast section and how thick will the bricks be? Resulting in what total thickness for the dome?

[PsychDoc]

Re: dome shape?

Logman- I feel your pain about wood vs brick. I can make lumber do what I want it to, bricks not so much. So do what you feel confident doing and learn along the way. its only time and money, right? That being said I think this concept has merit.

The calculations for any size dome should not be terribly difficult. There must be some programmer/math wiz/ computer guy that could make it happen. But for us pencil and paper types it might go like this.

Each course defines two circles inner and outer. For example a 36" cooking surface has an interior diameter of 36" ,duh, and a circumference of 113" (remember pi x circumference) exterior diameter of approxamitly 41" circumference of 128" (let us just imagine that is accurate )
Now then, the difference between the two circles is 15". From there you can determine how much to remove from each brick. So if the bottom course is 28 bricks, 15" divided by 28 bricks = just over 1/2 inch from each brick. I suppose a person could take that from one side but probably better to take 1/4" from each side- tapering to zero on the exterior.
I know that doesn't create a dome, but a cylinder. And this is where my geometry 101 class falls short. The 'top' of each brick could be beveled at an angle that is determined by the domes circumference, I suppose that is 41" in this example. The two circles of each course are also becoming smaller by an amount determined by the size and orientation of the brick.
Okay maybe the calculations are terribly difficult. However once determined and your saw is setup you could cut all your bricks- or at least each course all at once. The result would be near perfect. The surfaces of each brick could be glued together with almost zero gap/mortar. Practically freestanding.

Now have a little grace as you destroy my theory.

[logman]

Re: dome shape?

My inner form is the stryofoam 1/2 sphere. I'll put the floor firebrick inside after the pour to lower the effective dome height by a couple inches. The outer wall for my pour is going to be 3/4 MDF board triangles made up to form a trapezoid dome. I'm shooting for a 3 inch gap between the forms which i'll fill with KS-4V and vibrate as i fill it. Planning for a one peice precast instead of laying up bricks, hoping it works as well as some of the other hand-built forms I've seen here.

[david s]

Re: dome shape?

You will need to coat the inside of those mdf forms with some release agent. I use a mix of 50/50 motor oil and kero. Add 2% by weight stainless steel needles to the castable for added strength (optional) and if you make up your own castable brew add some fine polypropylene fibres, if using a proprietary castable these should already be contained in the mix. They assist in the elimination of water when doing the curing fires.