#11  
Old 03-30-2008, 09:30 AM
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Default Re: 48" oven about to start, need advice about buttressing

This is the tower. It's a page from wikipedia. Take closer pictures later.
Imageumaguete Belfry.jpg - Wikipedia, the free encyclopedia
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  #12  
Old 03-31-2008, 02:48 AM
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Default Re: 48" oven about to start, need advice about buttressing

Kulas,

Apologies for not replying to your PM earlier... I just noticed it..

With regard to buttressing your low dome oven, I followed the recommendations of a professional builder and all worked out fine... It's overkill, but I sleep well at night too... think about it... if you're dialed in a on a lower dome oven, do you want your project to be an "experiment" or do you want it to be a guaranteed success... you really only want to do this one time, right??

1. Once the dome is up (before any firing please...), measure out about 3" around the dome and mark a large circle on the hearth slab.

2. Drill approx 5/8" holes every 12 inches or so about 2" deep.

3. Insert approx. 12" lengths of 5/8" rebar into holes (or whatever the height of your soldier course is plus about 4"). You may need a hammer to sink them all the way down.

4. Bend and wire tie 3/8" rebar crosswise around the network of rebar stands to create a single framework of rebar reinforcement.

(In addition to this, outside the professional advice, and I would certainly recommend this if you're not using a professional insulating castable, I wrapped & wire tied the entire rebar framework in 1/2" galvanized fencing wire... This serves to "hold" all the contents inside the framework...)

5. Measure another 3" outside this framework and mark another large circle on your hearth slab...

6. Frame up a form using approx 1/4" fiber board.. I secured the form at the bottom/hearth slab with angle brackets temporarily drilled & screwed into the hearth slab (and the fiber board of course). I secured the mid level and top with a racheting strap commonly used to secure large materials to the roof of your car or truck...

.. you now should have a form built up around the outside of your dome with a rebar reinforcing framework in the middle all the way around the dome... Make sure you've closed the form where it wraps around to the front near the oven door...

7. Pour a "stiff" mixture of insulating castable and let cure the required time.

I can see from your post that it may be a challenge to locate a source of commercial insulating castable... keep trying... (Somebody on that island has to be doing some sort of refractory work...) It may be expensive, but it's worth it.. It's much more structural in nature than the perlcrete mix, which is really only castable to the extent that it holds it's own form...(no flaming.. it does have good compressive strength).. little teinsile strength at all though... If there is absolutely no source of insulating castable in your locale, I would work up a mix of perlite, concrete (not just portland cement) and perhaps some fiberglass or metallic strands to increase structural strength.. In this case, I would definitely mix up a few batches and test... Using concrete, you definitely will experience a loss in insulating performance with a gain in strength... The main idea is, you want all the the insulating power you can get while maximizing the structure's ability to maintain a cohesive whole.. a couple cracks here and there are fine.. a crumbly mess certainly is not.. At the end of the day, the rebar framework is what really guarantee's your dome won't cave in...

** All measurements are approx and can be converted to the closest approximate metric sizes...

My 42" oven has been up and in use for about 8 months now.. we use it weekly for friday night pizza and a couple times a month on saturday's with the residual heat from friday for sourdough bread and roast... The 14" dome height works excellent with pizza... There's a good balance between the heat coming off the hearth floor and the heat coming off the flame licking across the top of the dome... At first I thought I'd limited myself a little too much with the 8.75" door arch height, but I cleared a 25lb turkey, sans v-rack of course... :-) , with a little room to spare (came out fabulous!)

Feel free to shoot over any additional questions you may have. Good luck.

JB
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Old 03-31-2008, 09:58 PM
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Default Re: 48" oven about to start, need advice about buttressing

Thanks JB,

Good to hear that your oven's working great. Congratulations!

I've located all sources except the castable insulation. I will keep trying though, as per your advice. Nice post by the way!

Thanks and good day to you all!

-Kulas
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  #14  
Old 03-31-2008, 10:00 PM
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Default Re: 48" oven about to start, need advice about buttressing

dmun,

I've posted the link above of a bell tower with buttressing. Maybe you would like to check it out if it was what you had in mind. Thanks.

best regards,
Kulas
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  #15  
Old 04-01-2008, 06:02 AM
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Default Re: 48" oven about to start, need advice about buttressing

I saw your picture, and I just saw a tapered tower with a dome on top. I didn't understand what it had to do with the topic at hand - buttressing. I think either I didn't understand something you were saying, or vice versa, so I think it would be helpful to more fully explain what I was saying.

Everyone who builds an arch, of any form, should read the following page:

Auroville Earth Institute is a research, design and developing agency for vaulted structures, construction of various Vaults, Arches, Domes (VAD).

It talks about the construction of arches and the shape of the forces acting on the arch. I've taken the liberty of copying a few images from the page I linked to:



The forces acting on a dome are in the shape of an inverted catenary the form that a chain assumes if draped from two points. For maximum strength, a dome shape must be either in that shape, or be thick enough to contain that shape in the center third of the dome thickness.



The bad news is that the full catenary shape forms a dome that is way too high for use in brick ovens, particularly ones for the baking of pizza, the shape shown on the right. The good news is that you don't have to use the entire curve to take advantage of catenary theory. The illustration on the left shows an arch formed by a chain stretched more taut, giving an arch more like a low dome pizza oven. In order to avoid stress on the upright soldier course, you need buttressing around it, to encompass the shape of the curve. You could of course build massive side walls that would be hard to heat up, but I think there is a better solution.



This drawing shows a catenary form ceiling on a square room, where the weight, and the forces, of the dome are brought down to the four corners of the square, instead of the entire circle. This suggests a method of buttressing a low dome oven at intervals, with a minimum of mass to heat up.

To be continued...

Last edited by dmun; 04-01-2008 at 06:05 AM.
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Old 04-01-2008, 06:03 AM
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Default Re: 48" oven about to start, need advice about buttressing

This reply had to be split up because of the limit of the number of images.



Here's an oval shaped low dome oven section, showing two buttresses on either side of the dome. I can't easily draw a catenary curve in my cad, so you will have to imagine it. You can see the forces are spread out, bearing down on the brick floor, instead of pushing out on the side walls.



I would envision six buttresses, two of which would form the side walls of the entry door. For the sake of simplicity I've just shown some bricks leaned up against the side of the dome. It would be much more sound structurally if these buttresses were tied into the first courses of the brickwork.

As Johnrbek said above, this is strictly theoretical. You can take it or leave it. His solution to the problem is a good one if you can find a castable refractory insulating material that is also structural to the extent of being able to be reinforced with rebar, and to withstand the huge forces of a unitary dome that's hot on the inside and cool on the outside. I'm interested to hear exactly what this material is, and to see some specs on it, but in the meantime, I think my solution is one that would work with minimal mass and traditional materials.

Sorry to be so long winded in my reply. I hope I've explained what I'm talking about.
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Old 04-01-2008, 06:47 AM
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Default Re: 48" oven about to start, need advice about buttressing

Awesome answer dmun. Thanks for taking the time and effort to get it out here for us plebes. I though that this is what you were getting at, similar to the external brickwork that you seen on many cathedrals throughout Europe. I didn't realize a the time that they were buttresses engineered to counteract the outward forces caused by the domes.
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Old 04-01-2008, 09:05 AM
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Default Re: 48" oven about to start, need advice about buttressing

This is an excellent thread! Thanks for the drawings DMUN! I would guess that the issue with the extra Thermal Mass from buttressing could be minimized by using the Insulative Firebrick. Any thoughts here?
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Old 04-01-2008, 09:22 AM
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Default Re: 48" oven about to start, need advice about buttressing

Quote:
I would guess that the issue with the extra Thermal Mass from buttressing could be minimized by using the Insulative Firebrick
This is a good idea. Insulating firebrick is often on sale, is cheap to ship, and has the same coefficient of expansion as regular firebrick. It has good insulating properties as well, almost as good as perlcrete, iirc.
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  #20  
Old 04-01-2008, 05:36 PM
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Default Re: 48" oven about to start, need advice about buttressing

dmun,

In re your interest in seeing specs on an insulating castable, please have a look at the link below.. It includes strength & thermal conductivity measures:

MatWeb - The Online Materials Information Resource

I used Kast-o-lite 22.. I believe I used 4 55lbs bags priced at .60/lb:

http://www.hwr.com/products/datashee...-O-LITE_22.pdf

Here are some pic's of my work:
Low Dome Neapolitan Oven Update - Dome Construction. Photo Gallery by John Bek at pbase.com

As you can see from the images, my oven most closely resembles the "segmental arch" depicted in your diagrams...

In the matter of insulative capacity, typically, I fire on a friday afternoon to 1000f dome temp... let the oven cool to a floor temp of approx 750f... move fire to side and keep logs burning till pizza cooking is complete.. then just throw the oven door on and leave it as is for the night... Early in the morning, temp is about 550 with some coals still in the oven... Sweeping out the oven for bread baking, I'll let it sit for another 3 hours or so till it's down to around 475... After baking the oven will slowly move down into the 350 range late in the afternoon and may hit somewhere around 225 the next morning. So, not sure whether that's good performance for the commercial type oven Kulas is building, but it works fine for me.. Oh yeah.. I opted for approx 3" firebrick walls to keep heat up times lower (never fire more than once a week), so factor that in..

Again all of this was recommended by a professional builder... I didn't make it up.. cannot take the credit...

BTW, my opinion is that insulating firebrick, on it's own, in a segmental arch dome, would not be a sufficient buttressing material. Too light weight. The rebar/castable in my structure guarantee's the dome is not going anywhere.

JB
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