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-   -   Stepped dome? (http://www.fornobravo.com/forum/f9/stepped-dome-7801.html)

bobby769 09-06-2009 10:34 PM

Stepped dome?
 
Hello all,

I'm new to this forum and masonry work in general.

This post got me wondering if some some sort of a stepped dome would work well for an outdoor brick oven:

http://www.fornobravo.com/forum/f8/e...tion-5253.html

dougrappe 09-09-2009 10:20 AM

Re: Stepped dome?
 
If you mean stepped on the inside, just rings of flat courses smaller and smaller as you go up, I think you lose the structural value of a dome with angled courses of brick. If you imagine the dome as an arch rotated around the keystone, you have a self supporting structure, so even if all of your mortar falls out, the dome stays put. (not that anyone's mortar is falling out, just cracks for me so far:))

Of course it would be much easier to build flat courses, but what fun would that be!?

Doug

bobby769 09-09-2009 10:29 AM

Re: Stepped dome?
 
I visualize it as initially the inside and outside of the dome would be stepped. after the layers are in place refractory cement would be layered on the outside so that process would wind up smoothing out the outside steps. but yes, when you look at the inside of the dome from front opening you would see the "steps".

I haven't quite figured out what to do with the hole that would be left in the middle of the dome. Perhaps a round "plug" that would be angled almost like a cork in a bottle. Thought about using the hole as the place for the flue but IMO a flue at the front of the oven is much more effecient use of the heat.

As for the fun in building; I love DIY but for this project my interest is more about being able to have my own wood burning brick oven and the fun I'd have in experimenting with cooking.

I was a brick oven pizza guy for about 6 years and that really was the most fun I had cooking (and eating for course).

cynon767 09-09-2009 11:45 AM

Re: Stepped dome?
 
I think that, in order to make a stable vault, your ceiling would have quite a high, steep peak... possibly too high and pointy to be an efficient oven.

bobby769 09-09-2009 12:31 PM

Re: Stepped dome?
 
I think I understand what you are saying.
Is it this:

One brick sitting squarley on top of another brick will be quite stable. That top brick will continue to be stable even if it is offset, until the offset starts to become greater than 1/2 the width of the brick. Beyond that point the greater the offest, the greater the instability.

So, if I offset each consecutive layer by less than 1/2, let's say 1/4, I will need more layers for the top of the dome to start closing up. This would result in more of a conical dome.

If I understand correctly, my next question is how much of an offest would be needed for the dome to be proportionate? For that question you kind of have to put the stability of the dome to the side. Which of course is not something I would do when (if) I were to use this design.

cynon767 09-09-2009 01:09 PM

Re: Stepped dome?
 
A bigger problem than just one brick on top of the next is that the force on the lower courses are increased as you go up... each brick ledge is supporting the weight of all the bricks above it. It will only be stable if the back edge of each brick is loaded with as much weight as the front. This means that the entire structure would end up more or less cylindrical with a conical hollow in the center; a lot of mass to heat up, even without considering the less-than-optimal shape of the chamber.

see this wikipedia article on corbelled arches for more info.

dmun 09-09-2009 01:10 PM

Re: Stepped dome?
 
The Pompeii oven, as designed, is easy and fun to build. We'll of course cheer you on if you decide to do something different, but there may be a reason this design has persisted for thousands of years. At least you decided that a flue in the top of the dome was a no-go. That's been tried and tried again.

Read the "why round" section.

bobby769 09-09-2009 01:15 PM

Re: Stepped dome?
 
Thank you so much for your replies.
Many important points have been made so I'll stick to the tried methods.
Stepped dome is out but me being the way I are :) stepped dome will be in the back of my mind. I find it very interesting how the balance between a stable dome, one that is proportionate and in this case one that will hold in heat but not be so massive as to take forever to heat up interacts w/ each other.

fxpose 09-09-2009 02:10 PM

Re: Stepped dome?
 
Since the bricks are squarely stacked on top of one another you can perform a quick experiment by assembling a scaled down dome and see how it works out structurally, perhaps by standing on it. My assumption is the dome will be structurally sound. I thought of doing something like this for a temporary oven.
But I'm not so sure about the thermal efficiency or dynamics of staggered courses.

ThisOldGarageNJ 09-09-2009 05:57 PM

Re: Stepped dome?
 
welcome to the beginners club... If your worried about building the dome, Its not half as difficult as I thought it would be. I dont know either if a stepped dome would properly circulate air and reflect heat ?? Keep reading until you either go blind or make up your mind... One or both will happen, Good luck and welcome
Mark


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