48" oven about to start, need advice about buttressing

[dmun]

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.

[gjbingham]

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.

[Kemo]

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?

[dmun]

Re: 48" oven about to start, need advice about buttressing

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.

[johnrbek]

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

[dmun]

Re: 48" oven about to start, need advice about buttressing

John, that's a great looking dome. And that "C" ring looks solid as a rock. Is your soldier course laid right up against that, or is there an expansion space?

I couldn't access your matweb data sheet, but I didn't try very hard. You apparently need to register.

[johnrbek]

Re: 48" oven about to start, need advice about buttressing

dmun,

Actually, it's the c-ring (or buttress) that is laid up against the soldier course... the soldier course first, then the "angle setting" first row, then the rebar reinforced buttress (c-ring) was poured.. after that was solid, I continued with the chains in the dome.

There is no expansion space between the buttress pour and the soldier course... You don't want one... You want that thing solid... I did have two hairline cracks along 11 & 2 o'clock in the buttress, so that's pretty much how the expansion thing played out. No biggy.

The matweb link was just one I found via a google search to get you some data points.. Just google "kaolite 200 data sheet" and you'll get a lot of hits..

Hope all this helps. Take care guys.

JB

[Kulas]

Re: 48" oven about to start, need advice about buttressing

Sorry guys, I've been busy so I wasn't able to reply to your posts.

Dmun,

Thanks very much for the nice posts.

The picture I posted was maybe not exactly what I had in mind, but it had the buttressing you were describing right at the very bottom of the structure.

The link you posted from Vaults, arches, and domes is of great help. And it is very easy to understand in terms of its basic principles. I may need to take this direction.

The buttressing with six sides including the door columns is also a very feasible alternative but I am not sure I can do this. I can do this alright, but I do not understand the very basic principles of this type of buttressing, thus, working in this direction will be like treading unchartered territory for me. And the probability of me making a mistake is higher than using a solid type of buttressing around the dome wall.

However, I do not discourage those more adventurous ones to try out buttressing the wall with separate columns around the walls instead of a solid single piece of mass around the whole wall.



best regards,
Kulas

[Kulas]

Re: 48" oven about to start, need advice about buttressing

Bricks and insulation are coming next week. Will start at the end of the month with the hearth stand.

John,

I ordered calsium silicate insulating cement. It did not say "castable" but I think this is it. It's a ready mix and I just have to add only water. The supplier said it will set 'like' cement. I'm going to make a 4 inch thick layer of this insulation (with rebar and tie wire inside) around the dome wall and check if the heat still goes through, if it does, I will pour some loose perlite around it.

Thanks.

-Kulas