48" oven about to start, need advice about buttressing

[Kulas]

Good day everybody!

I have been planning and thinking about building a round oven for quite some time now based on the pompeii design from this site (great site to say the least!) and the progress of construction in my head as well as some on paper is taking shape already. I am planning a low dome oven 48" or around that size (depending on minimal brick cutting).

Oven diameter: 48"
Dome Height: 15-16"
Door height: 10 inches semisphere
soldier course: standing on the cooking surface so as to create more airspace for fire: 9"
Door width: 19 inches

Insulation, stand, housing, and vent, I have almost figured out in my head. I have previously built a Hand design oven and it's working just fine and makes good pizza. fire brick wall of 4.5", with cement/lime cladding of 4" with rebar. Insulation of 4" perlite on sides and on top. 6" layer of sea salt under firebrick floor of 4.5" and under that 2 inches of perlite boards. Oven internal dimension is 4' x 5'. Yes, it's big! and I use tons of wood. Fortunately, I use it everyday so I'm not using as much wood as you could imagine with an oven this big. ... a truckload of wood may go for as long as 1 month and 3 weeks if its dry. 1 month and 1 week if its not seasoned. This truck is a six wheeler by the way.

I built this oven for bread. But I mainly use it for pizza since I am only beginning to learn how to bake. And due to the mass, keeping it at pizza temps is quite easy just as long as I manage the fire well and have tons of wood. Once I have a smaller round oven I can use this for bread exclusively and for lower temp cooking such as roasts.

Anyway, with this oven I have gained a bit of understanding of how to build an oven and the principles of the design. A lingering question I have now is with the buttressing of the sides of the oven. My guess (and from what I read in the relevant threads) is that I need to buttress the sides of oven because of its higher tendency to collapse due to the low dome. I can do this with 2 inches of cement cladding with rebar, but that will drastically add to the thermal mass. Another option I have is to pour perlcrete with rebar outside the firebrick walls as a buttressing. Now, from what I see from perlcrete is that after it has dried it will be flaky. I am not sure if it will hold the pressure from the brick wall. I have also read j bek's thread and saw that he used something else for this purpose. That product he used, I do not have access to. Would a mix of 6:1 perlite and cement, 4 inches thick, effectively hold the walls in place? I would deeply appreciate it if anyone has had experience with this and would share some knowledge.

Insulation on top would be blanket and loose perlite. Only the sides will have perlcrete. Under would be board of some kind. I'm pretty sure I will be starting next month so this is getting exciting for me. Would really need some help with any or all of you guys.

-Kulas

[Kulas]

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

I'm almost finished locating all the materials I need for insulation: cal sil board and ceramic fiber blanket and some perlite over the insulated oven. I've also received a quote for a bag of calcium silicate insulating cement for $41 /25k bag.

I would have ordered the cal sil boards from the fb store, but the shipping was over $500 since I'm in the Philippines. So I decided to look for sources here though they may be of lesser quality.

The calcium silicate cement specs says service temp of up to 650 Celsius. bulk density 350 kgs/cubic meter. I'm thinking of using this for the buttressing of the soldier course and the angle setting brick of the dome. Do you guys think this would work as an insulator or would it be just be a heatsink wicking heat away from the oven chamber? I'm thinking of using perlcrete but this might seem as a more solid buttressing since it's cement.

Thanks in advance for any insight.

[Dutchoven]

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

Kulas
I might suggest that instead of buttressing as you mention....eliminate the soldier course and lay the equivalent number of courses of lets say 6" long as opposed to the 4.5"...could stand them on edge for 2 courses to get to 9"...or something else...even could miter the last one to "spring the courses for the dome
Best
Dutch

[Kulas]

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

Dutch,

That's very interesting. It might work, though it may be too much thermal mass on the sides? 6" on flat side four bricks is 10"... I'd angle cut 1" off to get a wall of 9"... sounds like a good idea. There may still be some possibility that the dome would push the walls out (I'm certainly guessing here ) since that wall has more strength from bottom up as opposed to side to side? But I guess that's why it should be thicker than 4.5". I definitely have to think about this first before I write further, I haven't thought of this. I'll be back.
Thanks!!

best regards,
Kulas

[dmun]

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

As you know from running your oven, the top of the firing chamber burns white first, and it takes a long while for the carbon to burn off the sides. I suspect that your buttressing might exacerbate this problem.

I'm thinking that having brick buttresses, at intervals, with the door opening functioning as two, and maybe four others to carry the stress of the dome weight to the floor, without adding to the overall mass of the oven. Cathedrals are built like this, with stout columns bearing the load, leaving space for windows and "curtain walls" in-between.

Forget about perlcrete having any tensile strength. Even fully cured you can dig your fingernail into it like cork.

[Kulas]

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

Thanks dmun.

I have noticed that with century old churces made of stones. I am, however, doubtful with my skill with brick and mortar and first hand engineering knowledge to achieve something like that. Maybe I could do something more simple for my skill.

Perlcrete, as you have confirmed, has no tensile strength. But will building a round brick housing to hold the 4 to 5 inches of perlcrete around the soldiers make the perlcrete rigid so that it can support the pressure from the dome?

After laying the soldier course and the angle setting brick, I could make a brick housing 4 to 5 inches larger than the outside diameter of the oven and same height as the soldier course and angle setting brick, so I can pour perlcrete there and wait for it to dry before continuing with the dome construction.

I think that the perlcrete will be strong enough if it sets between firebrick and outer brick housing. Also, in my experience, the insulating properties of perlcrete improves several months or even a year after first use of the oven if you put straw, or even rice ears, in the mix. (I'm not sure if rice ears is the right word, this is the outer covering of rice grains taken off during milling) It will burn or disintegrate over time and insulation is better than new. So with this in mind, I think even with a bit of compression of the perlcrete from the dome, it would have enough strength IF it is also being supported by a brick housing. What do you guys think?

I am very grateful for your patience in reading my posts and for everybody's help. It is very much appreciated. Thank you.

best regards,
Kulas

[Dutchoven]

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

Fantastic idea here! Door jamb bricks act to buttress the dome at the front buttress at the opposite poles at the back and maybe two on the sides! Excellent should distribute the thrust perfectly! Kulas I think you should go this route!
Great thinking Dmun!
Dutch

As you know from running your oven, the top of the firing chamber burns white first, and it takes a long while for the carbon to burn off the sides. I suspect that your buttressing might exacerbate this problem.

I'm thinking that having brick buttresses, at intervals, with the door opening functioning as two, and maybe four others to carry the stress of the dome weight to the floor, without adding to the overall mass of the oven. Cathedrals are built like this, with stout columns bearing the load, leaving space for windows and "curtain walls" in-between.

Forget about perlcrete having any tensile strength. Even fully cured you can dig your fingernail into it like cork.

[Kulas]

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

I'm off to a nearby church for site seeing!!

[gjbingham]

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

Kulas,
You must be cooking a ton of pizzas to want to build that size oven. Though smaller than the first, its still plenty big to feed a lot of people. Are you commercial? A truckload every 6 - 9 weeks sounds like you must be.

[Kulas]

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

gjbingham,

yes, I'm commercial but in a small town where wood is still available (and cheap) Dumaguete City, Negros Oriental to be exact. A ton of wood maybe, but not a ton of pizzas. I can probably use a smaller oven right now, although I hate to build again in the future if its just not big enough. Who knows, maybe in a few years there are more people here. I hope with this round oven I consume less wood for pizza and less wood for the bread oven also.

Anyway, I went to look at an old tower yesterday. It was built with stone and it had buttressing as what dmun described for old churches. I'm going back to look at it again, to have a better idea. Though its not a dome, I think the buttressing is the same. I will take pictures tomorrow of that tower and also some baguettes! Best ones I've made so far! (That's because I've just started learning bread baking, not that it's great, but its an improvement.)

best regards,
Kulas

[Kulas]

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

[johnrbek]

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

[Kulas]

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

[Kulas]

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

[dmun]

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...