Solid model of Pompeii design

[Alan]

I've been working on my Pompeii design and I've made some solid models using SolidWorks that I thought I'd share with the group and also post some questions that I have...

First the dome geometry, looking in from below. Assuming standard firebricks, the dome is 4.5" thick. It starts out with a vertical section 4.5" high (half bricks). The remainder is exactly parabolic. Diameter is 42" and interior height is 21". I framed the opening 20" wide and assumed that the angled cuts would leave 4.5" of brick, so that the framing for the opening interrupts the dome at the point where it is 29" wide on the inside. I made the straight portions of the inside opening 8" high, and added a parabolic arch with its peak at 12".



Lo and behold the arch appears to intersect the dome nicely. However, here are my first two questions: The opening height is slightly under 3/5 the dome height, even at its highest point. Should I try to make it higher? This would mean pushing the opening back into the dome a little more; otherwise, the parabolic shape of the dome drops slightly below the opening. Also, the dome, where it's interrupted by the opening, lies against the opening wall rather than on it. Is this OK in people's experience?

Here's the dome and entrance on the base, without the exterior finish.



Starting from the bottom, I show the grade-level slab, then block walls (12" block along the side and some extra block about halfway back just to set the size of the firewood compartment), then supporting slab, insulating layer, thermal slab and oven floor. I didn't make the insulating or thermal slabs any larger than necessary, and I may go ahead and make some other cuts to the thermal slab but probably won't go as far as shaping it to match the floor. I may also in the actual build just lay the floor without cutting the curves. The square "columns" at the entry are meant to be two rows each of 4.5x9 fire brick. They're set 2" out on each side from the outer edge of the opening. Does anyone foresee problems with shifting of the less than full size slabs? I could always pour insulating mix around the periphery to fill in the extra space with rigid material.

Finally, here's a cross-section that includes the outer finish, which will most likely be brick or stone.



The outer finish walls sit upon the 4" ledge on the grade-level slab and supports the porch at the opening.

I'm thinking through the chimney detail and this is where perhaps I'd hope for the most guidance. In the design as shown, I can corbel in from the sides at the entry tunnel (the 9x9 "columns") and when I reach the appropriate width place a piece of angle iron across the front and support a terra cotta flue pipe on the angle iron and the interior wall. This means that the front wall of my chimney throat is just the brick or stone finish material. Does this seem OK, or do I need to add a firebrick wall or use a steel throat instead?

And my final question: I'm assuming a slip plane where I go from firebrick to plain brick or stone - right behind the face. Do I need one somewhere else, such as right in front of the inner arch wall?

Comments and questions appreciated as well as answers. Thanks for a great forum.

Alan

[Alan]

Embedded pictures

I wanted the pictures in-line but couldn't figure out how. Maybe just because they were too big?

[Fabio Allodoli]

Nice job.

Hi Alan;
My name is Fabio, I have started the process of building a complete outdoor kitchen, with the brick oven and cover.
I and laborers have finished pouring foundation for BBq pit, sink, fridge, and of course the Pompeii oven with cement blocks.
Finished pouring footing and layed first coursre of the entire kitchen, one day later the entire kitchen walls including the oven. Friday poured a 30x22 foot concrete pad, and within the inner walls of the bbq etc, including oven slab.
I went and poured a 5" slab with rebar, with no insulation slab at all, I live in SoCal and I am going to double up on the fire bricks on the floor. THis will also give the height I like. I have been taking pictures of the entire process of the work and it is a long process since I am limited to 10 Lbs, since I have carpel tunner since i fell at work.
Like the detail work and tank you for the chimney detail, I was concerned since I have not seen much detail or mentioning of the chimeny.
I have a few questions about the pictures where you show thew courses that you laid out with thew spacers.
1. What thickness are the spacer @ the thickest edge toward the outer of the dome, I looks like about a half inch by the picture. Do all the courses have the same spacing, or does the spacing change as you get closer to the top?
2. What is the total lenght of the opening from the very front to to beginnig of the inner part of the dome.
3. Is the flute opening right after the steel bar you have showing Picture 34.
4. What is the peak height of the entire opening.
I know I said a few questions, but want to make sure I cover all the bases.

As soon as permissable I'll post the pictures I have been taking.
My E-mail is:
fallodoli@yahoo.com

How did you construct the actual dome curveture. I amthinking on using some heavy duty but plyable rubber sheetswhen I get close to the top, and just put a broom handle toward center and that will givbe me the correct curveture for closure, I think. One other option I might get a friend of mine make a Insta-pack form for me before I start to do the curveture of the dome when it's finish I'll just pull down the instapack?

Thank you for pictures.
Fabio.

[Alan]

The attached shows half bricks going up the side of the dome profile. These are placed by eye in the CAD system, so measurements are very approximate, but it looks like almost 2.5" for the first course, then the spacings go from less than 1/4 inch from the first course to the second, all the way to about 3/4" at the top. Being parabolic, I think the spacings will increase linearly as you go up - in other words the space will increase by the same amount on each course - but I'm not certain.

The length of the opening "tunnel" as shown in these drawings is 4.5 (dome opening" + 9 (flue area) + 4.5 (outside wall) = 18". Since then, I've revised my drawings a little, adding a firebrick wall between the flue and the outside wall (adds 4.5") but decreasing the 9" flue area to 7.5" to match the inside width of an 8" flue pipe. I will probably make further revisions.

The flue is directly above the two square cross-section "spacers" that lie between the inner and outer openings. These are to be corbeled in to the width of my flue pipe. The flue pipe is to lie upon a horizontal wall built up from the inside arch, and either an arched firebrick wall on the other side of the chimney spacers or a piece of steel layed between courses of the outer finish.

I'm still working on this and haven't even started digging yet, so don't follow my direction until there are comments from more experienced people!

[paulages]

it seems that there are several ways to decide what the spacing will be between each course. as you mentioned a parabolic design will mean increased spacing as the angle becomes more acute, and decreased spacing as the top of the dome flattens out. when i layed out my dome template, i wanted a flatter top, a bit more like the napolitan design, so i just layed my bricks out how i wanted the shape to be. then, i measured and made a different shim for each course, so that i could keep the exact shape i wanted....

then i started actually doing it, and all of this work ended up being pointless. this is mostly because i ended up just following my styrofoam templates instead of using the shims, but it is also due to the fact that theory and reality are two different beasts.
several people on here have used a variation on jim's method, using a standard shim. it also seems that, though this should also create the same dome that was layed out in planning, a few people have ended up with a slightly higher dome than planned.

the point is that finding the exact measurements on a computer model is a good idea, if you have the skill of a professional builder to follow blueprints to precise detail. otherwise, however, that model will likely turn into something a bit different when it becomes reality.
nevertheless, you seem to have a much better vision of how everything peices together than i did before i began, and that will surely be very helpful.

[Alan]

I agree completely that things will change a bit in the actual execution, due at least to brick sizes and the need to fit things together at the transitions. My experience in watching masons is that little is planned out on a brick-by-brick basis. One of my main reasons for laying this out in a solid model was to understand how a straight arched opening intersects a parabolic dome. My pictures show the interrupted portion of the dome butting up alongside the arch, and I'm not sure that's OK. I think it will extend the arch bricks into the dome and cut angles on both sides to match the dome shape the way you did, so that the dome is better supported by the arch.

The styrofoam appraoch really seems ideal because it allows you to draw your profile accurately but also adapt to reality. The old-time equivalent is to use thin wooden strips, which if they're flexible enough would come close to parabolic.

[paulages]

i think my approach to tying the arch in with the dome worked well. however: one unexpected thing did happen. one side of the arch opening thrust forward about an 8th of an inch--right at the point where the arch curves quickly downward--once everything had settled. i assume this was due to the thrust of the dome pushing outwardly before the arch transfers the load downward, as the point of intersection was pretty high up. i think my only mistake was not continuing the next section of arch (where the vent will be) immediately, to help bear the load. it seems to have settled where it is (the oven is at about 800-900 deg as i write this), so it shouldn't be a problem.

[Alan]

Good lesson. Get things all tied together and/or provide temporary reinforcement. It's hard to see it being a continuing problem for you.

The advantage of the flat opening with steel angle is that the angle wouldn't allow that movement. But I love arches.

[Alan]

Opening heights

I neglected to answer one of Fabio's questions in my previos post.

In these drawings, the peak of the arched opening to the dome is 12.7". At about 13" the opening would have to be moved further back toward the center of the dome to avoid having the dome drop into the opening. Making the straight vertical portion of the dome flattens the dome curvature and raises the opening height that can be used. Lowering the vertical portion of the opening does the same thing. Of course, one can also depart a little from the set dome profile in the neighborhood of the opening.

I'm planning my opening to widen in two steps - inner to "entrance tunnel" and entrance tunnel to outer opening - in order to facilitate access and to allow for insertion of a plank door. But I've left the outer arch height the same as the inner arch height to help assure that smoke goes up the chimney.