Starting new 36" build

[Les]

Re: Starting new 36" build

I would think that the thin edge will spall easier. But after a point it will stop - where that point is I have no clue. Why are you tapering them so much? If you are willing to cut the angle why not do it to a full size brick.

[deejayoh]

Re: Starting new 36" build

Next course, the dome starts taking shape

I need to angle the shoulder by a few degrees. I guess I could have done this with soldiers, but I didn't

Does anyone see any problems with a course of angled bricks like these?

[deejayoh]

Re: Starting new 36" build

Made progress today, and not just in sketchup! I got the bottom three courses in. Ran out of daylight just as I was getting to tie the top course in the second side of the arch.

After a day of working with bricks and mortar, I am certain I did not miss my calling in terms of being a mason. It's a lot harder than I thought - but what I laid up seems to be sturdy enough, so I am hoping I'm ok!

[deejayoh]

Re: Starting new 36" build

Of course, this is total conjecture, but lastly, does a flatter dome and the potential for a perfect pizza outweigh the benefit of a slightly higher dome ideal for roasts etc?

John

I think I am still going to have a flatter dome. Targeting 18" high with a 39" diameter. I think the only real change I am making is that the dome will be a continuous arc of 21.25" from shoulder to shoulder, instead of two quarter circles @ ~18.5" joined by a flat top.

I did rework it one more time since my last post - in order to incorporate two sailor courses (instead of the one in my last sketchup version) so that I'll have slightly more height at the edges - and I am pretty happy with that. That is what brings the dome radius in at 21.25 inches - which I am going to cut out on a plywood form in order to ensure the shape stays on track. I have a vague idea about building some sort of slider for a bracket along the edge of the form so that I can consistently manage the height - which may or may not happen. (see drawing)

If I could build this thing in sketchup I'd be golden, but I think the weather is finally going to cooperate this weekend so I can get started on mortaring!

[GianniFocaccia]

Re: Starting new 36" build

It looks like the "thrust line" that the article refers to would probably cut through the inside third of the arch.

I have several thoughts on this. The application of the ideal dome shape as it relates to stability is noble from the educated engineer's viewpoint as well as the dome's potential for longevity. However, when I first saw your lower, flatter dome profile I was envious, not just because that shape is sexy, but because in all practicality, now that I've built my dome, I don't think the forces at work here (line of thrust) would overcome competent construction skills and modern materials and cause your dome to fail in your lifetime. Of course, this is total conjecture, but lastly, does a flatter dome and the potential for a perfect pizza outweigh the benefit of a slightly higher dome ideal for roasts etc?

John, I am guessing this is why you went with the plywood form instead of the IT?

Exactly. My original dome shape was slightly flatter as well (19.5" radius and 18.5" ceiling) and was based on a freehand template drawing that allowed for a slightly taller 'shoulder' than a true hemisphere. My plan was to build an adjustable IT and use the dome template too but decided to go with just the template. As I progressed upwards through courses 3 & 4 I discovered that I only had to use the template two or three times per course to stay 'on profile'. Merely cutting the bricks consistently and careful alignment kept the dome true and the sticky homebrew meant I didn't have to prop the bricks up at all. The only error I made was getting about 1/4" outside the template at the shoulder which left me with a 19" ceiling, 1/2" higher than planned.

John

[deejayoh]

Re: Starting new 36" build

After reading through the link that John provided, I realized that the arch profile that one gets with the "lazy susan" style IT is probably more rounded than is ideal. It looks like the "thrust line" that the article refers to would probably cut through the inside third of the arch.

I'm sure that for all practical purposes - the design would be strong enough - but I figure I am only building this thing once, and I'm not so far along that the changes will really cost me any time. So I have reworked both my dome profile and my arch profile and guess what? They look remarkably like what John and Aidan have done. Just took me a while to tumble to the logic...

Anyway, the only remaining bit of confusion that I have is what to do for the IT. I have a 19.5" radius on the floor, and am targeting an 18" internal dome height. It turns out that the way the math works, when the radius of your dome is greater than the height of your dome (in my case, the radius is 20") - so the center point for the arch is below the surface of your cooking floor. I can see no practical way to use a single length arm keep everything all nice and even.

John, I am guessing this is why you went with the plywood form instead of the IT? How did that work for you? I was thinking of rigging up some sort of off-axis jig - but not sure it is worth the time.

[Amac]

Re: Starting new 36" build

Hoping I can figure that out! thanks Aidan

Sorry Dennis I should have made that a bit clearer. I used the template shown to cut all the arch bricks and as you can see outside the arch projects 1" from the dome - and the inner side of the arch is 3.25" (not 3" as I said above) but still I thought it was a little narrow. If I made it flush with no projection the inner arch would be 2.25"
The lines on the brick in the previous pic show that I could have cut 2 arch bricks from one brick - using that same template just slid towards the middle of the brick.
This would have made the arch project a little more and thickened the inner side of the arch by the same amount.

I should add that that would have been varied had I chosen a different arch radius.
Sorry for cutting across the discussion you and John were having. I know how easy it is for this to be confusing, and I hope I am not adding complications.
Aidan

[deejayoh]

Re: Starting new 36" build

You could, but it'll take a while and may not translate to your arch geometry unless its perfectly hemispherical. I cut all my arch bricks first, but only basic blocks. Once I had my TDC brick in place, I drew a circular line down each side to derive my final curve. As you progress upwards, will find that the inward-facing angle of each arch brick changes not only between bricks, but within each brick themselves.

with that description + the pics, I think I finally have it.

I wouldn't worry about the 4" surface bonding area. For projects the size of our ovens, the line of thrust shouldn't be an issue given the bonding strength. You may find this interesting reading (if you haven't already): Auroville Earth Institute

Thanks for the link. Looking at the dome shapes on that site is helpful. I see that mine looks a little like the one they show as being weak... I'm going to revisit the height of the vertical portion of the sides and the flat plug. I think I'm going to play around with starting the angle at the second course, and a more rounded top.

Thanks John

If I did it again I would have cut the arch bricks like I showed in my thread (see pic below) which would have made that almost 4" with almost 2" projecting. Angles would stay the same, but fewer cuts and less waste, and a bit more thermal mass.

Hoping I can figure that out! thanks Aidan

[Amac]

Re: Starting new 36" build

John

I wish I had made the face of my inner arch end up flush with the front of my oven

My arch projects just 1" from the dome all around. I feel this is slghtly too little as the dome narrows towards the inner part of the arch to just 3". If it was flush that would reduce to 2". Probably wouldn't make a lot of difference to the operation - jut 2" seems narrow from 4.5".
If I did it again I would have cut the arch bricks like I showed in my thread (see pic below) which would have made that almost 4" with almost 2" projecting. Angles would stay the same, but fewer cuts and less waste, and a bit more thermal mass.

But I agree with John DJO your projection part looks too much - just I don't think I would make it flush.
Another thing is that you will not get an even projection for the entire arch unless it is a pure semicircle in a pure hemisphere. It may be possible to match an elliptical arch with an elliptical dome, but it hurts my head just trying to imagine it.

[GianniFocaccia]

Re: Starting new 36" build

But it sounds like I can use the sketchup I put together for the dome profile to figure out the angle.

You could, but it'll take a while and may not translate to your arch geometry unless its perfectly hemispherical. I cut all my arch bricks first, but only basic blocks. Once I had my TDC brick in place, I drew a circular line down each side to derive my final curve. As you progress upwards, will find that the inward-facing angle of each arch brick changes not only between bricks, but within each brick themselves.

See attached - The yellow trapezoid would be my arch brick. Does this look right?

Almost. You want the depth of the (TDC) brick's inside face to match the height of the courses around and above it. This will give you enough height to work with on the bricks that flank the highest bricks in the arch. Also, your arch extends too far forward. I wish I had made the face of my inner arch end up flush with the front of my oven - I would have gotten a better angle for my vent given I will have a thermal break in the entryway.

I wouldn't worry about the 4" surface bonding area. For projects the size of our ovens, the line of thrust shouldn't be an issue given the bonding strength. You may find this interesting reading (if you haven't already): Auroville Earth Institute

John

[deejayoh]

Re: Starting new 36" build

Another thought: Since I have to mess with my arch again, I am thinking I will raise it by a 1/4" to get a perfect 62% ratio (11.25/18).

If I do that, it seems to make more sense to have the top of the arch intersect with the next tier of brick, as pictured.

What I don't like about that approach is that there is only a 4" surface on the face that intersects the dome. I guess I could fill in the gap with mortar - but it looks messy.

Recommendations?

[deejayoh]

Re: Starting new 36" build

Thanks John. That is super helpful.

I am thinking I got a little ahead of myself when I cut the backs of my arch bricks, because I hadn't yet calculated what the angle should be.

But it sounds like I can use the sketchup I put together for the dome profile to figure out the angle. I just need to match the height of my arch (11" at the bottom, 15.5" at the top) to the angle of the bricks in my picture and wa-la, I have my angle.

See attached - The yellow trapezoid would be my arch brick. Does this look right? That angle carries all the way across?

Thanks!

Dennis

[GianniFocaccia]

Re: Starting new 36" build

I don't even want to attempt drawing a scale model of how that interacts with the arch. Makes my head explode. I will just figure it on the real thing

I know exactly where you are, DJO and your approach is good. Don't kill yourself thinking about it - it will come. Consider some ideas for your gameplan. They worked for me:

1. The most important factor in arch integration is it's placement (how deep). Draw a profile of your dome and regardless of your arch size/shape, place the arch so that the top-most center brick (TDC) matches the profile of the dome. Because the angle of the dome at this level is shallowest of all arch bricks, this brick will extend the deepest into the dome. Proper placement of the TDC arch brick will also ensure consistency with the curvature of the dome (not the profile) and once the arch is complete, allow you to proceed on up in round.

If you will be following the FB plans and not angling your arch bricks, it doesn't hurt to build your arch first. If you will be angling your arch bricks, cut each of the arch base bricks first, then the TDC brick to match the dome curvature and profile. You can then cut each successive arch brick to match the front of the arch and be consistent with the inner arch curve and profile for each course.

This approach works with hemispherical as well as semi-straight-sided arches.

Hope this helps,

John

[Amac]

Re: Starting new 36" build

That is an ambitious and impressive project. Excellent - may the oven gods go with you.
Aidan

[deejayoh]

Re: Starting new 36" build

Are you using a "lazy susan" type design for the IT to achieve the flattened top?
I think with the lazy susan IT the flattening happens naturally as you get higher up depending I guess on the offset of the pivot.


Ideally the slope where the dome wall meets the arch will be exactly the thickness of the dome wall (1/2 a brick), so the inner edge should coincide with the inner radius as measured by the IT, and the outer edge with the outer radius.
With those straight sided arches I am not sure that can be achieved for the rounded part.

Yes, I am using the lazy susan IT - so the vertical portion of my walls should match to the vertical portion of my dome - so that the dome meets up with the arch. If not, I've wasted a few bricks and some time cutting. I will be able to check with the IT before I lay any bricks.

It took me quite a bit of figuring and sketching up - but I think I finally have the geometry right for the dome shape I want and the offset of the IT from the center. I've attached a screen snap.

I don't even want to attempt drawing a scale model of how that interacts with the arch. Makes my head explode. I will just figure it on the real thing