Compact 36" in Seattle

[ohthetrees]

Thieves!

The bricks were beautiful and brand new, neatly stacked, and left alone for no more than 25 minutes. I just can't bring myself to believe it was an honest mistake. Nonetheless, I did put up a couple of signs, but I'm not hopeful.

The heartwarming part of the story is several neighbors heard about it, and wanted to start a paypal donation fund to help me pay for new ones! Very sweet of them, though I declined since the expense isn't that much of an issue. It is more the annoyance of having to go buy more!

[Tscarborough]

Re: Compact 36" in Seattle

There is no single best design, just certain parameters that should guide the design.

It should be a smooth transition from the total area of the entry to the area of the flue.

The sides, if possible, should be at no more than 33 degree angles. This will dictate, in part, the height to width ratio of the smoke chamber (the name for the area above the inner arch to the bottom of the flue).

This area is the most flexible part of the build, though, so you can get away with almost anything, so long as the structure will support it.

[ohthetrees]

Re: Compact 36" in Seattle

I've cut bricks for the first two courses, and my floor, out of 12"x24" skagits, which are hard to handle.

Now I need to make my mortar. I'm planning on either 3:1:1:1 or 3:2:1:1. If I can't decide, I'll do 3:1.5:1:1
Any advice?

I have some gardening "dolomite" lime, any idea if that will work? Or do I need some sort of special lime suited to mortar?


Thanks!

[GianniFocaccia]

Re: Compact 36" in Seattle

Hydrated lime is the recommended product. Here is the thread:

http://www.fornobravo.com/forum/f2/n...tar-14114.html

[Tman1]

Re: Compact 36" in Seattle

In the second picture, you have the bricks standing on end, and it seems like it's a pre-fit situation. I'm not an expert by any stretch, but if you do what your showing, won't the dome be much higher than recommended?

[ohthetrees]

Re: Compact 36" in Seattle

GianniFocaccia, thanks for the info and link about the lime.

Tman1,
Lord help me, I'm messing with the design. If I used a true hemisphere, the little area where the dome met the floor wouldn't be very usable, because it would start sloping right away. This way, the dome starts vertical, then dives over more abruptly than it would in a hemisphere. Think of it looking like a squashed hemisphere, like someone very heavy sat down on top of it. The attached picture is NOT my final profile (just messing around), but it illustrates what I'm getting at.

[Mike D]

Re: Compact 36" in Seattle

I think others have said that a full brick height first row is not so stable. I have read over many threads and most don't recommend a full first course. But it's your oven, so don't let me get in the way. I have seen people use a full brick but on the outside of the floor. I think a half brick height on the first row is standard.

I am at the same stage as you right now. It's getting exciting to see some actual progress isn't it.

Mike

[GianniFocaccia]

Re: Compact 36" in Seattle

OTT,

A lot of Pompeii builders would love to have the dome shape that you have laid out in your pic. Your design, however, suffers from inherent weaknesses caused by gravity (precast domes exempt). Your flat dome ceiling cannot provide an acceptable line of thrust and is susceptible to collapse. Also, your soldier course is not ideal to withstand the side thrust forces a flatter dome will place on it unless it is buttressed.

The conventional compromise is to shorten the height of your soldiers or build a hemispherical dome and place it on top of two horizontal courses.

This explanation of dome geometry may help answer some questions for you:

Auroville Earth Institute, training courses, workshops on Vaults, Arches, Domes(VAD), stabilized rammed earth walls, compressed earth blocks, vaulted structures, compressed stabilised earth blocks, rammed earth.

[ohthetrees]

Re: Compact 36" in Seattle

GianniFocaccia,
I certainly appreciate your input, and your advice. I am well aware that a perfect dome is a stronger shape than my squished hemisphere. My idea was to distort the shape just a bit, but it is possible that I've gone too far. One detail.... That picture I posted of the dome shape was not my final shape that I settled on, it simply illustrated my point pretty well. The shape I settled on is a little less agressive, but now I'm worrying that it is still too far from a perfect hemisphere.

My existing plan was to have full height soldiers (as depicted in my picture of me pre-fitting my bricks. Those bricks are cut on top at 22.5?. Then the next brick is also cut 22.5?. That gets us half way to 90, and my plan was to make that curve as gradual and hemisphere-like as possible.

What I'm hearing from you, is that you don't think that my plan is sound...

What do you think of soldiers, but instead of standing them at floor level (as depicted) I start them outside of the cut floor, which would make the dome 2.5" shorter before it started to curve in.

I'm still pretty attached to the idea of distorting the shape of the hemisphere, at least a bit.

Also, all those bricks are cut at 22.5?. I could recut, I suppose. What is the maximum angle you think is reasonable?

Thanks for the advice!

Cedar

[azatty]

Re: Compact 36" in Seattle

The quick planning tool I got from John's link is to get a piece of chain, let it droop down a distance equal to your dome height with the ends of the chain at a distance equal your hearth diameter, and the curve it traces is the best dome profile for those measurements. Anything flatter will probably collapse without buttressing.

Of course, you could carry the flat top of the dome with some steel rings, but that introduces different expansion characteristics into the mix. Not to mention additional fabrication.

[fxpose]

Re: Compact 36" in Seattle

Instead of using tall soldiers I ran my first 3 courses vertical, on top of the oven floor.

[Tman1]

Re: Compact 36" in Seattle

GianniFocaccia,


My existing plan was to have full height soldiers (as depicted in my picture of me pre-fitting my bricks. Those bricks are cut on top at 22.5?. Then the next brick is also cut 22.5?. That gets us half way to 90, and my plan was to make that curve as gradual and hemisphere-like as possible.

What I'm hearing from you, is that you don't think that my plan is sound...

What do you think of soldiers, but instead of standing them at floor level (as depicted) I start them outside of the cut floor, which would make the dome 2.5" shorter before it started to curve in.

If I understand correctly, and am processing correctly, this would make your dome diameter bigger, but not really change anything but looks. The forces are still there (actually a little more due to the increased size), but when you look at it from the door, it would seem, or look to be lower.
Also the problem would still exist requiring buttressing. The only way to get out of that is to make the dome more hemispherical (round).
Am I making any sense?

[GianniFocaccia]

Re: Compact 36" in Seattle

I'm with Tman's assessment. A full-length soldier placed on the outside of the floor is still a full-length soldier! A half-brick soldier relieves some of the thrust forces, but if you go with a 4 1/2" sidewall, you might as well go with horizontal courses.

I too wanted a slightly modified hemisphere so I drew out a semicircle (39 1/2"diameter) and took 1.5" inches off the top. According to mklingle's analysis, this modification may prove pointless, but I learned something in the process: My slightly-lowered dome curve precluded me from using my indispensable tool.

Whatever you do will prove to be effective, if you follow known practices and established guidelines.

[ohthetrees]

Re: Compact 36" in Seattle

The chain trick is fabulous. I love it when simple little physical "calculators" like this can replace a bunch of hard math. I remember in college a chemistry prof pulled out an old gas chromatograph recorder that would draw curves on a piece of paper. Nowadays, the computer integrates the area under the curve for us. Back then, the instrument would draw the curve on paper, then they would cut out the curves with scissors, and weigh the paper on a precise scale! The weight of the paper was proportional to the area under the curve! Anyway, I'll think about using the chain trick, and I'm also considering going back to a perfect hemisphere, which has the advantage of allowing the indispensable tool to come into play.

[ohthetrees]

Re: Compact 36" in Seattle

Finally got started "for real". I decided to scrap catenary domes, squashed domes, low domes, and just go with simple old hemispherical. That way I could use the indispensable tool, and it just cut down on too much thinking about it. I did two courses yesterday, and one today. Today was the first course that started to angle inward, and was more challenging than the previous two. My cheap-n-cheerful indispensable tool really helped, and I feel good about how it is going. I'm really happy with the performance of the mortar I'm using... And by performance, I mean workability. I have no idea how it will stand up to heat.

I'm doing 3:1.5:1:1 being sand, fireclay, lime, portland.

The sand I'm using is this really nice, white, clean silica, #50 . I got all my materials from salmon bay gravel, and I have to say, cheaper, faster, friendlier, and better selection than the big box stores. All lowes had was "sand". It didn't even mention the fineness on the bag, and was a dirty, dusty brown. At salmon bay, I chose from several different sizes and grades.

I'll post more about building my tool on the indispensable tool thread, but I think mine might be the easiest to construct that I've seen yet.