Chimney and Arch Questions

[ftaylor74]

Hi All!

With my dome finally complete, I am preparing to add the the landing arch and chimney/flue.

I've checked various masonry (arch building) sites, as well as various projects here in the forum, so I think I am approaching this construction the right way. But I'm having nightmares that as soon as I pull my arch frame, the whole structure will come tumbling down.

So, I'm posting some pics of my progress in hopes that someone with way more experience than I have will chime in, in the event that I am that I really am falling off the sled.

The entry into the oven is 17 X 12 inches (204 sq in). I plan on a nominal 6X6 sheet metal flue (18% of the oven opening area), with about 1 inch of perlcrete insulation between the flue and the brick chimney. I would have gone with a 6 X 6 terra cotta liner, but the 24 -30 lbs is putting me off.

Chimney and flue will rise about 30 inches above the arch.

Right now, I'm using uncut (no mitering) firebrick, set on their sides. So they rise just under 4 inches. I'm considering adding a second row of half height bricks to bulk it up and give it more strength. Does that make sense, or is it overkill.

Also, I'm concerned about building on the frame itself. I don't see any way to avoid that. But does anyone know of a way to keep the arch bricks in compression while building?

Finally, the piers for the arch are about 9 X 9 X 12 of solid firebrick. Is that sufficient mass to resist the compressive forces of the arch and chimney?

Thanks in advance
Regards

[jeeppiper]

Re: Chimney and Arch Questions

To me, the arch looks very solid. you should have no problem whatsoever.

The only thing I see is that your spacing between bricks might be a little tight at the inner radius of the arch. You dont want the bricks to touch because you want to get mortar in there and all around the bricks. You may have considered this already and it is just the way the photos appear. If you do want to increase the spacing, a simple solution would be to raise the arch ever so slightly, thus increasing the radius. I would leave at 1/4 inch between corners of inner arch.

[ftaylor74]

Re: Chimney and Arch Questions

thanks for the feedback. to your point about the tightness of the joints, i agree with you. for the purposes layout, i cut a slightly overlarge keystone to hold everything in place. I will be using spacers to insure minimum clearances as i set them with the heatstop. the 5 degree spacers should insure a 3/16 inch space between the inner arch bricks and about 3/8 inch at the top. i will trim the keystone to maintain approximately the same clearances with the final piece.

Do you have any idea how much weight this will bear?

thanks for your insight

[ftaylor74]

Re: Chimney and Arch Questions

also, i forgot to ask, what is the advantage of the wider mortar joint? you said to try to keep 1/4 inch vs my planned 3/16. if it will strengthen the arch, i'll definitely increase it. but i've always heard, that you shouldn't use any more than you need. And i'm pretty sure that the heatstop instructions say use a 1/8 spacing.

Thanks again for the input!

[jeeppiper]

Re: Chimney and Arch Questions

First, I am not a Mason....so take my advice with extreeeeeme caution!!!

I dont think you are going to have ANY issues in terms of strength. Everything in your arch is in compression. As far as the 1/4 inch goes, I was just thinking out loud; seemed like a good number. If you can do 1/8 inch...go for it; as long as you can get mortar in there. The mortar joint is supposed to be slightly weaker than the firebrick; you want the mortar to break before the brick.

Seriously, you have a beautiful arch and nothing to worry about w.r.t. strength. I would pick my mortar joint thickness based on cosmetics.

[ftaylor74]

Re: Chimney and Arch Questions

thanks Jeeppiper! And so it proceeds... i'll post pics after i've done some cement work.
Regards

[DavidApp]

Re: Chimney and Arch Questions

Hello ftaylor74

My entryway is similar to your design. I have a 5 brick by about 4" opening into a 6" chimney. I do get some smoke coming out the front of the vent area.

You may have a wider opening front to back than I had. You can get more opening by angling the bricks on both sides of the opening.

David

[ftaylor74]

Re: Chimney and Arch Questions

Thanks for the response. I have built the arch, and the chimney opening is 6 1/2 inches by 9 inches, giving me nearly 60 square inches of outflow from the oven. I think the ratio of chimney area to oven opening is 15 to 25%, so I'm hoping I will have a fairly smoke free landing area. I,all know in about two weeks when I fire this baby up!
''''''''

[dakzaag]

Re: Chimney and Arch Questions

I don't know how to calculate the resistance strength of your arch, but just by looking at what you have going on there, I would estimate that 350 lbs would not be a problem (static load.) If your plans are to bring additional masonry up to the sides of the pillars, then you will be able to support 5 times that much weight.
Tighter joints are less likely to crack when heat starts causing expansion and contraction. Follow manufacture recommendations on joint size as much as possible.

Nice looking oven if you ask me.

[Greenman]

Re: Chimney and Arch Questions

I don't think that your arch will be a problem. I have built 3 ovens with that same arch and buttress set up and they are fine. I didn't put any additional weight on it other than the flue though but there is strength enough to cope with more.

This might be interesting. Auroville Earth Institute

[ftaylor74]

Re: Chimney and Arch Questions

I don't know how to calculate the resistance strength of your arch, but just by looking at what you have going on there, I would estimate that 350 lbs would not be a problem (static load.) If your plans are to bring additional masonry up to the sides of the pillars, then you will be able to support 5 times that much weight.
Tighter joints are less likely to crack when heat starts causing expansion and contraction. Follow manufacture recommendations on joint size as much as possible.

Nice looking oven if you ask me.

Thanks for the info. I feel a whole lot better about the strength of this now. 350 lbs!?! Wow!. I will be adding two more layers of bricks at the top of the piers, and then a chimney using 2 inch wide bricks (to save a little weight and expense). I think this is going to work!
Regards

[ftaylor74]

Re: Chimney and Arch Questions

I don't think that your arch will be a problem. I have built 3 ovens with that same arch and buttress set up and they are fine. I didn't put any additional weight on it other than the flue though but there is strength enough to cope with more.

This might be interesting. Auroville Earth Institute

Thanks Greenman. I used a method to calculate the line of thrust of the arch that is very similar to the funicular method shown in the article. According to my calculations, the actual LD remains inside the center 1/3 of each the bricks of the arch, so i expect it is stable. That said, I didn't attempt to clculate the laod bearing capacity of the arch. But I will minimize the weight of the chimney and keep my fingers crossed.
Thnks for the reply!
Regards