Arch question

[blip]

I'm nearing the beginning of my earth oven construction (thought this forum might have more info than the "Other Ovens" one, and I'm planning on having a brick arch doorway. I've read a lot about this, but have a couple of questions:

1. I was thinking of using mortar instead of a cob mixture. It might break up the continuity of things, since everything won't be cob, but thought it would hold better. Good idea?

2. How can I make a keystone? I'm trying to avoid buying/renting any fancy cutting tools, and I've read about people having a lot of trouble getting a brick into that shape. What's the easiest way? Can you buy keystone shaped bricks? Some people seem to just make a large area of mortar/cob to function as a keystone. Will this really work?

Thanks!

[Tscarborough]

Re: Arch question

You do not really need a keystone, it is more for looks than anything else. I would not lay brick with cob, use mortar for the arch and make sure you have sufficient buttressing to hold it.

[Gulf]

Re: Arch question

I did look into cob ovens before I found FB and I am wondering one thing. If you are looking into adding portland or a portland/lime mix to a cob oven why would you need a cutting tool of any kind. Can't you just fab the dome and entry arch in place over a sand form?
Edit: sorry, I misread you post, must be my beer goggles .

[azatty]

Re: Arch question

You do not really need a keystone, it is more for looks than anything else. I would not lay brick with cob, use mortar for the arch and make sure you have sufficient buttressing to hold it.

Well, you don't need a beveled keystone, I think is what T means. I didn't cut any of the bricks for the horizontal portion of my decorative arch. I would try to make it a true arch rather than the flattened arch you see on some ovens.

[Tscarborough]

Re: Arch question

No, I mean you don't need any type of keystone at all. An arch built with consistent units is just as strong as a keystone arch.

[azatty]

Re: Arch question

Ah, I see what you're saying. I always think of the brick(s) in the center of the arch as the keystone(s), whether denominated as such or not.

[brickie in oz]

Re: Arch question

I would try to make it a true arch rather than the flattened arch you see on some ovens.

Built correctly they are no different.

[azatty]

Re: Arch question

True, but the flat arch has to be wider to avoid buttressing. On a cob, I would think he's trying to reduce his footprint, so buttressing might be out.

[blip]

Re: Arch question

Thanks for the help. Is a flat arch the kind where a few bricks are stacked vertically first, and then an arch with a small curvature is made? That wasn't what I was going to do. And, no, no buttresses.

Glad I don't need a specially shaped keystone, but I will have a brick acting as a keystone. I'm figuring 6 bricks on each side, plus one in the middle. I just wasn't sure if the regular shaped brick would work, since the pressure might push it upwards somehow. But, then again, the arch won't really be supporting much weight at all, mainly just its own weight.

Oh, one more questions, for the spacers in between bricks to hold their angles whilst mortaring: anything special? Will regular old rocks work?

Thanks for the help!

[GianniFocaccia]

Re: Arch question

True, but the flat arch has to be wider to avoid buttressing

Maybe I have this backwards, but when plotting the line of thrust through a vault (arch), I understand that a taller arch, ideally an inverted catenary arch, distributes thrust downward more than a flatter arch. Wouldn't a flatter arch require more buttressing?

[Tscarborough]

Re: Arch question

Yes it will require more buttressing.

[blip]

Re: Arch question

Speaking of catenary arches...I'm thinking of using a catenary arch template for my dome. I've read that you should use a chain. Any size or type of chain? Will rope work? I'm guessing that the chain has weight to it, which makes it work, not that chains have any special properties which make them form perfect arches...

[Tscarborough]

Re: Arch question

Actually, when calculating arch loads, it is less when there is a minimum of at least the width of the arch of masonry above it. That is to say, when there is more than the width of the arch of masonry above it, all the arch supports is a triangle of masonry with corners at the springlines and the center of the arch an arch-width above the peak of the arch.

If the distance is less, the arch will carry the weight of the masonry vertically from the springlines to the top of the masonry.

[GianniFocaccia]

Re: Arch question

Chains do have magical properties in that each of the individual links act independently and are not influenced by horizontal forces (like stiff rope fibers) that might distort the true shape.

I too initially thought about a catenary arch shape but quickly moved on to the thrust considerations involved in building a flatter-shaped dome suited more for pizza. I feel the size of a WFO dome and with the materials involved, the forces exerted by a flatter dome would not be prohibitive.

If you're still interested, you can find catenary data here:

Auroville Earth Institute

John