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azatty 02-22-2011 12:50 PM

How much arch buttressing?
Some of the vent arches I've seen don't look like they'd hold much weight without the "legs" kicking out. Is there a secret formula or rule that masons use to determine how much buttressing is needed for a particular arch shape given the estimated load on the arch? Or is it like hillbilly fabrication--just weld on more angle iron until it feels solid?

GianniFocaccia 02-22-2011 01:27 PM

Re: How much arch buttressing?
Not sure of the formula for calculating line of thrust on an arch with vertical sides, but here is the prevailing document regarding stability calculations for vaulted structures:

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.

Dino_Pizza 02-22-2011 04:10 PM

Re: How much arch buttressing?
Hi attaty,
My answer to your questing would be: yes, definitely a little 'hillbiilly':D and no, not too much 'fabrication'.

Here is a pdf that shows some options. Keep in mind, it's my 1st and last oven and I know nothing about brick structural forces. So here's my humble opinion based on some experience:
1.) the lower your arch, the more outward force gets directed down. My arch walls were 3 brick high, not 4 so there would be slightly less force (1/4!) out the side walls to worry about. Have you seen kebwi's and a few others wfo that have an almost perfect arch?/no vertical arch walls? No buttressing is needed there.

2. If you are building an enclosed non-igloo style then do what I did: I used the stud framing around my oven for the cement board that eventually enclosed it. I used that as a buttress and wedged FB insulation blanket tightly between the outer arch and stud. Easier than an angle iron.

3. IMHO: outward thrust is not an issue for these ovens we build unless the arch walls are very tall, the base is not firm or solid. Also potential weakness: if you don't build a gradual 'flu-box' or vent transition which will A: draw smoke better and B: spread the load/weight of the vent better. Also, as shown in my pdf pic #3, interlocking the side arch bricks into both arches adds a lot of structural strength although it precludes an air-gap or heat-break which most of us don't have (although I greatly admire it in others:))

Still, it's up to you: I fully understand the desire to make this wfo build of yours as strong as possible but I've never seen a 'failure' on the arch-walls yet. If you do install angle brackets make sure you insulate between so the metal doesn't transfer into your ovens concrete base.

good luck, Dino

SteveS 02-22-2011 10:31 PM

Re: How much arch buttressing?
Great Post Dino! I'm at that stage with my build in Tanzania and this has helped a lot!

Cheers, SteveS

azatty 02-23-2011 09:01 AM

Re: How much arch buttressing?
John and Dino:

Thanks for the information. It confirmed what I thought--a true arch will hold weight, but a poorly designed arch will fail (probably at the "fourth brick" in Dino's .pdf). Dino's arch diagram is exactly what I was wanting to see.

Volongo 05-19-2015 07:19 PM

Re: How much arch buttressing?

Originally Posted by Dino_Pizza (Post 108509)

Hi Dino,

Regarding your diagram above, where does 2 brick high arch fits in? I see only 1, 3 & 4 bricks high analysis of the line of trust. Thanks.

I find the construction of this dome very impressive. Auroville Earth Institute

david s 05-19-2015 08:07 PM

Re: How much arch buttressing?
If you want to reduce the sideways thrust, reduce the radius of the arc. The flatter the arch the greater the thrust. Semicircular is very strong, a large diameter arc produces the greatest sideways thrust.

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