How much arch buttressing?

[azatty]

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]

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]

Hi attaty,
My answer to your questing would be: yes, definitely a little 'hillbiilly' 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]

Great Post Dino! I'm at that stage with my build in Tanzania and this has helped a lot!

Cheers, SteveS

[azatty]

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.