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]

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]

Re: How much arch buttressing?

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]

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]

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]

Re: How much arch buttressing?

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]

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.

[Dino_Pizza]

Re: How much arch buttressing?

Volongo,
1st: thx for the compliment. 2nd: there is no real reason that I did not show a 2 high arch-wall in the example. 2 high would be a fine way to make it. And as David said above, a perfect arch is the strongest. It also looks great, works for pizza but sliding in a wide casserole pan with the 2 big chickens may bump into the arch at the lower sides.

[nissanneill]

Re: How much arch buttressing?

I have always been lead to believe that the ultimate arch strength is achieved when you imitate the arch using a chain but it must be done inverted to your actual planned ash.
This is achieved when you hold a chain in the air at the same dimensions (width of arch and radius or height above the vertical arch support walls). The shape of the chain drawn on a card/board will provide the ultimate shape for your arch.

[Tscarborough]

Re: How much arch buttressing?

NissanNeil, that is true, but only from the point of the springline, and it makes no reference to the required buttress to support the arch, only the optimal shape.

Ultimately, anyone that is not an engineer, or anal enough to learn engineering concepts has only one choice: over build the hell out of it.

[david s]

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.

The strongest arch is the catenary arch and because of its form is a popular design for kilns, but unless it is quite tall (height exceeding span) the base walls angle in way too much to make it useful for an oven.
I am using catenary arches for my kiln enclosure.
http://www.fornobravo.com/forum/43/p...tml#post187669