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Support question - Forno Bravo Forum: The Wood-Fired Oven Community


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  • Support question


    on paper, my deviationist design for the hearth support structure looks good, with the concrete blocks laid along the axes of a hexagon.

    BUT, I just roughed out the 1850 mm diameter circle and placed 400 x 200 x 200 mm concrete blocks and it feels a bit cramped and over-engineered. So I laid the blocks in a simple cross shape (they don't meet on purpose, so that our abundant little red-bellied black snakes can escape if disturbed). Pls see attached snap.

    My question is this: will this provide sufficient support for the round hearth slab and central dome? My hunch is that it's ample, but I'd welcome any input from the knowledgeable people on this excellent forum!


    Attached Files
    "I started out with nothing, and I've still got most of it"

  • #2
    In the 70's glass round glass table tops with a stand similar to your were very popular. They were not pretty in my view.

    The reason for building the stand with four walls or building a circular stand are two-fold. The first is that it provides an nice location for temporary storage of some wood and oven tools. It sounds as though for you it would also provide a very nice den for the red bellied snakes. The second reason is for support. The weight of the dome is supported by the outer walls of the dome and is transferred to the floor. Thus your floor bears the weight from the dome and the floor transfers this to the stand a short distance away. Due to the relatively small diameter of the dome 42 inches or less (1070 mm) there will not be much sag in the reinforced floor.

    So now with this modification the outer perimeter of the dome is supported at four locations and the load is transferred toward the center. Your greatest unsupported location is located at the four (4) places along the perimeter that is at a 45 degree angle form the walls that are at 90 degrees.

    The recommendations that this site supports will tend to be on the conservative side. We tend to stick to what has been proven to work. The last thing we want is a catastrophic failure of an oven. That just looks bad.

    So that being said. the saving grace is the fact that your oven hearth is made up of a reinforced concrete layer (followed by an insulation layer and then the cooking floor). As you are about to enter into uncharted territory make sure that your reinforced rebar extends from cinder black wall to cinder block wall and cover the open space.


    • #3
      not uncharted

      This is a picture of my father's brick oven:


      I don't know how he reinforced the stand, but he used two semicircular rebar reinforced pieces for the hearth which he poured in forms and then lifted in place. I think if you integrate the stand rebar with the hearth rebar you should be pretty solid, but then I'm not an engineer. I think my dad made his oven more with passion than engineering. If his fails he'll live with that and redo it. I'm not endorsing that approach, but you need to determine where your values lie on this. I built pretty straight from the pompeii plans, but for many this is a backyard project where creativity is part of the joy. You could always run your plans by an engineer rather than solicit the input of an internet forum. For what it's worth, I'd go for it. I'm sure you'd be able to salvage a lot for the rebuild if it does fail .


      • #4
        Originally posted by jengineer

        So now with this modification the outer perimeter of the dome is supported at four locations and the load is transferred toward the center. Your greatest unsupported location is located at the four (4) places along the perimeter that is at a 45 degree angle form the walls that are at 90 degrees..
        Thanks for the reference to the Seventies, jengineer!
        But seriously: I tend to think holistically, and the hole (actually four 'holes' of unsupported spans between the radials) in my 'design' would seem to make up LESS of an unsupported hole under the dome than in the square design...

        And of course the weight of the dome wall sits very close to the centre of those four radial walls (which in turn would have grouted reinforced cores reaching down to the reinforced foundation which in turn sits on my proposed reinforced pile footings). But then, I better defer to the engineer, and mull this over a bit more... Perhaps my hexagonal idea gives it just that extra edge in support?

        Cheers, and thanks for your input!

        "I started out with nothing, and I've still got most of it"


        • #5
          Originally posted by maver
          I think my dad made his oven more with passion than engineering. If his fails he'll live with that and redo it.
          Thanks Maver! Just one quick question: how long ago did your Dad built his oven? I assume it is still standing?!

          Re the creativity bit: I built our house, such as it is (unfinished) with shed roofs of some 6.50 m clear span, resting on concrete reinforced bond beams atop the 3 m high concrete block walls. I used hardwood rafters at about 1.1m centres - which my building inspector later expressed more than a little surprise with. I pointed out to him that after some 20 years there was no rpt no deflection in those exposed rafters - but eventually agreed to eventually put up a strutting beam and place some sticks up at an angle for support (this hasn't happened yet, and the roof and walls still stand!).

          I guess in the old days I'd have flown by the seat of my pants :-)

          Have fun!

          "I started out with nothing, and I've still got most of it"


          • #6
            Centered flue?

            Originally posted by maver
            This is a picture of my father's brick oven:
            How did he vent the oven? Not through the top of the dome, I presume.
            My geodesic oven project: part 1, part 2


            • #7
              Carioca, his oven is about four years old now, and aside from battling with cracks in the stucco (he seems to have finally won) it is standing and in good shape.

              Yes, Dmum, it is vented through the top of the dome. It's unfortunate, because he did a lot of book reading and ultimately built it based on his recollection of ovens he grew up with (in Naples). It also has poor insulation. He heats it for several hours prior to making pizza and it takes 3-5" per pizza depending on the preheat quality. Still good pizza. We don't really talk about design, but he was surprised by my oven's short preheat and faster cook times.


              • #8
                my design is taking wings...of plywood!

                Hello Maver,

                good to hear about your old man's oven bearing up!

                In the meantime, I've dug more soil from the round foundation hole during a 30 C heat wave, which fired my brain enough to start thinking Brazilian, to wit: Oscar Niemeier! (I had been inspired by him in the mid-fifties in Brazil, and wanted to become an architect, but my parents saw fit to exile me to a fancy boarding school in my native Germany. As a result I didn't finish high school but absconded after two years in that joint and bummed around Europe for some 5 years or more... I wouldn't have hacked the math, anyway!)

                Anyhow, I am now considering reverting to my original hexagonal axes idea, but casting pilings, footings and support walls together with the hearth slab into formwork built mainly from old two-ply plywood which should bend nicely. I'll have to experiment with that stuff.

                This would allow me to cast the support walls only 100 mm thick - which increases the available space between the 'legs' compared to 200 mm wide concrete blocks. But since everything is reinforced (galvanised wire mesh) and hangs together, including with the hearth slab. I think it should work fine.

                And the aesthetics of it! Soft curves soaring under the dome, reminiscent of the presidential palace in Brasilia and the gull wings of that classic Merc...

                Back to the diggings. I'll keep you all posted.

                Cheers for now,

                "I started out with nothing, and I've still got most of it"


                • #9
                  Il Forno dell' Gallo Chi Canta...

                  is the name Bianca today bestowed on my previously 'secret' project.

                  Bianca - whose forebears made charcoal in the woods north of Genoa - has finally agreed to accept and cooperate with this stone oven scheme - after taking more good earth from the diggings for her garden and sharing half a bottle of 1998 Hunter Valley Elizabeth semillon with me. (In fact she sealed the pact by pouring us a tiny glass each of her egg liqueur, made from the eggs of her free-range chooks and a bottle of brandy.) We even agreed to plant an olive tree to guard against cold southerlies, and a whie Genoa fig tree to shield the site from the westerly sun...

                  So, it's official and out in the open!

                  The name honours Bianca's boss rooster Goldie (we assumed it was a hen, initially), who has a beautiful drawn-out warning chant when kookaburras swoop near his hens...

                  Ci vediamo!

                  Last edited by carioca; 11-12-2006, 12:46 AM. Reason: additional information
                  "I started out with nothing, and I've still got most of it"


                  • #10

                    Per questo tipo di occasione, che cosa si dice ? "Complimenti", o "buona fortuna"? O qualcos'altro differente? C'e una occasione felice. Spero che suo forno e buonissmo.

                    Great name, and congratulations on the agreement. We will expect photos.
                    Last edited by james; 11-13-2006, 01:58 PM.
                    Pizza Ovens
                    Outdoor Fireplaces


                    • #11
                      grazie per i complimenti/auguri, James, but...

                      I'm having sleepless nights gnawing my fingernails down to the wrist while I wait for news on my pallet with oven materials. The supplier decided to send it by road to a rail station near me, where a forklift would be needed to unload it.

                      Being a bit paranoid, I drove up to the station Monday (120 km round trip) to learn that for many years they've been a pure passenger rail stop, without any freight handling facilities!

                      Frantic phone calls and e-mail exchanges trying to re-route/intercept the truckie, hoping to direct him to a brick depot 30 km north from me - but total silence since yesterday afternoon. I used the energy created by my frustration and anger to fell a largish eucalypt (122 cm girth) near the oven site, managing to drop it just clear of any and all nearby fruit trees...

                      Scrivo in Inglese perché il mio Italiano è quasi non-esistente!


                      "I started out with nothing, and I've still got most of it"