Barrel vault in ferro cement

Discussion in 'Designing, building, making and powering your life' started by mark anstice, Dec 29, 2013.

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  1. mark anstice

    mark anstice Junior Member

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    Greetings All



    I am about to put a barrel vaulted roof on a 4x8m (13'x26') room here in Morocco and have a dilemma.

    The exact span measurements will be 14.4' wide and 5ft high, so a relatively flat arch! The plan is to do it on a metre or yard wide moveable form and to lay locally fired bricks down first so we have a really stunning interior for our permaculture classroom. A thin layer of cement will go over the top of those, then chicken wire, then 10mm rebar (3/8") in a welded mesh, then expanded metal lathe, then the cement in one go. A final layer over something for insulation, probably polystyrene, will follow.


    The problem: My walls are not strong, being built of limestone with an earth and lime mortar, 1.3' or 40cms thick by 3 yds/m tall and split by 3 large arched doorways on one side. There's compressive strength but cohesive strength is minimal and there is no possibility for buttressing. I could put a couple of steel bars, say 1" thick, across the bottom of the span but would like to avoid that and achieve a clean space if possible.


    There is an existing 8"x8" reinforced concrete ring beam atop 8 x embedded vertical pillars. I decided on ferro cement as I would like this roof to be self supporting with regard to shear forces or sideways thrust. The added weight of 2400 bricks for purely aesthetic purposes is a risk but they amount to less than 2000kgs/4000lbs which, spread out over 26' doesn't seem too much??


    I have been advised to solve thermal expansion issues by simply having the roof sitting, unattached, held there by it's own weight and inertia, on a strip of asphalt along the ring beam. This seems to make sense until you come to the question of whether or not this roof will actually be self supporting or not. Should my ring beam be imparting strength to the roof as well as the walls, or will the roof take care of itself?


    I have a tendency to over engineer things and am putting another ring beam on top of the first (because the first wasn't made very well in my opinion) but the more I get into shear forces and the like the more bamboozled I become and I don't want to knock these walls over. If any of you can illuminate me a little I'd be very grateful. I'll post up a picture of the project on the facebook page below tonight when the internet connection allows.

    BTW, before anyone suggests I post this up on ferrocement.net, I have been trying to register with that site and forum for 2 years and am still awaiting approval. I have no idea why my attempts have been unsuccessful. All emails to them go unanswered.


    Many thanks and happy New Year.


    Mark
    Fertile Roots Foundation
    www.facebook.com/fertileroots
     
  2. Rick Larson

    Rick Larson Junior Member

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    Wish I could help you, but I generally figure out things as I go along, much like you are doing. Bill Mollison would chide people like us who do things before we think about them! Ha! Well, I'm learning to think more about what to do before I do it instead of what I use to always dream about.

    Anyway, why couldn't you build "cohesive strength" into the walls? And which way will they tend to lean, if you put weight on them, in or out? Weak at the bottom, middle, or top?
     
  3. Pakanohida

    Pakanohida Junior Member

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  4. 9anda1f

    9anda1f Administrator Staff Member

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    Especially check out the shear wall section: https://www.flyingconcrete.com/shear-walls.htm
    From what I'm understanding of your project, there will be considerable lateral force "outwards" from the barrel dome roof. Although I'm not quite sure what a ring beam is in your instance, merely perching a barrel dome roof on it will not account for that shear.
    If your "ring beam" is an integral piece that will resist outward forces, it may be enough to counter the force generated by the barrel roof if it's structurally tied to the ring beam. Otherwise I would surely consider a system of shear walls against the outside of your structure to account for the outward directed barrel roof forces.
     
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