#11  
Old 08-16-2007, 06:12 PM
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Default Re: Cantilever Engineering

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Originally Posted by Hendo View Post
Sorry - I grabbed the wrong end of the stick! Was thinking of the suspended slab between the blocks, not the cantilever out the front. So yes, for this section, the tension face would be at the top and the rebar should be closer to the top.
We replied within seconds of one another. I'm glad we agree!!

Ken
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  #12  
Old 08-16-2007, 06:15 PM
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Default Re: Cantilever Engineering

Yep - see my previous post.

Duh!!!
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  #13  
Old 08-16-2007, 06:15 PM
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Default Re: Cantilever Engineering

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...the rebar should be closer to the top.
How close can the rebar be to the surface of the slab?

Ken
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  #14  
Old 08-16-2007, 06:48 PM
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Default Re: Cantilever Engineering

I don't get why it would be different for the cantilever - gravity is gravity whether it's in the middle of the hearth or off the edge?

Yes, I'm dumb. I just can't visualize this.
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  #15  
Old 08-16-2007, 07:03 PM
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Default Re: Cantilever Engineering

Imagine a wooden ruler fixed to a table top, and hanging over the edge of the table. Now put a brick on the overhanging bit. At the table edge, the top face will be in tension, and the bottom in compression. Imagine what the wood fibres are doing at the top face if you will. Trying to pull away from each other.

Alternatively, place a wooden ruler on two bricks and put another brick on top of the ruler in mid-span. This time it's the bottom face in tension and the top in compression.
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Old 08-16-2007, 07:30 PM
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Default Re: Cantilever Engineering

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Originally Posted by Ken524 View Post
How close can the rebar be to the surface of the slab?
You'll probably have your own code for such things, but I can only quote the SAA (Standards Association of Australia) Concrete Structures Code, which specifies a minimum 30mm cover for concrete slabs "exposed to the weather or ground/fresh water" and 20mm if not so exposed. You could argue that the underside is not exposed to the weather or water, but theoretically a cover of 20mm would still be required. So it looks like 20mm cover underneath the rebar in the 'suspended' section, and 30mm cover on top in the 'cantilever' section would be the closest you should go.

The code also specifies a couple of "general requirements", which may be worth thinking about too: "The cover shall not be less than the nominal diameter of the bar ... being protected." and "... in walls and slabs the cover shall also not be less than the nominal maximum size of the aggregate."

Others might also want to weigh in on this one - your code could well differ from that here.

Hope this helps,
Paul.
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  #17  
Old 08-16-2007, 08:27 PM
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Default Re: Cantilever Engineering

Quote:
Originally Posted by Hendo View Post
Imagine a wooden ruler fixed to a table top, and hanging over the edge of the table. Now put a brick on the overhanging bit. At the table edge, the top face will be in tension, and the bottom in compression. Imagine what the wood fibres are doing at the top face if you will. Trying to pull away from each other.

Alternatively, place a wooden ruler on two bricks and put another brick on top of the ruler in mid-span. This time it's the bottom face in tension and the top in compression.
Ah! I get it - they're the reverse of each other! Thanks!
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Old 08-16-2007, 08:32 PM
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Default Re: Cantilever Engineering

No engineer here either but, I am a huge Frank Lloyd Wright fan and I remember reading in his material that a cantilever is very stable until it reaches out more than 1/2 the distance of the supported slab or I guess about 1/3 the total distance. Looking at your diagram and thinking of those words I believe your cantilevered counter area will be fine.
Best of luck to you!
Dutch
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Old 08-16-2007, 10:54 PM
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Default Re: Cantilever Engineering

At 1 foot even straight across not a tapered curve as in this case you haven't yet reached a point of tension. If we are talking tension factors here the center of the hearth slab with the weight of the oven resting on it has way more on it than your 1 foot over hang will ever have on it even if you line up your guest at the pizza party and have them sit on it AFTER they have had their fill of pizza.

As I stated in my previous post this is a non issue (even at 1' all the way across ) no alterations of rebar and placement need to be made. On the 4" slab the rebar will be fine in the middle of the form height so 2".
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Old 08-17-2007, 08:42 AM
enz enz is offline
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Default Re: Cantilever Engineering

Your right on and Hendo corrected himself , ahhh, correctly. Cantilever is in tension on the top and compression on the bottom. The rest of the slab is the opposite. I actually put a z bend in the rebar to bring it from the bottom of the main slab, to the top of the cantilever.

One advantage of that was that I was able to have a locally thin (3") hearth slab right under the oven and inset the perlcrete (4") into the slab. You really only need insulation in a circle right under the oven. There are some pictures in my "Casting Refractory, this is not a drill" thread in tips and techniques. At any rate I've been baking all summer and this arrangement works perfectly.

Enz
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