Cantilever Engineering

[Hendo]

Re: Cantilever Engineering

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.

[Archena]

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.

Ah! I get it - they're the reverse of each other! Thanks!

[Dutchoven]

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

[Unofornaio]

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".

[enz]

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

[Ken524]

Re: Cantilever Engineering

Thanks everyone for the great discussion. In reference to Frank L Wright's "Fallingwater" house, did you see where his cantilever has been sagging for several years and was forecast to eventually collapse? Apparently, poor workmanship (not poor design) was blamed. Engineers were able to design a fix a couple of years back using a series of high tension cables. All is well at the Wright house now.

Paul, your code research helped quite a bit! My rebar is about 2" below the surface. 30mm is about 1.5 inches. So I'm good there. While U.S. code may differ from Aussie code, the laws of physics are identical on both of our continents!

Anyway, after wondering about this last night, I ran to the Depot this morning and grabbed 4 more pieces of rebar and added them in at the point of max tension. As I was finishing up, the concrete truck arrived so the rest is pretty moot!

Thanks Unofornaio and Dutch for the confirmation that I'm in safe water here.

I attached a pic of my extra (and likely unnecessary) rebar and the fresh pour.

Thanks again all!!
Ken

[Archena]

Re: Cantilever Engineering

...

While U.S. code may differ from Aussie code, the laws of physics are identical on both of our continents!

...

Are not! Everything is upside down down there.

[Unofornaio]

Re: Cantilever Engineering

Thats China....isnt it?

[Archena]

Re: Cantilever Engineering

Nah, China's just the place you get to if you tunnel straight through the Earth. You have to take a right turn to get to the Southern Hemisphere where everything is upside down.


:teeth:

[Hendo]

Re: Cantilever Engineering

Paul, your code research helped quite a bit! My rebar is about 2" below the surface. 30mm is about 1.5 inches. So I'm good there.

Pleasure ? glad to help out. Always good to know you?re not getting too thin with the cover. As Uno has stated, it ain?t going to move ? even without that additional rebar (but what price peace of mind, eh?).

Cheers, Paul.

[asudavew]

Re: Cantilever Engineering

I spoke with my father last night.
He was a commercial carpenter for over 30 years.
Lots of concrete work, planning, forming, etc.

I asked about a cantilever.. (i couldn't remember the name, but he spouted out what it was called when I said concrete overhang.)
anyways...

He said a 12" over hang is no problem at all, he built one before that was 4 foot long and built out of 3 1/2 inch concrete. It was designed by an architect. Of course the slab was much bigger so the overhang could be larger, but he said I didn't even need rebar in the slab, mesh would work just fine, but I think I will use rebar just in case.

Anyways.. I have no fears about a shelf anymore

[Ken524]

Re: Cantilever Engineering

He said a 12" over hang is no problem at all...
Anyways.. I have no fears about a shelf anymore

Thanks for adding to my piece of mind! Now that the curing is done, it looks like one very solid piece of concrete.

Keep us the good work on your oven, I'm enjoying watching your progress.

Ken

[asudavew]

Re: Cantilever Engineering

Thanks for adding to my piece of mind! Now that the curing is done, it looks like one very solid piece of concrete.

Keep us the good work on your oven, I'm enjoying watching your progress.

Ken

You are welcome and thanks.
I was interested in building a cantilever too, so I think my hearth will look very similar to yours. Well hopefully!

I hope you don't mind.

[Ken524]

Re: Cantilever Engineering

You are welcome and thanks.
I was interested in building a cantilever too, so I think my hearth will look very similar to yours. Well hopefully!

I hope you don't mind.

I don't mind a bit. Imitation is the sincerest form of flattery
I'm really happy with the way mine turned out.

Ken