how far can I cantilever a 4 inch concrete slab ?

[fastfredracing]

I bought a small mixer, and have learned just about enough to be dangerous . I am making a small patio off the back of my house . Almost done, but I need to overhang a retaining wall by about 11 inches to meet where my other deck will be .
This will be a 4x8 slab, and the long side is where I will need the overhang.
There is a similar slab on the other side , that is 6 inches thick, and overhangs by 6 inches .
Any ideas ?

[Iciclehead]

Any rebar in it?

Dennis

[fastfredracing]

Of course! I was thinking about doubling up on rebar. I also have a bunch of sheets of heavy wire mesh, that I have been laying into the slabs .

[island911]

Originally Posted by Iciclehead

Any rebar in it?

Dennis

And where (top or bottom) does it exist?

[Gretch]

only pre-stressed concrete slabs can be cantilevered, Fred.

Poured concrete shears at a 45 degree angle.

[red-beard]

^^^This

[flatbutt]

Somewhere at sometime I recall reading about a length over depth ratio for concrete overhang....I think I'm remembering this correctly but don't bank on it.

[rusnak]

Maybe instead of doubling up rebar, space the lattice half the distance and dowl pin it into the deck and go with 6" slab.

[MBAtarga]

I don't understand the vision:

[fastfredracing]

Ill snap up a picture tonight.

[jgreen]

Frank Lloyd Write knows....

[908/930]

Not sure if this would work but, if you drop in 3' x 3/4 dia threaded rod with bent J end into the slab about every 12" starting 4" from the end, and cap the outside cantilevered end with 1/2" steel plate and tension it after the concrete has fully cured (30days). Possibly would keep it under compression so it will not break off. Pretty sure you want to be near centre of thickness of the slab and at least 2" from the surface, so real tight with 4" slab.

[vash]

Can you tie it to the other deck?

I’m not quite picturing it. How high is this overhang? (Off the ground).

[TimT]

only pre-stressed concrete slabs can be cantilevered, Fred.

Now that's just not true ...

Many bridges you drive over every day have cantilevered components.. I.e. outside stringer to fascia is a cantilevered component.. The wheel path is designed to be closer to the support...

However.. considering a 4 in slab there is not enough depth to install the reinforcement needed..

And an 11 in overhang is nothing.. hardy a cantilevered deck..



If you're worried increase the depth f the slab to perhaps 6-8 in prior to the overhang to allow for proper placement of reinforcement

[wdfifteen]

You can’t. Even a slab well reinforced by rebar is going to flex. A cantilever will flex in tension (the top will try to get longer). Concrete is great in compression but sucks in tension. It will crack and crumble and fall apart and you will be sad. You’ll need a thicker slab. In other words, “Yer gonna need a lot more than four inches, bub.” You’ve probably heard that before.

[A930Rocket]

To my feeble brain, I just don’t think a mixer making small batches of concrete would help either.

[vash]

There is also failure in creep.

I think it can work if the failures are simply some cracking. But Tim is right. I’ve seen overhangs. We usually post tension in the transverse direction. Plus we have lots of rebar. Lots. Lots.

I’d love to see a pic of what Fred’s got cooking over there.

I always joke with my concrete venders. If I do a pour at my house, I’m pulling a mix design from my files. I’m pouring a high strength mix. Driveway cracks? Sure. My tree roots are gonna struggle.

[daepp]

Originally Posted by jgreen

Frank Lloyd Write knows....

And of course, at Fallingwater his cantilevers sagged after time. And his patron (the furniture guy - can't remember his name) actually had him double the amount he'd been planning to use.

[rusnak]

from Bill Doorley on Vimeo.

[Crowbob]

Originally Posted by jgreen

Frank Lloyd Write knows....

Almost every one of his homes is unlivable, outrageously expensive to maintain and ridiculously expensive to build.

But they certainly are beautiful.