Cantilever Engineering

[Ken524]

I'm going to be pouring a 4" thick hearth slab on top of my block stand. I would like one side of the slab to cantilever 12" (diagram attached).

Any hints or tips to make this work? Building the form is easy enough, I'm just concerned about making it structurally sound so it lasts longer than me .

Is rebar enough, if so how much?
Do I have too much cantilever?

Thanks!
Ken

[dmun]

Re: Cantilever Engineering

I'm no engineer, but my protruding shelf is just two inches of slate, held by a couple of little brackets. It's as solid as a rock. I laid a lot of bricks standing on it.



I think the regular rebar grid, what, twelve inches, should do fine. It's just a shelf, right?

A hint: The edge of cast concrete slabs look pretty rough. If you are planning a tile or stone edge, double-face-tape the tiles backwards to the inside of your forms before you pour. Should work fine as long as you remove the forms and scrub the next day.

[Unofornaio]

Re: Cantilever Engineering

Hey Ken,

It looks like the overhang is just a counter space no? This will without a doubt be fine. You do not need to bump up your rebar size or anything like that just extend it out to the curve and remember to stay at least 2" away from the forms.
If you would like a smooth finish on the overhang area you can line the form with plastic where you want it really smooth.

If its gonna be covered with tile or left plain make sure to poke around in the forms and do a little tapping to make sure you get a nice clean edge. Not too much banging because this will bring the rocks to the bottom just hear and there give it a little whack. I don't know if it would go with your plan or not but it would look nice for the whole front edge of the slab to be rounded over, you know like a bull-nose. Just a thought.

Dumn- I was typing while you had replied addressing the same issues..just want to let you know I wasn't tying to 1 up ya.

[Ken524]

Re: Cantilever Engineering

I think the regular rebar grid, what, twelve inches, should do fine. It's just a shelf, right?

A hint: The edge of cast concrete slabs look pretty rough. If you are planning a tile or stone edge, double-face-tape the tiles backwards to the inside of your forms before you pour. Should work fine as long as you remove the forms and scrub the next day.

Yes, In theory, it's just a shelf. We'll make it a family rule never to sit on the cantilever! . Only beer bottles, pizza and pot roasts (maybe a wine bottle on "Fancy Night").

My plan is to run the standard rebar grid, then have a few extra pieces extending over the block edge into the cantilever.

I don't think the rough concrete edge will be an issue. I'm planning on adding a few inches of cement board and tile to the overhang to bring it up to the level of the oven floor. The front, curved edge will be stuccoed with decorative tiles embedded in the stucco.

Thanks for the advice... BTW, your slate looks super... your whole project is amazing! I'm only hoping I can build something that isn't an eyesore to the neighbors .

Ken

[Ken524]

Re: Cantilever Engineering

Hey Ken,
You do not need to bump up your rebar size... and remember to stay at least 2" away from the forms. ...line the form with plastic where you want it really smooth.

Glad to hear another confirmation that this will work! The plastic is an excellent idea - will do.

If its gonna be covered with tile or left plain make sure to poke around in the forms and do a little tapping to make sure you get a nice clean edge.

We did the poking & "tap-tap" routine with the foundation forms and are *very* pleased with the results.

...it would look nice for the whole front edge of the slab to be rounded over, you know like a bull-nose.

I love the idea, but a full bull-nose would mess up the flat surface to mount the tiles we bought in New Mexico. I could put a bit of curve on the bottom edge to soften it up a bit. Thanks for the idea!

Thanks for all the great help guys!

Ken

[enz]

Re: Cantilever Engineering

One thing that I did was put the rebar at 1 inch from the top surface of my 3-1/2 inch cantilever, as the concrete is good in compression and the rebar is good in tension. This arrangement doubles the strength over having the rebar in the middle. You can sit on it all day long.

Enz

[Ken524]

Re: Cantilever Engineering

...put the rebar at 1 inch from the top surface of my 3-1/2 inch cantilever, as the concrete is good in compression and the rebar is good in tension. This arrangement doubles the strength over having the rebar in the middle.
Enz

Thanks Enz! I set up my rebar today. I think it's set at about 1/3 from the top of the 4" slab. As soon as these storms blow through, I'll measure it and make sure.

Concrete arrives at 1:00pm EST tomorrow. Pictures to follow!

Ken

[Hendo]

Re: Cantilever Engineering

Sorry to introduce what might be a red herring, but isn't the tension face of a suspended slab at the bottom, rather than the top? So shouldn't the rebar be closer to the bottom?

[Hendo]

Re: Cantilever Engineering

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.

[Ken524]

Re: Cantilever Engineering

Sorry to introduce what might be a red herring, but isn't the tension face of a suspended slab at the bottom, rather than the top? So shouldn't the rebar be closer to the bottom?

Hopefully we will come to a consensus before the concrete arrives tomorrow.

I'm thinking that the top of the slab at the cantilever point is under a stretching tension (from the weight of the cantilever) and the lower part of the slab at the same point is being compressed. Being that concrete is strong in compression, the lower part of the slab is fine. It's the upper part that's under the "pull" force that needs the reinforcement. (That's my theory and I'm sticking to it! )

[Ken524]

Re: Cantilever Engineering

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

[Hendo]

Re: Cantilever Engineering

Yep - see my previous post.

Duh!!!

[Ken524]

Re: Cantilever Engineering

...the rebar should be closer to the top.

How close can the rebar be to the surface of the slab?

Ken

[Archena]

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.

[Hendo]

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.