How do you determine amount of rebar needed?

[soilsailor]

Hello all.

I'm trying to pull together my estimates for our shed project. We are planning on a 40x60x14 building (most likely red iron frame). I intend to subdivide and heat half the building with radiant floor heat. The other half is for cold storage.

What I'm trying to figure out is what the size and spacing should be for the rebar in the concrete slab. For that matter, what thickness the slab should be as well.

Does anyone know a good resource for figuring the size and quantity of rebar and slab thickness?

Intended use for the shed will be equipment storage on the cold side, a service bay (no pit nor a lift) and a shop on the heated side.

Thanks.

 

[SJMullin]

In Canada we have a National Building code that provides basic guidelines that each province in turn tweeks so they can charge a fee for their expertise! the the township or county that you live in changes it and in turn can charge for their expertise as well. Now I know this because my wife and I just build out house on a floating pad to save money over a full foundation, Ha! $ 14,000 later we have a pad that is 40' x 40', 8" think in the middle and 3 ' thick around the outside with rebar grid with no more than a 6" opening in the grid in any direction in 3D also the cement has fiberglass fibers in it as well for the extra strength. And lastly NO sawcuts anywhere in the concrete.
You could land a Huey and drive a tank over our pad and not hurt it, these are the new earthquake regulations, I didn't know we lived in a earthquake zone!
Good luck!

 

[Raspy]

There's the minimum amount of rebar to meet structural requirements and then there is the practical amount that will minimize cracking and provide a grid pattern to tie your radiant tubing to.

The minimum will be approximately one number three (3/8") bar each 24" on center, both ways. Radiant heat and reduced cracking means 12" spacing instead of 24", each way. Or, even better, 9" OC for very cold areas. If you have an L shaped pour with an inside 90 degree angle, it is guaranteed to crack radially from that point. Lay a row of bars at a 45 degree angle in that area, maybe 6 bars on 6" centers.

These suggestions don't give you some actual calculations, but it's a common configuration and is fine for 4" through 8" slabs.

Don't worry about putting in too much bar, except for the cost. Structural concrete systems usually have about 10% steel, if I remember correctly. That's way more than needed for slabs, but too much doesn't hurt. Even if you do see cracks, they will be stable and won't affect non-metalic radiant piping, like PEX.

 

[BeezFun]

Hello all.
Does anyone know a good resource for figuring the size and quantity of rebar and slab thickness?

Yes, there's lots of design books online, but you'll probably go catatonic trying to read them unless you've done this before.

I worked concrete for a few years, you don't say what you're storing, but assuming it's just tractors and farm equipment I'd pour a 4-6" slab, put a vapor barrier under it, use 3/8" rebar spaced 12" and tied every joint. You could use wire instead of rebar for something like this, but you need to believe that the contractor is really going to pull it properly during the pour so it gets in the concrete. You can't use wire with radiant heat so that will force you to rebar in the heated section. Also with radiant and rebar you might be wise to go to a 6" slab just to make sure there's plenty of room for everything and you don't get thin surface cover. Like the other poster says, if this is subject to building permits they'll most likely specify what you have to do. People tend to go overboard with reinforcement in concrete, we poured factories bigger than Walmarts with wire and no rebar, and they had big machines and fork trucks running all over the place. It's more important to get a good level compacted base and then pour/finish it properly.

 

[Beltzington]

$ 14,000 later we have a pad that is 40' x 40', 8" think in the middle and 3 ' thick around the outside with rebar grid with no more than a 6" opening in the grid in any direction in 3D also the cement has fiberglass fibers in it as well for the extra strength.

WoW - Is that US or Candian Dollars?

Good info everyone, thanks for sharing!

 

[Tractor Seabee]

Do not use wire mesh, please, you are wasting your money, it is very difficult to get it placed in the proper place in the slab. I have seen guys with a hook trying to pull it up while standing on it, go figure. Put up on chairs and the placing requires not walking on it while dumping the concrete, again you are walking it down. I work industrial work and for this type slab on grade; smart designers are using fiber re-inforced concrete instad of mesh or rebar. Key is get those control joints cut with a soft saw as soon as possible after finish, even if you are working at midnight. Next morning is way too late, cracks are already started. I know, the old timers will say different "I've been doing this for 40 years don't tell me how to do it, and all concrete cracks. Not so, talk to the real experts as a lot of new research and new products are making the old timers look bad. I do quality control on military contracts and fight this syndrome all the time. Pay a little extra and make that form of 2X6 and get a 6" slab, you will be glad you did, especially with the radiant heat. Fiber, no interference with the heating pipes. Put at least 6" of styrofoam or 4" of urethane board under the cooler and heated area. You will save it shortly in utility cost. Insulate the vertical edges of the stem wall also.
Ron

 

[mjncad]

We always designed for a minimum of 3" of cover over any reinforcing (sidewalks excluded). I'd go with a minimum 6" slab, 1/2" (#4) rebar 24" O.C. both ways on chairs. If you're planning on radiant heat, 6" would be a minimum to get the right amount of cover.

All concrete cracks...how much it cracks depends on the prep work involved. A poorly prepared base is going to cause problems. Also affecting the cracking is how skilled the crew is and the quality of the concrete, and the temperature it is poured at, and additives if any. Concrete is simple in concept; but complicated when it comes to getting the right stuff.

Now if you really want a true 6" slab, you could rip your own form boards out of plywood or OSB instead of relying on a 5-1/2" wide 2x6.

 

[Raspy]

I agree that a 6" slab is a very good thing. But don't even consider "cutting" control joints in after the pour if you are installing radiant piping. In spite of what some will claim, you really don't know where the tubing is after it's covered and you will be very unhappy if you have to jackhammer it up to fix a cut tube. One you cut with a saw.

The act of pouring concrete and walking all over in it during the pour leaves the tubing probably where you intended it, but not worth betting on. I've personally dug up tubes that were under the slab, pushed into the gravel bed, and others that were right at the surface and barely covered. Don't risk it. But do your best to support the bar adequately and be careful during the pour. One of the advantages of spacing the bars no closer than 12" is that you can step between them as you work. As previously mentioned, 6" mesh is a nightmare in radiant structural slabs, but it works very well in topping slabs.

 

[flusher]

Hello all.

I'm trying to pull together my estimates for our shed project. We are planning on a 40x60x14 building (most likely red iron frame). I intend to subdivide and heat half the building with radiant floor heat. The other half is for cold storage.

What I'm trying to figure out is what the size and spacing should be for the rebar in the concrete slab. For that matter, what thickness the slab should be as well.

Does anyone know a good resource for figuring the size and quantity of rebar and slab thickness?

Intended use for the shed will be equipment storage on the cold side, a service bay (no pit nor a lift) and a shop on the heated side.

Thanks.

My 24x42 ft shop has 6" thick 4000 psi concrete floor with #4 rebar on 24" centers.





Good luck

 

[soilsailor]

... Put at least 6" of styrofoam or 4" of urethane board under the cooler and heated area. You will save it shortly in utility cost. Insulate the vertical edges of the stem wall also.
Ron

Wow, I had heard that a minimum of 1 inch was needed, with 2 inches being better, but 4 to 6 inches seems like a lot. I realize that by adding more, we are certainly increasing the resistance to conduct the heat to the ground. I haven't checked locally, but a 4' x 8' x 2" sheet of foam board insulation was about $35. This could get expensive.