Thinnest slab for residential 40 to 60 psf? 1

[boonmec]

For a single span, end supported floor and/or roof of 29.5 by 53.5 feet (inside dimensions) resting on concrete walls (ICF poured), what design will give the thinnest overall depth of slab?

Does the inclusion of PEX tubing, along main reinforcement direction, for hydronic heating require an additional effective layer away from the main compressive section?

Thanks

 

[dicksewerrat]

Put all your data together and call a structural engr.

 

[boo1]

The PEX tubing will AFFECT the slab design

 

[MikeDB]

Is this slab actually spaning 53 feet or is it a slab on grade?
I had a 1 1/2 story house with hydronic radiant heat in the floor on both levels. The main floor was a slab on grade (chat, insulation and maybe 5 to 6 inches of concrete. The upper level was traditional wood joists with a 1 1/2 inch light weigh concrete that contained the water tubes.
It was a very comfortable and economical heat system, if you don't count initial construction cost.

 

[boonmec]

Thanks Mike,

The slab is actually intended to span the 29.5 ft dimension, probably as a "one way slab" type of design. Since the posting I downloaded a structural demo from Enercalc, which is very easy to use. A fixed-fixed end assumption was used, although I do not know if this is the best guess: the floor will be resting upon and tied into the concrete walls. Simulated as an 8 inch thick by 12 inch wide beam, it yields a slab with 10 psf dead load and 40 psf live load having a deflection less than the L/360. The hydronic tubing will be placed above the effective slab.

Although a little knowledge is dangerous, it can also be handy if only to approximate for conceptual planning before going to see a structural engineer for official design. The floors will be in a duplex for my own use, located in a height restricted zoning area, hence the need for thin floors.

Any comments on end conditions and loadings?

 

[MikeDB]

If I understand what you are suggesting, it does not sound economically feasible. You are going to have living area below this slab? Make sure the program is including the weight of the slab itself. For fixed end conditions the walls would have to be able to carry a significant moment and be reinforced accordingly -- very unusual for residential work. I would have assumed pinned end connections and a heavily reinforced slab over 6 inches thick. The slab alone will have a dead weight over 75psf. The cost of formwork and temporary shoring will be high. Suggest you check with your structural engineer and a local contractor before you get to far.

 

[wsheppard]

First of all notice that I'm NOT a strucural engineer. But I am building a mostly concrete home, and have faced some of the same issues. A few things to consider -

1. Overhead slabs without insulation will transmit a lot of contact noise. Not much ambient noise though. Tap shoes suck.
2. Moving outside the 'norm' in any way is expensive. Contractors will likely hedge their bid 20% or more.
3. Too far outside and your building inspector will want your structural engineer to come out perform the inspections.$$$
4. A combination of honeycomb and beam sounds like it would well for you. We formed ours using shored EPS and laid in the steel and radiant tubing and poured it easily. It looks like ___|---|__|---|___ hopefully you get the idea. But it was labor intensive and definitely NOT inexpensive. It was important to stay under the height limitations, and this helped.
5. Your structural engineer better be your friend and drinking buddy or you should bring your checkbook because you probably can't carry that much cash.

My 2 cents and worth every dollar. Good Luck, sounds like you'll need it, and I hope you find more of it than I had.

 

[boonmec]

Thanks wsheppard,

Congratulations on your unique home, and thanks for your observations.

In the intervening time period, by fooling around with the enercalc demo, a T beam (T joist) configuration similar to that which you suggested has shown to be the lightest and thinnest for reasonable forming costs, as well as being supported (mainly) on the two end walls.

I am curious about the honeycomb material you mentionned, or is this like a waffle slab instead of T beams? Please let me know more.

 

[wsheppard]

Thanks for the kind words. Here's more specific info -

Flat Roof -

I can buy EPS foam in 8" thick sheets for almost exactly $1 per foot. EPS gives an R value of 4 per inch, so 8x4=R32. Lay a solid layer of this on top of the shoring. Next we need beams. It's trivial to setup an inexpensive foam cutter with some NiChrome wire and a variable amperage battery charger. This contraption can to temp. clamped to a table saw top or saw horses. Cut strips of the EPS sheets at (get a number from engineer) say 10 inches. Depending on the eng. requirements you might even use 6" foam for this part. You will then glue these strips to the existing layer of foam with 2-4" spaces between them. This is what will form the beams of concrete. You will then need to install some rebar on chairs at the bottom of this channel. It will be something like two #4s that look like this |---|..|---|..
Again, your engineer will tell you what the depth of the channels needs to be, the direction they need to run, etc. But the point is that it's easy to install. There will of course also be some tie in with the steel that should be poking up out of walls. Be careful as you can not to create a thermal bridge from the roof directly into the wall system .
Ok, now that gets poured, and you wait 28 days. Then you spray DensiCrete on it. This is a single application mixture that soaks into the entire slab and beams, and chemically increases the density of the concrete, simultaneously increasing the psi from say +-3000 to +-6000, and waterproofing it. You can still put a color coating over that (after another 28 days) if you want, but it's strictly for asthetics at that point. Structurally you're done, and you have a waterproof roof system with an R value of something like 40-45. On the inside you remove the shoring, have the easy to work with foam into which you will install electrical, etc. and then what we're doing is shooting a stucco like finish over that. ONE NOTE: if you need to hang something heavy, like maybe a giant ceiling fan from the ceiling, be sure to drop a connector from within the concrete roof into the ceiling area. I know, people will tell you that it's not needed, I disagree, and for small cost of proper planning a non metallic bolt or hanger, why worry about it?

Floor system -

Very similar to the roof, except that you don't have to worry about snow load and stuff like that, and it will therefore be able to be made much thinner, especially if your engineer will give you credit for the DensiCrete, if you want to use that on the floor levels too. I am. But not too thin, or you will here footsteps overhead. The tie in to the walls can be ledger style or whatever, but again watch out for thermal connections to the walls if you're in a climate with extreme temps. I'm in Colorado at 10,300 ft. so this is an issue for us for sure, but it can be done.

Hope this helps. If you want to contact me offline - 720.849.9980. Good Luck and don't forget to have fun.
Cheers,
Wayne Sheppard