concrete ceiling/floor; feasible?

[jklingel]

We are going to build a single story house, 44 W x70 L, slab-on-grade. One end will require filling and compacting as much as 8' of gravel. Instead, I am now considering digging the "deep" half out and having a 35 x 44 root cellar, if you will. That would mean the dug-out end will have a concrete ceiling (the floor slab). There is already a concrete foundation wall across the house at the 35' mark, so I'd dig out from there to the 8' foundation wall. I can envision the whole thing as being relatively easy to do, but the span of the ceiling concerns me. What kind of span can a typical 4" slab make when said slab has #3 rebar on 6" centers in both directions? I could easily cut the clear span down the middle from 21' to 10' 4" or so w/ two 35' beams, posts, and footers. What about a 6" floor w/ #4 rebar on 8" centers, etc? Is it economical to even consider a concrete ceiling in a "normal" house? I know I can build that half with 16" BCIs, 1 1/2" of plywood subfloor and 2" of gypcrete (for the radiant PEX), but then I have BCIs below grade; maybe not cool? Options, options. The cleanest, to me, would be to keep the original slab and support it from underneath, and/or beef it. Any ideas on what I'd have to do to the original 4" slab to have it make a span? I've Googled for concrete span info, but am not finding much. I WILL be running this by the engineer, but I thought I'd try to learn a bit before I talk w/ him. Thanks. john

 

[zeug]

Hi, the only way to do a suspended slab would be post tension. There is plenty of info on the internet to explain this further. Just make sure you have an experienced contractor.

Typical layout here in CO would be concrete basement walls 1 foot past grade and engineered wood floor on top of that. puts you up about 2 feet above grade. Wood product rim joist is no problem. Long spans supported underneath with steel I beam.

You could also do a supported concrete floor with steel I beams and corugated steel sheet underneath the concrete slab. This is done quite a bit on comercial sites.

Kent

 

[noslo04]

Wow man. That's a long paragraph.

My aunt and uncle have a 10 year-old one-story house with walk-out basement. There is an unfinished room beneath the garage - he calls it his shed lol. That space is underneath the garage slab and it has a corrugated steel pan ceiling. It's likely 22'x22' with a column in the middle, but I don't know the exact dimensions.

Not sure if this answers your question at all. All I'm saying is that it has been done.

 

[jklingel]

Zueg: Thanks; that's a good start. I'll Google it. Noslo: Yep, a long paragraph, but you digested all of it. Thanks for the info. Now that you mention it, I think I remember seeing concrete passages under streets done that way as a kid.

 

[Kevin54]

The "Bunker" thread shows how he built a garage with concrete floor above the lower room. THere are quite a few members on here that have over and under garages.

 

[jklingel]

The "Bunker" thread shows how he built a garage with concrete floor above the lower room. THere are quite a few members on here that have over and under garages.

•• FYI, I just read the first 10 or so pages of the bunker thread; lots and lots of deleted posts, and nothing much but "hoorahs" from others. I imagine I'll find stuff via Google. later. j

 

[purpony]

its doable. A friend of mine did this. He has a single story ranch on the side of a hill. When he built the attached garage, he dug it out and now has a basement/storage under the garage concrete floor. He had an engineer design it and has a steel I-beam or maybe more holding the concrete ceiling up. Its pretty cool.

 

[Justanoldguy]

Easily done.
Just typical commercial construction procedure.
Many ways to skin a cat on this.
As opposed to poster 2's "the only way to do a suspended slab would be post tension"
Google for an idea that satisfies you.

 

[Costner]

Look into "corefloor" panels. They are prestressed concrete panels that are placed over the top of concrete foundation walls. Then a layer of standard concrete is poured over the top. The end result appears like a "normal" garage from the top and nobody would suspect you have an entire room below.

This is quite common in my area - I see it with two stall and three stall garages all the time, so a 35 foot span isn't all that difficult. Looking at the load tables I found for a local supplier, they show the max at a 37 foot span, but when you consider the depth of a standard garage being somewhere close to 24 or 26 feet clearly that wouldn't be an issue.

http://www.gagebrothers.com/corefloor.html

I will tell you that it isn't exactly cheap. I was quoted the total cost to be about $10,000 per stall to do a corefloor setup, but that includes costs of additional excavation, more concrete for the foundation walls due to their increased height, the added concrete in the floor of the lower level, thicker foundation walls (they can include a "lip" for the corefloor panel to sit on), added costs for drains, engineered drawings, requirement to install electrical in the space below for lighting (code requirements) and labor to install.

When you consider the potential of having a 700 sq foot shop under a two stall garage for $20k, maybe that isn't such a bad deal afterall... but obviously costs will very greatly depending upon many factors, and if you don't have to excavate as much due to the slope of the lot as well as you already needing tall foundation walls, the added cost could be significantly less making the decision fairly easy.

 

[jklingel]

Thanks for the replies. That CoreFloor looks pretty cool, though the shipping up to me would be spendy. I'll check further on this; it looks like money will be the only restriction, not materials.

 

[Snap50]

There are all types of possibilities.

Many clients have used precast concrete planks with a 2" poured in place concrete topping, but you need some money and a producer in your state. It's an ultimate and durable solution.

Poured in place reinforced slabs work too, with or without steel beams depending on the span, layout and intended use, but you need a commercial concrete contractor because most residential builders are not knowledgable about reinforcing steel practice and it would be critical in your case.

Avoid using wood for a garage floor because sooner or later it will deteriorate. And even before that point there will probably be noticible deflections when you drive on it.

And I'd pass on post tensioning; you are getting into very high tech for residential contractors. That equates to $$$$$ and big potential for things to go wrong due to inexperience. Keep it simple.

And then there is always the straight forward approach of having retaining walls designed and constructed to restrain compacted structural gravel fill to make it slab on grade. But again, residential excavators are not too savy on compacting backfill and you may end up with floor settlement.

Best suggestion is to spend a bit of money hiring a suitable Engineer to advise and design a final solution. Tell him your intended usage and financial priorities.

 

[rburke65]

Ok....what does "about $10,000 per stall" mean? a stall?

 

[W_A_Watson_II]

Precast concrete planks with a 2" poured in place concrete topping. I looked at doing this several years ago and had I not changed the plan/location would have done it in a heart beat. the issue is finding the "local to you" supplier.

I plan on doing it with the retirement home still.

 

[jklingel]

I'm glad I asked about this; it's been a learning experience. I am now out of the loop, though, as we are going to drop the house 4' into the earth (which will be way cheaper and relieve me of another huge project, ie, this under-ground cellar deal). The plusses are I get to rent the JD 700 cat to excavate 2500 more yards of soil, plus I'll save a ton or time and money on building. Pushing dirt beats building any day!