Engineered suspended garage slab

[Tundara Homes]

I have a question about spec'ing a room below a garage. I realize the garage floor needs to be engineered, and it will be after my design process is complete, but just for interest sake, what might this look like? We are designing a 22 x 24 garage with a music studio below. I have no idea what a floor like that would look like, or even how show it on the plan. I will have a lot of notes and vagueness in this area and divert design to an engineer, but I would like to at least be close.

If it was just spanned with concrete, how thick would the concrete be. If anyone has any ideas regarding this, and they would like to help, please do. Thanks.

[Gene Davis]

Cars drip water and here they drip tons of slush, so waterproofing is as big a deal, construction-wise, as the floor structure itself.

Autos, light trucks, and SUVs don't require as much structure as you might think, but certainly some engineering is involved. If you can stand a post or two, and a dropped beam, in your space below the two-stall garage, you'll need less structure.

Some use prestressed hollowcore concrete planks and run full span. In many cases, 10" plank depth will do. Such a floor gets topped with a thin concrete slab. Others use something like LiteDeck forms to do a reinforced concrete full span deck. Alternatively, one can do it with structural steel, corrugated steel deckform, and reinforced concrete slab atop.

In all cases, one must design for drainage by pitching the slab, etc.

Then there is insulation to deal with, unless you are in a place like maybe Hawaii where it is wondrously temperate year round. The LiteDeck forms solution is a good one for insulation.

[Joe Carrick]

FWIW, I have built many garages above grade using 4x12's @ 12" o.c. (12' span) and 1-1/4" T&G Plywood Decking. You can cover this with just about any finish flooring you want - Tile, Vinyl, Hardwood or Concrete. The main thing is to put a good waterproof/oil-grease resistant sealant.

As Gene said, having a Column or two and a Beam down the center of the space below helps a lot.

Structurally, a vehicle is generally not as big a load as you would have to design a balcony for. The problem is spot loads at the wheel locations.

[Tundara Homes]

The LiteDeck forms solution is a good one for insulation.

I do like this idea. I have been looking into quad deck, a similar product, but this site has much more info. They actually show spans. Thanks Gene.

[Tundara Homes]

Joe, that's also good to know, then you are not limited by how wide or long it is. As long as the space works around the posts, I like that idea.

[marty]

I have this situation quite often and usually set the floor depth at the maximum likely to be needed which should 12".
If the engineered design comes in less - say 10" then I just raise the basement floor 2" rather than change all the upper levels.

Another consideration is acoustic inulation. If 12" is allowed for it gives space for the ceiling to be isolated from the structure so you could end up with an 8" precast slab with 4" for thermal and acoustic insulation.

[perryh]

I use the wood structure floor with a lightweight concrete and waterproofing above, can be easily modeled with the new floor layers. Of course the sizes of flr. jsts. are the engineers call. You also need good shear walls below.

[Joe Carrick]

If you think about it, a car that weighs 2200 lbs + 4 adults at 200 lbs each and occupies a space of 10'x20' is only exerting an average load of 15 lbs/sq.ft. - less than a normal live load for a living space. The point loads at the wheel locations would be 750 lbs each which is why I use the thicker plywood and 4x's @ 12" o.c.

It makes for a really stiff floor - shear wall requirements are no more than for any other occupied space.

[Allen Brown]

I've done this on a couple of houses. One was 24" steel web joists 24" O.C., with steel ribbed and 8" of concrete poured over. (that's only because the guy got all those web joist for free from a job.)

The other was 6" of reinforced concrete, ribbed steel, and a couple of LVL beams under. I don't remember the details, it was several years ago, but it's fairly common.

[perryh]

If you think about it, a car that weighs 2200 lbs + 4 adults at 200 lbs each and occupies a space of 10'x20' is only exerting an average load of 15 lbs/sq.ft. - less than a normal live load for a living space. The point loads at the wheel locations would be 750 lbs each which is why I use the thicker plywood and 4x's @ 12" o.c.

It makes for a really stiff floor - shear wall requirements are no more than for any other occupied space.

That must be a small car, what about a motorhome at 22,000 lbs or a truck.

[Joe Carrick]

OK Perry, so lets take my Nissan Exterra at 5400 Lbs plus 800 lbs of adults = 6200 lbs.

We're still talking about only 30 lbs/sq.ft or 1500 lbs per wheel - with the people in it. Not really a very big deal structurally. Typical Live Load requirements for Residential use is 40 lbs/sq.ft minimum.

[perryh]

I know, I just wanted to here from you. still alive

[marty]

From my discussions with engineers on this is that there is a difference between static loads and the rolling/braking loads of a vehicle. If the vehicle was lifted up and placed on the floor it would match typical live loads but thats not what happens in reality.

[dshall]

FWIW, I have built many garages above grade using 4x12's @ 12" o.c. (12' span) and 1-1/4" T&G Plywood Decking. You can cover this with just about any finish flooring you want - Tile, Vinyl, Hardwood or Concrete. The main thing is to put a good waterproof/oil-grease resistant sealant.

As Gene said, having a Column or two and a Beam down the center of the space below helps a lot.

Structurally, a vehicle is generally not as big a load as you would have to design a balcony for. The problem is spot loads at the wheel locations.

Yep, been there done that.

[Joe Carrick]

From my discussions with engineers on this is that there is a difference between static loads and the rolling/braking loads of a vehicle. If the vehicle was lifted up and placed on the floor it would match typical live loads but thats not what happens in reality.

That's true for example on a bridge, but the speeds, exceleration & deceleration of a vehicle in a residential garage are not sufficient to be considered as dynamic loads. Static analysis is quite sufficient for this.