Raised Floor System

[bouk715]

I'm working on a project where the client would like to have a raised floor area (roughly 2 feet tall or less) constructed with a concrete slab on foam. The project is a renovation, so the foam would be placed on an existing 4" concrete slab.

I'm looking for any information (design guides, tips, etc.) regarding the design/detailing of this type of system. Doing some initial research, it looks like EPS Geofoam is appropriate and there are some compressive resistance tables provided for different densities. I've designed slabs-on-grade, but is the design any different for slab-on-foam? Also, how do the blocks typically get anchored together or to the existing/new slab?

Any advice or just getting pointed in the right direction for documentation would be greatly appreciated.

Thanks.

 

[TDIengineer]

I would design your raised slab similar to a 1-way slab. Assume the styrofoam is simply formwork that is left in place and provides no structural value. The new slab placed above is designed like a raised slab, possibly a 1-way slab where you can provide a 6" curb/wall element that acts as a short bearing wall and dumps the load to your existing slab on grade. You can check bending and punching shear for this existing slab and soil bearing assumign some effective width of the existing slab on grade.

Not sure what your design loads are above, and how much settlement is an issue.

 

[TopKnot]

Why would the foam not be structural? Just pick the right density and type foam. a 5" slab with 100 PSF leaves you a resultant compressive force of 1.1 psi. lots of good info on

http://www.geofoam.org.

Even at the lowest density of 0.7 lb/ft^3, the compressive resistance at 1% is 2.2 psi. It claims no significant creep below 1%.

I haven't used this myself, but I would consider it. I'd use a taped HDPE vapor barrier around everything, to help mitigate against gas spills, acids, oils, etc.

_________________________
TKE

 

[XR250]

Regardless of what the manufacturer claims, I would be worried about settlement of 20" of foam.
As TDI say, design it as a structural slab.

 

[steellion]

Is this for a slab-on-grade? We use this foam without requiring the topping slab to be structural. The load is going to get into the existing slab below. You can add knee walls to the slab below and design as one way like the other posters have stated if slab deflection is critical.

 

[paddingtongreen]

@bouk, you don't say why your client wants the foam, is it to absorb vibration? Some responders, above, assume that there will be concrete walls around the foam, will there be concrete walls?

If there are walls, then make it a structural slab, one way or two way according to the length/width ratio. If there are no walls, then it must be slab on grade.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[bouk715]

It's for a pavilion area next to some bowling lanes (raised bowling lane area is by others). The client typically has wood-framed the raised floors in the past but said most are going the slab/foam route. I had assumed it would be designed as a floating slab on grade supported by the foam and the existing slab beneath it. Looking at the geofoam tables, even relatively low density units seem to have enough resistance for the live load plus additional concrete (1% compressive resistance). Since it is supported by another slab below, the "settlement" would just be the deformation of the foam, right?

There will be perimeter walls around the raised area, but I hadn't anticipated putting in intermediate walls. FYI - actual raised height is 17", so I'm thinking 12" foam with a 5" slab with WWF.

 

[bouk715]

It's for a lobby area next to some bowling lanes (raised bowling lane area is by others). The client typically has wood-framed the raised floors in the past but said most are going the slab/foam route (perhaps for vibration issues??). I had assumed it would be designed as a floating slab on grade supported by the foam and the existing slab beneath it. Looking at the geofoam tables, even relatively low density units seem to have enough resistance for the live load plus additional concrete (1% compressive resistance). Since it is supported by another slab below, the "settlement" would just be the deformation of the foam, right?

There will be perimeter walls around the raised area, but I hadn't anticipated putting in intermediate walls. FYI - actual raised height is 17", so I'm thinking 12" foam with a 5" slab with WWF.

 

[bouk715]

Sorry, accidentally posted twice. It's a pavilion/lobby area

 

[hokie66]

Is there a reason for using the foam, which I assume is polystyrene? I was of the understanding that polystyrene by volume is more expensive than concrete, but that may vary with location.