Design Concrete Slab on Grade 5

[pperr043]

I need help to design a slab on grade.

I have a two storey building, and my second floor and roof loads are transmitted to column footings below my slab on grade. Now, I just need to design my slab on grade for a relatively small uniformly distributed load (Live load = 4.8 kPa). I looked at a design book, but it discussed heavy loads, wheel loads, column loads, or other special cases. In those cases, they usually have grade beams and/or thickening of the slab below the concentrated load. I just wanted to deisgn a slab with a uniform thickness and no grade beams.

Does anyone know how to get my slab thickness for this type of loading, and the required reinforcment (if any are required)?

Also, should I use insulation below my slab?

 

[dik]

If I understand you correctly, you have the superstructure supported on foundations independent of the slab on grade.

In general, a slab on grade strength is largely dependent on the quality of the soil beneath it. Your uniform load of 100 psf will generate only minimal flexural stresses in the concrete, largely due to irregularities of the slab thickness and non uniform compaction of the soil beneath it. Other than these flexural stresses, it's only when you have non-uniform loading, point loads, wheel loads, etc. that the slab receives significant flexural stresses. That's why your reference material notes point loads, etc.

A three or four inch slab, on well compacted soil can easily accommodate a uniform load of 100 psf. It's the lack of uniformity of the load, slab, or soil that causes problems. 100 psf is a reasonable load for light storage and I would normally use a 5" thick slab for commercial buildings and a 4" slab for residential. I would retain a local engineer, either structural or geotechnical to review this as well as provide a spec for the concrete to be used as well as any sealers, reinforcing, sawcutting, etc.

Insulation under the slab can help in areas where frost heave susceptible soils are present; the thickness of insulation and type is dependent on climate, soils, and loading. Again you should consult a local consultant.

Dik

 

[JAE]

This is from an older version of the CRSI manual:

Sub-slabs under other slabs - 2"
Domestic or light commercial (<100 psf) - 4"
Commercial/Institutional/Barns (100-200 psf) - 5"
Industrial/gas stations/garages (<500 psf) - 6"
Industrial (600-800 psf) - 7"
Industrial (1500 psf) - 8"
Industrial (2500 psf) - 9"
Industrial (3000-3500 psf) - 10"

with the following reinf:

2" - none
4" - one layer WWF 6x6-W1.4xW1.4
5" - one layer WWF 6x6-W2.1xW2.1
6" - one layer WWF 6x6-W2.9xW2.9
7" - two layers WWF 6x6-W2.9xW2.9
8" - two mats of #4@12"o.c. ea. way (top and bottom)
9" - two mats of #5@12"o.c. ea. way (top and bottom)
10" - two mats of #5@ 8 to 12" o.c. ea. way (top and bottom)

Now these are very empiricle and I don't really ever use WWF in slabs on grade anyway. Just a rough rule of thumb for slab design that was once suggested by CRSI.

 

[dik]

JAE... thanks for the cite... I've used similar thicknesses as a rule of thumb and never been challenged, but it's good to have a reference...

Dik

 

[pperr043]

Thanks for your help guys.

I realize that the soil does impact the design. Just to let you know, the soil report that we received for this location is a stiff clay fill of 4-5 ft deep, followed by a stiff clay. Also, it is said that the bearing capacity for spread footings of approximately 37,500 psi.

Note that the location of this project is in Ottawa, Canada, where frost heave is possible. The frost layer is approximately 4 ft. Do you have any reference for the required insulation thickness.

 

[GeoPaveTraffic]

Can't help you with the insulation thickness, but the footing pressure of 37,500 psi is incorrect. I don't know what it should be, but unless it is good bedrock, that isn't correct.

 

[cvg]

if in Canada, bearing capacity is probably cited in metric. the conversion from metric could be off...

 

[pperr043]

Sorry, maybe my conversion was off.

Bearing capacity = 180 kN/m2 (or 3,760 psf)

 

[dik]

Dow Chemical has an excellent *.pdf publication for insulation required for different freezing conditions... If you can't dig it up... drop a note.

Dik

 

[pperr043]

Dik,

Thank you for your reference, but I can't seem to locate it on the Dow Chemical website. Would you have a direct link to that pdf?

 

[dik]

This is one for Celfort...

http://www.owenscorning.com/worldwide/admin/tempupload/pdf.3-74495-199_HighDensity_E.pdf

I have a *.pdf file for the Dow publication, but cannot find a link on the internet. It's titled "Insulation for Geotechnical Applications". The one I have for Celfort is titled "High Density Extruded Polystyrene Insulation". Both are good publications.

If SRE is reading this, I can upload the DOW publication to his site if OK.

Dik

 

[dik]

I have another tekkie publication titled "Design Guide for Frost Protected Shallow Foundations" published by HUD and having instrument numberU100K000005897

I can send them all to SRE. Check my user profile for a list of my interests.

Dik

 

[CarlB]

A DOW styrofoam link is

http://www.dow.com/styrofoam/na/pro-us/applications/

"Builders Guide to Frost-protected shallow foundations" at:

http://www.nahb.org/publication_details.aspx?publicationID=437

HUD publication for FPSF:

http://www.huduser.org/publications/destech/desguide.html

 

[dik]

CarlB... the NAHB publication is quite good... added it to my collection... thanks, dik

 

[wcl77]

I need a bit of guidance in tackling my most recent work assignment. I have a RC rectangular box storage tank (60-ft long and 20-ft wide and 13-ft deep) that contains sulfur. The storage tank is located in a refinery. I need to punch a hole (18-inches x 30-inches) in the top slab (13-inches thick with No. 6 @ 8" e.f. e.w.) to permit the installation of a submersible pump. There is no additional load on the top slab as top of slab is 6-inches above grade.

I am interested in methods of analysis (ACI 318?) to determine if the existing slab will support the load from the new pump (approx. 4.5 kips dead weight and 2.5 kips operating weight) and possible retrofits to the slab in the area of the new opening to ensure a safe and reliable design (already plan on sealing cut concrete with chemical resistant epoxy paint). My gut tells me there is plenty of flexural and shear capacity in the two way slab.

Any other pointers will also be appreciated.

 

[dik]

Keep the hole, if you can, approximately 10' from the exterior wall; this will take you out of regions of -ve moment. Have the long dimension of the new opening parallel to the 20' span.

You can check the reinforcing cut, likely a max of 3-#6 bars... The #6@8 T&BEW seems like real overkill but you can do the sums since I don't know what the loading is. If necessary to reinforce the slab because of the opening, then carbon fibre and epoxy might be in order. Check to see that there is no adverse reaction of the concrete and repair to sulphur... there shouldn't be... but check.