Slab On Grade

[Canada580]

Hello !

I have to design a slab on grade foundation for a small shelter. I have attached a sketch for this. The load due the shelter will be only around 30 KPa. If I design the slab for cantilever action that can cause due to frost upheaval, do I still need worry for some other stuff? It's no a problem if the shelter as a whole move upward or downward uniformly. Your thoughts on this will be much appreciated.


Thanks.

 

[spieng89]

How would you quantify the uplift moment from frost upheaval? I would just make the slab structural and turn down slab edge below frost depth. At least I haven't seen a good way to accurately quantify frost upheaval or accurately predict the displacements the slab would encounter.

 

[Canada580]

Thank you raspivey. I would consider all the weigh from the building on the pad. Then calculate bending moment with that cantilever action assuming slab will be supported at center only. If the upheaval force is more than building weight, the entire building will move upward. So the slab will not get load greater than the load from shelter itself. The shelter is for equipment storage used in communication sector. Please see attached picture for more clarification.

 

[hokie66]

Your sketch shows the centre of the slab heaving due to frost, but wouldn't it be the edges that go up?

 

[Canada580]

Thanks Hokie66,

Yes. The frost heave could be hugging or sagging. I just assumed center upheaval because it gave me higher bending moment. As I can design the slab for maximum consideration, if bearing pressure is OK, do you think that the foundation should be frost protected ?

 

[KootK]

- Unheated structure?

- Remote location difficult to access?

- No building services that would be sensitive to movement?

Your concept would work I think. And, while it seems extreme at first blush, it may be an economical solution depending on your answers to the questions above.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[CANPRO]

What you're showing is common in my area....we thicken the slab edges similar to what raspivey suggested but we don't carry the concrete below the frost line. The thickened edge gives you something to put your anchor bolts in as well. I suggest insulating the edges. It's fairly cheap and easy to do.

I've never designed for upheaval as you suggested....but also always show insulation. I suspect the insulation or even a frost wall would be cheaper than designing the slab as you have proposed. See link below.

http://www.ecohome.net/guide/slab-grade-technical-guide

 

[dik]

Where are you located, what type of soil and what is the depth of frost penetration? Do you have degree-day information?

Dik

 

[Canada580]

Thank you Canpro, KootK and Dik.

It is for Winnipeg. Frost dept aroun 2.5 m. Air freezing index 3251 degree F. About 5 feet of silt and clay fill. I am thinking to replace about 3 feet of the fill and replace with gravel compaction. My bearing load is low around 30 KPA. 8 inch slab is good enough for maximum load that can be there if frost upheaval occur.

 

[Canada580]

I am also thinking to provide 2" insulation that provide R10. But not going to check freezing stuff. Though?

 

[southard2]

One resource you could use to check your logic is a publication for designing monolithic slabs on grade for expansive soils published by the welded wire institute. If you have a site with expansive soils the traditional structural solution has been to design a stiffened slab on grade or a post tensioned slab on grade.

The document below is an excellent resource for designing stiffened slab on grades.

http://nebula.wsimg.com/8724fe0dc68...C50EBA80CBDB6E201&disposition=0&alloworigin=1

The method is essentially designs grade beams that deal with heave or loss of support at the perimeter of the building. When a portion of the soil loses support the grade beams act as cantilevered beams. This is mainly good for residential or light weight framed buildings. You can modify it of course if you have heavy walls like CMU once you understand the methodology.

This reference is actually still referenced in the Florida Building Code which means it is also probably still referenced in the International Building Code. But I figure the frost heave and expansive soil issues will be very similar since both involve volume changes of the soil supporting the building. Check it out it is a pretty good resource.

John Southard, M.S., P.E.

https://www.pdhlibrary.com/

 

[dik]

The silt is bad... particularly for frost heave susceptibility. Check with DOW insulation for thickness of insulation and extent away from the building. If the silt can readily be excavated you may not have an issue. How big a problem is it if the slab tilts a bit and possibly cracks a bit?

Dik, also from Winnipeg...

 

[Canada580]

Thank you Dik. I am thinking to replace about 3 feet of the silt fill and replace with gravel compaction. Also, The shelter structure is steel frame. I do not see any major problem with some tilt.