Slab on grade design 1

[stickframe]

I need help to design a slab on grade.

I have a one story commercial building (90'x150'), and most of the loads are on the perimeter of the foundation (point loads spaced at 15' on center). I just need to design for relatively small uniform load of 100 psf.

Will 4" slab work with crack control joints at 12' on center and with minimal reinforcing for temperature and shrinkage.

Also, the soil report indicates a maximum PI = 8. I have read some articles indicating that beam spacing should not exceed 20'. Do I need any beams since the area is big (13,500 SF) Does this apply only for slabs on plastic/compressible clay (with high PI)?

The soil is mostly sandy silt/silty sand.

 

[civilperson]

Yes, the slab on grade will work if the subgrade is properly prepared. Suggest 95% compaction, (Standard Proctor), for top 18" and 6" of granular base at 100%. Build slab in alternating strips of whatever width matches the joint spacing. Use perimeter foundation with isolation from the slab.

 

[miecz]

When you say you have point loads at 15 ft on center, do you mean you have 22.5 kip point loads?

 

[stickframe]

To civilperson,

Thanks a lot for your information because it is very helpful. Just one thing I would like to verify. When you say isolation from the slab, do you mean absolutely no dowels to tie the perimeter foundation with the slabs? Or just an isolation around each column? Thank you.


To jmiec,

The point loads are a result of the roof load spaced at 15' on center around the perimeter. Average point loads is about 15k.

 

[miecz]

Oh, so you're not landing any of these loads on the slab?

 

[civilperson]

The slab and perimeter foundation will be loaded differently and will settle differently. Tied together wil crack one or the other. An expansion joint material between the slab and the perimeter will prevent the cracks cause by different loading.

 

[kslee1000]

You might not be able to get away without providing reinforcement at columns to tie the slab. Your footing design is likely to be controlled by the horizontal loads due to wind, or earthquake, unless you provide a owefully large footing, overturning of the footing will be a concern. In such case, we used to tied the columns to the slab, and provide control/construction joint around the interface to minimize the effect of differential settlement.