Slab on Ground for residence remodel

[digrazi]

I am designing a slab on ground for a single story dimensional lumber framed residential addition. A 10x40 slab will be attached to the existing slab system with rebar dowels. The below grade geometry of the existing slab is unknown as is the reinforcing. There is no soils report for the site. I was planning on basing the design using guidelines from 1997 UBC Chapter 18,Div III section 1815 assuming moderately expansive soils. Is there any other guidance that I can reference. My only other references pertain to much heavier buildings (ACI, mat foundation design etc).

 

[fndn]

ACI has great publications on slab on grade design. Mostly. however all the inputs needed come from the geotechnical report. Be careful on this since you may have high soluble sulphate content-this will need special concrete mix design. You may also have moderately expansive or very sever expansive potential like you've indicated, in which case you'll need post tensioned slab or doubly reinforced slab with presaturated subgrade. The 1997 UBC requires you to check above parameters. You may just call a testing lab and ask them to sample upper 2 ft and test for soluble sulophates, expansion potential, Atterberg limit, PH, soluble chloride and sieve analysis and soil classification. You may then design your slab with Kv-subgrade modulus between 140 and 250 pci.

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[digrazi]

I have never done post tentioned design before. I am working with a design approach for slabs on expansive soils ubc 1815. The ubc section is directly based on a report published by the wire reinforcing institute. "design of slab on ground foundations" W. L. Snowden. It can be found at

http://www.wirereinforcementinstitute.org.

This approach uses a flexural analysis approach modified for slabs on ground. The result will be a slab with a full perimiter beam and minimal interior beam spacing (15-20ft). The slab steel will be two way 12-18in o.c and intergral with the beam steel. The beams will have two bars top and bottom and stirups. I think it will be pretty stiff...Is this what you mean by doubly reinforced?

I will further recommend that the top 6-12 inches of native, organic and deleterious material be removed and replaced by clean non expansive fill compacted to 95% relative.

I am stuck on the design process because I don't know how to calc cracking moment of inertia Icr. Any hints??!

The weird thing is that many of the engineers around here don't require soils tests for remodels of this type. I am hoping the building dept will....if not, I have assumed a high EI soil of 91-130 per ubc. I don't expect chloride issues here. We are in limestone country for the most part and this site is on a small ridge.

Thanks for the response!

 

[fndn]

digrazi-By douly reinforced, I was thinking of rebar both near the top and bottom of the slab. I like this approach, just use then minimum 5" compacted aggregate base below the concrete, 15 mil visqueen, and to avoid any sulphate problems in the soils, go with type V cement in the mix.

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[digrazi]

Interesting. I have not seen this approach for residential slabs, but I will consider it. We typically use a visqueen barrier on all slab jobs. The type V mix is a good idea. Thanks for the comments!

 

[DJYork]

Since no Geo. report, personal preference, but I would design more of a structural slab, using rebar instead of welded wire. Our soil tends to have more PVR, and rebar seems to be a practice around my area. Also, might can find a report relatively close to the same formation as your remodel.

 

[digrazi]

Without a soils report we are definately going to use a rebar grid. One interesting issue. Our local prescriptive method requires using an assumed PI and pre saturation based on that PI. But this needs to be observed by a soils engineer. However, they won't observe without an investigation for reference. So I am wondering what the risk would be to eliminate the pre-saturation requirement....the more I look into this the less I like designing foundations without soils reports!!! One engineer wrote disclaimers into his report. The language was...the owner is responsible, the engineer is not..etc...This could still end up with defense costs perhaps?

 

[digrazi]

Without a soils report we are definately going to use a rebar grid. One interesting issue. Our local prescriptive method requires using an assumed EI and pre saturation based on that EI. But this needs to be observed by a soils engineer. However, some soils engineers won't observe without an investigation for reference. So this prescriptive method almost requires some amount of soils testing-an EI at least.

The more I look into this the less I like the idea of designing foundations without soils reports!!! One engineer wrote disclaimers into his report. The language was..."since no soils report was provided, the owner is responsible, the engineer is not"..etc...The engineer could still end up with defense costs though...maybe? Of course this is probably always a possibility in this line of work!