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Re: House and detached garages on 24" slab

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Robert Rollo wrote:
> 
> From an economical $tandpoint, Brian is correct.  In these situations,
> we use a ribbed mat "raft" foundation with 12"x 24-32" beams at
> 16-20ft spacing poured monolithic with a 4-5" slab.  Do it all the
> time for Corps of Engineer projects in expansive soil conditions.  In
> fact, COE has a computer program that calcs both edge lift(positive
> beam moment) and center lift (cantilever beam moment) conditions for
> the expansive properties of the soil.  Input parameters come from
> geotech investigation and lab test values.
> 
> a 24" slab would be ridiculously expen$ive and unnecessary IMHO
> 

Robert:

I am interested in the COE program you mentioned.  I typically use the
PTI method of calculating edge and center lift moments, deflections, and
shears.  PTI has a program, but I don't want to spend $600.00 when my
old MathCad program works.  I also find it interesting that you spread
your beams as far as 20'.  We have PIs as high as 50 and 60 here, with
PI<35 being a welcomed material.  I typically will not space interior
beams farther than 14' or 15'.

Brian K. Smith, P.E.
Bossier City, Louisiana