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PT SOG on expansive soil

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Marlou,
 
While using the PTI method, you should consider that the PTI method does not really address the MILD reinforcing requirements and that currently in Southern California there is a significant problem with PT slabs on expansive soils, they are not being design for the forces and MOVEMENTS that they will be exposed to and the construction is another quality control matter.  Generally the building departments do see alot of PT slabs so they can not be depended upon.
 
Another thing is about the PTI method of design, the breaking up into rectangles is VERY important, clipped edges or non-rectangles creates alot of future problem.  The PIT method also by, by it's nature, creates a situation where the tendons create a bending moment that creates "edge lift" of the perimeter.  When PT slabs are located on expansive soils, the expansive soils will add the "edge lift" to the point where the floor elevations after a year or two can look like a bowl. 
 
Another consideration that is never quite considered is the friction loss of post tensioning between the slab and the soil and the post tension force that is lost between the passive soil pressure against the inside vertical edge of the exterior perimeter footings.  It has been determined that with this friction loss and passive soil resistance, the post tension force in the slab about 8 to 10 feet from the edges is close to ZERO.
 
Also the PTI method does not include a deflection check of the slab-footing to account for the EDGE CURL.
 
CRSI has a method that does consider these elements a little better.

Williston "Bill" L. Warren, IV - S.E.
Structural Engineering SOLutions
Newport Beach, California