PTI's 2004 Post Tension Design Values for soil seem high?

[inaz]

I've been using the Post-Tensioning Institute's new 2004 3rd ed. Design Manual (AKA Alternative Procedures). I'm finding the soil design values I'm getting are significantly higher than the previous methods

For example, a structural recently told me he calced a small, one-story, rectangular, residential home with a 20" P-T slab with tendons at 12" on center, in central AZ. Obviously, this sounds like way too much slab. The soil is a CH with LL=63, PL=23, clay=45% of total fines. Em center/edge = 6.7'/3.5' and ym swell/shrink = 2.85"/1.14".

I'm looking for some other people's experiences with the new design methods. Thanks for any comments.

 

[ptconcrete]

Glad to here you are using the new porcedures. I think there has been a great deal of misinformation and misunderstanding going around Arizona regarding the new procedures, both structural and geotechnical. One issue is that there seems to be incorrect assumption that the structural equations have not changed. This is incorrect, and using soil parameters developed using the 3rd edition procedures with the 2nd edition structural equations can result in overly conservative slabs. The Ym values you listed are big numbers and don't seem typical for Arizona. What part of the state is this in?
--Jim