monolithic slab-on-grade joints

huangyhg

monolithic slab-on-grade joints
i am interested in how other engineers detail their monolithic grade beam/slab-on-grade foundations.  i am in a highly expansive clay region and i design my foundations using the wri method.  how do you limit cracking for your monolithic foundations?  sawcut contraction joints would be what i would typically use for a slab-on-grade application if i was not on expansive soil, but it seems this contradicts the idea of a monolithic foundation.  aci 360r-06 recommends a minimum steel ratio of .5% of the slab cross-sectional area in order to eliminate sawcut contraction joints.  for a 4" deep slab that would put me at needing 0.24 in2/ft of reinforcement (#4 bars @ 10" o.c.).  this seems real high.  any thoughts would be appreciated.   

what part of the country are you in?  i am in the dfw area, which has varing levels of expansive soils.  i have had success with the equivelent area method in conjunction with saw cuts; which if i recall correctly, is #4's @ 14" for a 4" slab.  of course, some soil treatment was required to stabilize the soil.  so, the reinforcement was essentially crack control for shrinkage.  this is a difficult subject because most contractors (on residential projects) have quoted the foundation based on #3's or #4's @ 18" o.c.e.w.  so, you mostly likely will have a heated discussion with the contractor on the topic.  i hope that helps.
i'm in mississippi.  the p.i. can get up to around 60 in this area.  this is a relatively small commercial building.  the geotechnical report recommends cutting 7' down and 7' out and filling back with soil with a maximum p.i. of 24.  that seems to be the standard that all the geotechnical firms use around here unless you are going to a deep foundation.  
for crack control on a foundation where i was not overly concerned about soil movement, i would typically use sawcut joints and discontinue a portion of the reinforcement at the joint.  i don't think this method would be good for stiffened foundations that have been designed with the anticipation of soil movement.  
how do you detail joints for a stiffened foundation?  how do you lay them out?  similar to the example in the attached file?  

i appoligize to the tangent, but here in the dfw area there has been a few lawsuits relative to the "remove and replace" method.  the ones i specifically know of, had select fill (pi<=15) used to replace the existing.  the problem comes down to creating a "bath tub".  the select fill would allow water penetration and the moisture would leach into the adjacent expansive soils.  as a result, the surrounding areas would swell, causing more surface water to drain toward the building and providing more moisture.  the goetechs, typically, have a general statement in their reports to provide adequate subgrade drainage.  so, the engineer of the foundation was on the hook for the problem. you might study that a little bit. just an fyi, perhaps the pi=24 eliminates the problem.
as for our topic, i would refer you to a book called designing floor slabs on grade: step-by-step procedures, sample solutions, and commentary it was very helpful to me.
i have to go, but if you still need additional information, let me know and i'll find the methods suggested in the book.
i have had that conversation with a few of them in the past and their stand is that specifying  the fill to have a pi of 10-24 and more than 50% fines passing the no. 200 sieve will eliminate the issue.  
i appreciate your input and i will check out that book.   
mcqse,
removing 7' of expansive material below and out from the building sounds like a solution, and i suppose in your area it may be economical.  in other areas, i suggest a deep foundation with void formers under the slab would be preferred.
as to joints, your slab is either a stiff raft or it's not.  sawing joints in a raft makes no sense.