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.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]