structural slab on grade design

[hapunaterp]

Hello -
I am in the midst of designing a structural slab floor for a garage/studio area of a single family home. The builder decided that he does not want to compact the soil under the slab so I am designing it as a one way slab with walls supporting the exterior edges and two interior 'beams' of wall on footings. This is a 26' x 28' area. I have calculated the maximum positive and negative moments at the interior and exterior supports and determined the required min. reinforcement. The longest span for the 1' wide strips is 9'-3" so my plan is for a 6" t. slab. I was thinking that I would be placing the reinforcement at the tension sides of the slab depending on the location in the strip (at the supports or in the middle of the strip, top, bottom respectively) however for concrete cast against the earth, min. cover is 3". this means that all my reinforcement is going to be in the center of the slab. I may be over thinking things and I also do not design structural slabs every day so I am wondering if I take the top of the slab as not against the earth and use only 3/4" cover for the rebar at the supports that I wanted to place close to the top or if it really doesn't make a difference for the loads it will experience if all the reinforcement is towards the middle. it is only 6" thick so the distance between the tension and compression faces is not great but I thought I would look for some feedback on this.
thank you

 

[Lomarandil]

Placing the top steel with reduced cover (since that face isn't cast against earth) will give you some advantage, but I suspect the interior moments over your supports aren't much greater than your exterior (bottom steel) moments.

You may need a thicker slab, or to pour against another surface.

 

[DaveAtkins]

Put the reinforcing at mid-depth of slab. If you have a contractor that does not want to compact the subbase, I doubt he will want to mess with different depth bar chairs.

DaveAtkins

 

[ajk1]

If you must go this route, place a skim coat (2" to 3" thick) of weak concrete to form a proper level surface for properly chairing the bars. Then the cover on the bottom bars can be less than 3", unless the soil is corrosive and the corrosive agents will come thru the skim slab and eventually attack the bottom bars in the slab.

Your proposed design sounds like there must be more than just that "the contractor does not want to compact the base"...it sounds like there is poor soil.

Why would anyone go to so much expense and trouble as what you are doing, if it were not necessary? My advice is to retain an experienced geotechnical engineer to advise you. Yes he/she will charge, but his/her advice will save your client a lot of money and keep both you and your client out of trouble, and allow you to sleep at night.

You aren't doing your client any favour by skipping the geotechinical engineer. I have been called in to look at calamities that came about because some inexperienced person missed having the geotechnical engineer on board where soil conditions were not good.
Remember -- your client won't thank you if things go wrong -- rather he/she will sue you. I am a structural engineer, not a geotechnical engineer, but I know their value.

 

[Brad805]

Care to elaborate on the reasons for not compacting the soil? If for costs, what you are proposing will not go over very well. If for bearing issues, then it would be a different story.

 

[jike]

Don't forget that the top of the garage slab may have to slope toward a floor drain.