8 inch and thicker slab on grade - one or two mats of steel

huangyhg

8 inch and thicker slab on grade - one or two mats of steel?
i remember a thread here that included a statement that the usa corps of engineers requires two layers of steel reinforcement  for 8 inch slabs on grade,  where such slabs are designed to be reinforced,  and i cannot find this requirement now.
can a 8 inch slab on grade designed for forklift traffic have one layer of steel in the upper third and meet standards?
all help is appreciated -   thank you.

8" is little too tight for two layers of mat (actually 4 bars thickness). one layer should be ok, provides the subbase is well prepared and compacted.
i believe that recommendation for two layers of #4@12" e.w. appeared in an early version of the crsi handbook, for 8" slabs for industrial loadings (1500 psf).  crsi no longer includes any recommendations for slabs.  a footnote to the table said to place the top layer 2" from the top of the slab, second layer, 2" from the bottom.
if you have uplift on the slab, then you need to design it for such, which may dictate two layers regardless.
mike mccann
mmc engineering
if two layers required for strength. suggest to use 9" slab. the arguement is because the lower mat cover is usually non-uniform, thus, i prefer to maintain 3" cover at the bottom. now the space between bars = 9"-3"-2"(top cover)-4*0.5"=2". the above is assume the bars are aligned, you colud gain 1" by offsetting the bars. then 8" is doable.
for forklift traffic, i would not use reinforcing steel...just welded wire fabric for thermal and to keep cracks together.  for forklift traffic on a slab on grade, there is generally no need for reinforcement.  the stresses are generally not that high, and when they exceed 0.5xmr, you can enhance the mix to accommodate.
it's not a problem to get two layers in on an interior 8" elevated slab where the bottom cover is 3/4".  but to get two layers in an 8" slab-on-grade where the bottom cover is 3" would might violate the bar to bar spacing between layers. i've only ever called out a single layer in a 8" s-o-g.
8" - 3/4" (top cover) - 2" (4-#4 bars) - 3" (bottom cover) = 2-1/4" clear theoretically. this probably would be closer to 1-1/2" once the guys step all over the top mat.
i wouldn't go to a double mat unless you had 10" thickness.
if the slab is designed as plain concrete to resist bending and shear, then i add one layer 1" to 2" down from the top surface for t & s.
if it is designed as reinforced then i would use two layers for 12" or more.
what are the bars for? for flexural bending, or for crack control.
i have done heavy slabs (8 to 12" thick) that support rack systems and have forklift traffic. used one layer of bars (as = .005ag) for crack control only. worked fine.  
thank you all for the help and suggestions.  just a standard sog for a warehouse type building - but an older post was hanging in my memory - the crsi comment helps clear that.  thank you.
what about contraction joint spacing in these thicker slabs with 2 layers of reinforcing?  do you cut the top bars at these locations?  do you really want to induce a crack or should the reinforcing really be designed to control the shrinkage much like an elevated two-way slab?