slab-on-grade design for supported tank

huangyhg

slab-on-grade design for supported tank
i am dsigning a concrete slab for two granular activated carbon tanks for water treatment.  i am using the method described in "concrete floors on ground" published by pca.  each tank has four supports with base plates that will be located near the edge of the slab.  my questions are:
1)should the operating load (unfactored) or factored dead and live loads be used to determine the post load for use in the design charts?
2)can anyone recommend a value of "stress per 1,000 lb. of post load" where it may be better to design a reinforced concrete slab instead of an unreinforced slab.  the weight of each tank when full of carbon and water will approach 100 kips.  the conditions  (safety factors, concrete flexural strength, etc.) are giving me values that barely allow the charts to be used.  if i use factored loads and a safety factor of 2, i can't use the charts.  the design charts start at 10 psi on the "stress per 1,000 lb. of post load" axis.
3)if the loads are high enough that i can't use the charts, could i use a reinforced footing design approach?
any suggestions would be greatly appreciated.
thanks,
jack.