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Floating Stiffened Slab on Piers

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Greetings, fellow strucs.

We have a local geotechnical firm who typically gives a wide range of 
options as to their recommendations for lightly-loaded (i.e. light 
commercial) foundations.

One such option they call a "floating stiffened slab supported on piers."
It is essentially a slab on grade with a perimeter grade beam supported on 
drilled piers (typically at column locations for e.g. metal building) that 
is coincidentally stiffened at the interior, presumably with grade beams 
or ribs.

The geotechnical engineer's recommendations go further to say it should be 
"designed as a as a stiffened floating slab" in accordance with the PTI 
or WRI methods. Piers are there to resist the major loads, and shrinkage or 

In this scheme the drilled piers are not positively connected to the slab; 
the slab rests on top of the piers so that if there is uplift due to soil 
activity it is not constrained from lifting.

I was wondering how the PTI method is used to design such a slab. It 
doesn't seem logical to design it the same way you would as a "full 
floating" slab, unless you want to convince yourself that the piers are 
there solely to resist the gravity loads from the columns.

Any thoughts?

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