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Floating Stiffened Slab Supported 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 settlement.


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?


William L. Polhemus, Jr. P.E.

Polhemus Engineering Company

Katy, Texas USA