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> To all the PT gurus:
> I have seen many PT SOG designed for gravtiy load.  If a slab supports a
> three story building, as this thread suggest, how is the overturning
> forces from seismic forces   on the three story building accounted for in
> the footing design - especially if the bearing soil is very poor (say 500
> psf).  The overturning forces will create eccentric loading on the slab
> and therefore unequal soil pressure across the width of the SOG.

Don't forget that there are many areas of the world--and the U.S.A.--where
seismic forces are not a concern for design.

That may be a simplistic answer, but it is part of the answer anyway.

One area I know of where moderate seismic forces may come into play
(depending on the building type) when considering such a foundation would
be in the vicinity of Oklahoma City, OK, where expansive clay soils are
also a problem. However, keep in mind that PT SOGs are really more
appropriate when you have a lightly-loaded foundation (residential,
multi-family residential, warehouse, e.g.) on expansive soils. Such
structures will typically not be governed by seismic design considerations
(except perhaps in consideration of frame detailing per the IBC), even in
moderate seismic zones.

I do know that there are situations where "compressive clay soils" can
indicate the need for PT SOG foundation design, but I'm not familiar with
the particulars.

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