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RE: Site classification - IBC

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> I have a situation where rock is about 20' down and is good for
> 40 ksf.  The top 20' of soil isn't too good so the building will
> be on drilled piers.  IBC 1615.1.5 gives a procedure for
> determining the site class based on different strata of soil.
> I don't see it clearly spelled out when you have solid rock that
> you're bearing on and then soft soil above.  This is making the
> difference in lateral load of 350k versus 200k.  Anyone run into
> this?

Whether you bear directly on the rock or on the 20 ft. of soil shouldn't
affect the soil site classification.  The classification is based upon the
AVERAGE of the top 100' of soil/rock.

Some might argue that since you're bearing on rock that you should get to
use the rock coefficients, even if it's 20' down.  In that case, I'd say you
need to include that 20' of height, as well as the weight of the first floor
slab and grade beams, in your seismic analysis.  The piers and grade beams
would need to be designed as moment frames.

At any rate, any motion amplification that is caused by the soft upper soil
(the reason behind the higher Fa/Fv values for soft soil) would be applied
directly to the foundation.  It doesn't matter if the application is to
spread footings or to the sides of piers.

Jason W. Kilgore, PE, SE
Leigh & O'Kane, LLC
Kansas City, Missouri

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