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Re: Seismic Load on Residence

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Brian,

I believe that the base of the house is at the first floor.  That is the
level at which ground motion will be imparted to the structure of the
house - the 4 ft high foundation and retained soil under the slab will not
amplify ground motion significantly.

Section 1629.8.3.4 of the 1997 UBC allows the house to be considered
separately from the foundation system you describe provided the rigidity
below the base of the house is 10 times the stiffness of the upper portion,
which seem probable.

I would analyze the foundation system separately, follow the seismic forces
through the foundation system to the soil below.

Also see Section 1633.2.10.

Nels Roselund
Structural Engineer