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Re: Grade for Equipment Seismic Design Force

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You are correct.  The old "grade" was supposed to be changed to "base" in
the 1997 UBC.  We got it right in the 2000 INB.  :>)

Rick Drake, SE
Fluor, Aliso Viejo, CA


                      "Nels Roselund, SE"                                                                                              
                      <njineer(--nospam--at)>        To: <seaint(--nospam--at)>                                                                 
                      05/08/03 08:48 AM                                                                                                
                      Please respond to        cc:                                                                                     
                                               Subject:  Grade for Equipment Seismic Design Force                                      

Section 1632 of the 1997 UBC, Equation 32-2 relates Fp to the elevation of
the component in relation to grade.

In a building with a below-grade floor, where is grade for use in Equation
32-2.  Is grade the same as the base of the building, or is it adjacent
exterior grade?  'Base' is defined in Section 1627, but 'Grade' is not.  It
seems to me that if grade = base, the code writers would not have
introduced a new word, 'grade', here.

For a concrete or masonry building, I would designate grade as the lowest
floor level [not at adjacent exterior grade] -- that is where seismic force
acts on the structure.  This is treating the Equation as if Grade = Base.

For a wood-framed building constructed on a concrete basement wall, I would
designate grade as the top of the concrete wall, since there is a
significant change in rigidity of the lateral load resisting system at the
concrete/wood interface -- the wood structure will respond as if seismic
loads act at the top of the concrete.  This also is treating the Equation
as if Grade = Base.

Is something else intended?

Nels Roselund
Structural Engineer
South San Gabriel, CA

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