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RE: Grade for Equipment Seismic Design Force

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Don't know what the code writers intended, but my inclination would be to take the ratio hx/hr = 0 for any equipment mounted at ground level or below. The intent of equation 32-2 is to recognize that motion in the structure is amplified on the upper stories. I think that below ground level (minimum backfill elevation), the structure travels with the ground, and that motions will be equal to or less than motion at the surface. I would never assume hx is a negative number to take advantage of the minimum load level of equation 32-3.
-----Original Message-----
From: Nels Roselund, SE [mailto:njineer(--nospam--at)att.net]
Sent: Thursday, May 08, 2003 8:48 AM
To: seaint(--nospam--at)seaint.org
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
njineer(--nospam--at)att.net