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RE: Grade for Equipment Seismic Design Force

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I've interpreted this as exterior soil grade adjacent to the structure - I
think they intentionally used "grade" here to differentiate it from the
"base" of the structure.  

I once had an engineer try to convince me that "grade" for seismic design of
an elevated steel bin supported on a braced steel frame structure was at the
support points on the steel frame where it supported the steel bin. I don't
think this interpretation has any merit, but it shows that engineers do come
up with different interpretations where terms are not defined in the code.
William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

-----Original Message-----
From: Nels Roselund, SE [mailto:njineer(--nospam--at)]
Sent: Thursday, May 08, 2003 9:48 AM
To: seaint(--nospam--at)
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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