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Fp design value

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David:

The Seismic Design Manual does not present a "new" way to calculate Fp. 
Instead it clarifies the proper procedure for the unique situation of an
element with two different attachment elevations.

The Fp formula in the 1997 UBC is based on a similar NEHRP formula,
except it lost something in the translation to the UBC (not my fault). 
NEHRP's formula was based on the elevation of the equipment's or
element's "centroid", but the UBC's formula is based on the elevation of
the equipment's or element's "attachment".  This attachment elevation
becomes confusing when an element such as a wall is attached at two
different elevations, such as a floor and roof.

I am told from those who wrote this section, the UBC's intent is to
analyze the individual wall elements between supports for the average
force of those two supports.  This makes sense since the effective force
on a simple wall at its centroid would be the average of the attachment
forces (floor and roof). 

There was also some confusion as to when to apply the Fp minimum
threshold:  before or after the averaging.  Again, I was told the intent
was to apply the threshold before averaging, not after.  This way, the
out-of-plane forces on a wall are essentially the same as under the 1994
UBC (except for the new soil factor effects, near-source effects,
etc.).  This is the method illustrated in the Seismic Design Manual,
Volume I.

The 1997 UBC as strictly written does leave some of this to our
imagination, but the new SEAOC Bluebook to be released this month
provides a good explanation in its code commentary.

John Lawson
Kramer & Lawson, Inc.