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More About 1997 UBC Fp

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1997 UBC Section 1632.2 clearly defines h-sub-x as the element or component
attachment elevation with respect to grade.  The key words are attachment
elevations.  These are discrete points, each with a unique value.  F-sub-p
values should not be determined at elevations that do not involve attachments to
the structure, such as the middle of a panel.  1997 UBC Section 1632.2  goes on
to specify that design lateral forces shall be distributed in proportion to the
mass distribution of the element or component.

So how do we apply these requirements to panels attached at different
elevations?  The UBC is silent, allowing for engineering interpretation.  (No
matter how conscientious, code writers just can't anticipate each and every
situation.)  However, SEAOC made certain that panels were addressed in their
Design Manual Volume 1.   The example uses an average panel design value of the
F-sub-p values for each attachment elevation.  This is one rational engineering
interpretation for this application, and one that is relatively easy to use.
Another rational engineering interpretation would be to apply a straight line
(trapezoidal) distribution between the F-sub-P values.  This causes a little
more work for the engineer, and therefore, was not suggested in the Design
Manual.  I suggest that we all follow the SEAOC Design Manual Volume 1.

So how do we apply these requirements to block walls/fences?  Block walls have
one attachment point, at grade.  F-sub-p values should be determined at grade
level only.  There is no need to complicate the problem and determine F-sub-p
values at the top of the wall where there are no attachments.

Hope this helps.

Rick Drake, SE
Fluor Daniel, Inc.
Aliso Viejo, CA

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