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1997 UBC Equation 32-2

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A little historical background to 1997 UBC Equation 32-2.

Equation 32-2 was originally developed by the BSSC for incorporation in the 1994
NEHRP Provisions.  SEAOC modified the originally equation slightly for 
incorporation into the 1997 UBC.

The intention of the equation is to define a trapezoidal (not triangular) 
distribution of in-structure accelerations that reasonably agrees with 
earthquake response data from California earthquakes, including Northridge and 
Loma Prieta.  If you want to be conservative, and don't want to hassle with the 
trapezoidal distribution, just use the maximum equation 32-2 all the time.  It 
will give you higher design forces lower in the building, but maybe it won't 
affect the anchorage sizes.

Keep in mind that equation 32-2 applies to all sorts of nonstructural elements 
including equipment, piping, ductwork, ceilings, and cladding.  The 
interpretation of the equation for items fastened to more than one part of the 
structure is left to the judgment of the design engineer.

For a precast concrete panel in a low-rise building, adjacent floors experience 
measurably different peak acceleration responses.  One reasonable interpretation
is to design the panel for the average acceleration, and design the connections 
for the elevation related acceleration.  This is the approach suggested by 
myself and the other equation author for use in the SEAOC design manual.  This 
is not the only possible interpretation, but it is reasonable, and easier to 
apply than a trapezoidal distribution across the face of the panel.

By presenting an interpretation in the design manual, SEAOC has given its member
practitioners a reasonable method that should be acceptable to all building 
departments and be defensible in lawsuits.

A more complete explanation (including references) for the nonstructural force 
equations can be found in the 1996 SEAOC Blue Book, Appendix C, beginning on 
page 492.  This reference was distributed free to all SEAOC members.

Rick Drake, SE
Fluor Daniel, Alsio Viejo