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Re: 1997 UBC Seismic

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May 2, 1998
Re: Omega Factor in Seismic Provisions of the 1997 UBC

On April 27  George Barclay wrote:  "In Section 1630.1.1, there is a new
seismic force amplification factor "Omega" that is required in Em equation
(eq. 30-2),       "estimated maximum earthquake force that can be developed in
the structure.....It is not clear when to use this Em."   

Dear George:
As you know, the SEAOC Seismology Committee drafted these Provisions which are
also reflected in the 1996 Blue Book, Appendix C. 
The Omega Factor is similar to the (3/8)Rw in the 1994 UBC which was used as
an amplification factor for design of elements or components subject to the
maximum expected force that could be delivered into the system (i.e., bracing
connections, steel frame columns, discontinued elements under lateral force
resisting systems, etc.) so its application is (and was) limited to specific
cases as outlined in material chapters and Section 1612.4.  It was never
intended to use Omega in base shear force assessment for all elements of the
building.  

Perhaps a significant contribution in the 1997 UBC is to separate the Delta
(max) and Omega, both of which used to be represented by (3/8)Rw.  So whether
you were designing for building separations, deformation compatibility, or
connection design, one would use the same factor where as now maximum
displacements and maximum forces are being represented differently.
I hope this helps,

Ali Sadre, SE, Past Chairman
SEAOC Seismology Committee