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# Dynamic drift calculations (1998 CBC/1997 UBC)

• To: "SEAOC (E-mail)" <seaint(--nospam--at)seaint.org>
• Subject: Dynamic drift calculations (1998 CBC/1997 UBC)
• From: "Harper, Mark" <MHarper(--nospam--at)hdrinc.com>
• Date: Thu, 14 Dec 2000 16:27:53 -0600

```Questions: (Reference 1998 CBC VOL 2, based on the 1997 UBC)

1.	Section 1630.9.1 refers to section 1612.2 for load
combinations to be used with Allowable Stress Design.  Does this mean that I
take the dynamic base shear and scale it as required to the base shear
obtained in section 1630.2 without dividing by 1.4 for the drift
computation?
2.	When calculating the drift using a dynamic analysis, and
working stress design do I scale down the load to (80, 90, or 100% depending
on building type) of the static base shear (section 1630.2) to obtain delta
S  then multiply that value by .7R to obtain delta M.
That is Delta M = Delta S*.7*R (reference 1630.9.2)

Then when calculating the forces in the members scale down
the dynamic forces by an additional 1.4 to obtain working stress forces.

3.	When calculating "E" (section 1630) ROW is equal to 1.0 for
drift calculations and Ev =0 if designing for working stress.  However, does
Ev have to be taken into account for drift design?
Example; Dynamic base shear gets scaled down to E = Eh or
gets scaled to E = Eh+Ev, where Eh = the base shear obtained in section
1630.2?  Because the drift calculation "Delta S" appears to use ultimate
design loads I would think that Ev must be included in the design base
shear. However section 1630.9.1 refers only to section 1630.2.1 which would
make me think that the vertical component Ev is not to be included for drift
calculations!

Thanks for your help!

Mark Harper PE, SE
HDR INC.
mharper(--nospam--at)hdrinc.com
Phone: (626) 564-2820
Fax: (626) 564-2873

```