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Re: drift calculation, 1997 UBC using ASD

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I think you can/should use 2% or 2.5% drift *only* if the resisting structure can
actually provide some lateral resistance at this level. UCI wood shear wall tests for
APA and COLA show that max. shear capacity (strength limit states or SLS) occurs
about 1.5 - 2% of story drift.  For max. useable displacement, I think 1% story drift
is reasonable, but it is greater than yield limit state (YLS) which is used for LRFD
design. Some ascribe to reducing tested value of YLS by a phi factor. Phi factor in
ASCE 16-95 for shear walls (and diaphragms) is 0.65, but we think that is too
conservative.
John Rose/APA

Robert J Bossi, PE wrote:

> Thanks for alerting me to the printing error.  BUT does the errata make sense?
> If you compare Equations 12-6 (USD) and 12-10 (ASD), the USD equation will always
> provide a larger drift.  The drift limit of 2-2.5%H is a constant dependent only
> on building height. If the errata is correct then a building designed by ASD will
> always be permitted to have more drift than when designed by USD.  Regardless of
> the design method the drift used to compare to the maximum drift should be the
> same.  The only way to do this is to use the USD load factors.
>
> Incidentally, Appendix C (pg. 444) of the SEAOC Blue Book requires use of the USD
> (1612.2) equations when the design is otherwise ASD.
>
> Anybody got  ideas on this?
>
> Bob Bossi
>
> SDGSE(--nospam--at)aol.com wrote:
>
> > In a message dated 11/10/99 3:10:55 PM Pacific Standard Time,
> > rjbossi(--nospam--at)sonic.net writes:
> >
> > << Alex:
> >  My code (1998 CBC, first printing) says:": ... the load combiations of
> > 1612.2
> >  shall be used."  1612.2 load combinations are USD and do not contain a 1.4
> >  factor.  ie Eq 12-5 or 12-6.
> >
> >  Bob Bossi
> >   >>
> >  First errata to UBC97 changes the reference to 1612.2 to 1612.3, which means
> > E/1.4 for drift calculations using ASD. It may not be the intention of the
> > author, but there is also the Rho factor which got misinterpreted. You Loose
> > some, you win some.
> >
>