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Re: 97 UBC Drift: ASD vs Strength Design

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"Larsen, Mark" wrote:
> 
> I have a question regarding the determination of the Design Level Response
> drift, delta-s.
> 
> The May '98 Errata to the UBC appears to allow delta-s to be calculated at
> Allowable Stress level forces when using ASD.  Section 1630.9.1 states:
> 
> "A static, elastic analysis of the lateral force-resisting system shall be
> prepared using the design seismic forces from Section 1630.2.1....  Where
> Allowable Stress Design is used and where drift is being computed, the load
> combinations of Section 1612.2 (changed to 1612.3 per May '98 errata) shall
> be used."
> 
> The load combinations of 1612.3 use E/1.4.  Therefore, it appears that the
> delta-s for Allowable Stress Design will be lower than for Strength Design.
> Since neither delta-m, nor the story drift limits of section 1630.10
> differentiate between ASD and Strength Design, it appears that drift
> requirements will be easier to satisfy using ASD.
> 
> Does anyone have any insight into this apparent disconnect?  (Or does the
> ASD club score one point over the LRFD club?....)  If this thread has
> already been discussed, please let me know.  Thanks.
> 
> Mark H. Larsen, P.E.
> Phoenix, Arizona
> 
An earlier thread discussed the changes between the strength approach in
'97 and the working stress approach in '94.  See that thread for a
clearer defining of delta s, it will also give you clearer picture of
where the 0.025 story drift limit came from.  But as for the the load
combination: do as the code intends, not as the code says.  You may be
able to argue the exact verbage of the code, and many times this is all
we have.  But if you review the earlier thread, I think you find the
code intends you to use the full strength level force.  By the way, the
"s" in delta s stands for "Strength" not service.

Jake Watson, E.I.T.
Salt Lake City, UT