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Re: 97 UBC Drift: ASD vs Strength Design

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The erratum in question (which changes Section 1630.9.1) is itself in 
error; the original text was correct.  This point is clarified in the 
IBC which says "Where allowable stress design is used, [Delta] shall 
be computed using earthquake forces without dividing by 1.4."

Apparently the person(s) who produced this erratum misunderstood the 
section and simply tried to "coordinate" a sentence regarding 
"Allowable Stress Design" with the section titled "Load Combinations 
Using Allowable Stress Design."  While such "coordination" might seem 
appropriate to a non-technical copyeditor, the revised text does 
not reflect the intent of the codewriters.

> From:          SDGSE(--nospam--at)aol.com
> Date:          Wed, 13 Oct 1999 00:08:50 EDT
> Subject:       Re: 97 UBC Drift:  ASD vs Strength Design

> In a message dated 10/12/1999 8:34:36 PM Pacific Daylight Time, 
> Byainc(--nospam--at)aol.com writes:
> 
> << I believe the intent of the statement "Where allowable stresses....." in 
>  section 1630.9.1 is to clarify that once you determine that drift 
>  requirements are satisfied and you want to check the stresses in the 
>  elements, you can reduce the earthquake forces by 1.4 if allowable stress 
>  method is used. This does not imply that you can check drift requirements by 
>  reducing the base shear by 1.4. Delta, S is always determined at strength 
>  level regardless of what method you use for checking element stresses. >>
>  
> 
> Sorry Ben. I have to disagree with you. That's not how I read section 
> 1630.9.1:
> 
> "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."
> 
> Why would the code deal with the stress check of members in the drift 
> section, when it already covered it in other relevant sections?
> 
> Oshin Tosounian, S.E.

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
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