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ASCE 7-05 Section 12.2.3.2

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Jason -
 
This is commonly used in but not only used in residential wood-framed structures.  For instance we have a nice house on a bluff with a great view. The owner wants nothing but windows for the view but as an engineer we need a line of resistance so we stick a steel moment frame along that line in an otherwise wood shear wall structure.  The wood shear wall has a lower R value than the moment frame but using the exceptions below we can now design the MF using the higher R value associated for the MF while the rest of the structure uses the lower R for the shear walls and diaphragm.
 
Of course nothing is as simple as that but I tried to simplify for example purposes.
 
I hope this helps.
 
Andre Sidler, S.E., P.E.
Seattle, WA
 
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From: Jason Christensen <jason(--nospam--at)wcaeng.com>
To: "seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
Subject: ASCE 7-05 Section 12.2.3.2

Can anyone expand on the second sentence in this section? It seems to mean=
nothing, considering the requirement of the first sentence.

12.2.3.2 R, Cd , and =D90 Values for Horizontal Combinations.
Where a combination of different structural systems is utilized to
resist lateral forces in the same direction, the value of R used for
design in that direction shall not be greater than the least value
of R for any of the systems utilized in that direction. Resisting
elements are permitted to be designed using the least value of R for
the different structural systems found in each independent line of
resistance if the following three conditions are met: (1) Occupancy
Category I or II building, (2) two stories or less in height, and (3)
use of light-frame construction or flexible diaphragms. The value
of R used for design of diaphragms in such structures shall not be
greater than the least value for any of the systems utilized in that
same direction.

Maybe I am missing something, but it seems to say in the first sentence tha=
t you have to use the least R values (which makes sense), but the second se=
ntence seems to be giving an exception if the three criteria are met, howev=
er it still states that the least R values is to be used.=20

Any thoughts would be helpful.

Jason