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RE: A.S.C.E. 12.2.3.2

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The 2009 NEHRP Provisions contain an extensive commentary on the ASCE 7-05 Seismic Chapters. Here’s what they have to say about Section 12.2.3.2.

 

C12.2.3.2 R, Ω0, and Cd Values for Horizontal Combinations. For nearly all conditions, the least value of R of different

structural systems in the same direction must be used in design. This requirement reflects the expectation that the entire

system will undergo the same deformation with its behavior controlled by the least ductile system. However, where the listed

conditions are met, the R value for each independent line of resistance can be used. This exceptional condition is consistent

with light-frame construction that utilizes the ground for parking with residential use above.

 

Since light-frame residential was listed below, I highlighted the last sentence.

 

Gary

 

GARY J. EHRLICH, P.E.

National Association of Home Builders

D 202 266 8545

gehrlich(--nospam--at)nahb.org

 

From: erik [mailto:erik_g(--nospam--at)cox.net]
Sent: Saturday, October 02, 2010 1:34 PM
To: seaint(--nospam--at)seaint.org
Subject: A.S.C.E. 12.2.3.2

 

In A.S.C.E. 12.2.3.2 when talking about different lateral force resisting systems the second sentence says, “Resisting elements are permitted to be designed using the least value of R for the different structural systems founds in each INDEPENDENT line of resistance if the following 3 conditions are met…..

 

If those 3 conditions are met I take this to mean that if you have an ordinary moment frame w/ R=3.5 and a plywood shearwall w/ R=6.5 in the same line, say line ‘A’, and line ‘B’ is an independent line of resistance comprised of plywood shearwall’s, then the lateral forces in line ‘A’ are designed with R=3.5 and line ‘B’ can be designed w/ R=6.5!

 

As I read this section I see that they drew a distinction between the same direction & independent lines in the same direction. Why else would this be put into the code, especially since it applies to most light framed residential structures?

 

My boss says that this does not mean that, and that just because I might use a moment frame in the N-S direction of a single story residence, then all the other shearwalls in the N-S direction must be designed with R=3.5, but I disagree. For residential where the risk of life is fairly small, and the structure is fairly light  I think that this is way too conservative, since the forces that are designed for are only approximations based of statistical values. 

 

Any thoughts on this?

 

Thanks

 

Erik Gibbs