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RE: Dual Systems

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The only time you get penalized for choosing a dual system is when you
utilize a more rigid system such as shear walls or braced frames combined
with moment frames. The rationale for that is that, the SMRF is a much more
ductile system (or supposed to be!) so when you mix it with a system of less
ductility you should use a smaller R value. However, if your dual system is
comprised of EBF's and SMRF's you'll get the highest possible R, since both
systems have substantial ductility.

And, if you mix different systems along the same line of resistance you can
either try to qualify it as dual or choose the lower R value for base shear
calcs.

Ben Yousefi, SE
San José, CA

-----Original Message-----
From: J+R [mailto:jeffrandi(--nospam--at)adelphia.net]
Sent: Thursday, January 23, 2003 7:48 PM
To: seaint(--nospam--at)seaint.org
Subject: Dual Systems


I am looking for some insight why dual systems are penalized in regards to R
value over each individual system (I know this has been asked in the past
but I could not find any explainations).  It seems odd that a more redundant
system is penalized with higher design loads than if that axis had all the
same type of lateral resisting elements.

Once classifed as a dual system the shear is distributed based on the
rigidity of the elements regardless if the diaphragm is flexible or rigid
per UBC and IBC.  Is it required that the system be classified as a dual
system if say moment frames and shear walls occur along the same axis OR can
the lower R value of the moment frame/shear wall be used and the shear
distributed based on the classification of the diaphragm?

Thanks
Jeff Morris






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