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RE: Dual Systems

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On a related note...

In order to satisfy the 25% requirement, do you have to proportion your more
flexible system to be stiff enough to carry 25% of the load, or do you
design it separately for a value equal to 25% of your base shear?  

Back to the original question, my take is that if you have say MF's and SW's
in the same direction you must use the lower R value -unless- you
specifically detail the building for the special requirements associated
with dual systems.  But once you choose that SW R value you'd have to
design/detail so that only the SW's make up you lateral force resisting
system.  If you distribute the loads to some MF's based on rigidity and you
count it in your analysis I think you technically have a "Shear Wall Frame
System" which is only allowed in the lowest seismic zones and only applies
to concrete shear walls.

All of which leads back to my question about the 25% rule...  Any
clarification would be appreciated.

Peter

-----Original Message-----
From: Yousefi, Ben [mailto:Ben.Yousefi(--nospam--at)ci.sj.ca.us]
Sent: Friday, January 24, 2003 11:13 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Dual Systems


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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