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Thanks for the input.

Just to clarify:

The reason I mention excluding the flexible system is because the criteria
for a dual system states to 'resist total design base shear in proportion to
their relative rigidities'.  This then excludes a truly flexible system
which would be distributed solely on tributary area.

And to further clarify the previous question:

What would be the advantage of classifying a system as a dual system
(R=4.2), say for the case of masonry shear walls and Steel OMRF,when you
could use the lower of the two systems (R=4.5) per combinations of systems
along the same axis?

Jeff Morris

From: "Gerard Madden, PE" <gmadden(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: RE: Dual Systems

On the 25% rule, you take you base shear and multiply it by 0.25. Then
you make your shearwalls or braced frames inactive in your model leaving
only the flexible moment frame active in the analysis and design the
frames for this force.

You choose the R value from the classification in the UBC Table for the
specific dual system. You do not exclude the flexible system in your
model with 100% base shear. By the fact that it is much less rigid, it
will get a very small amount of load.=20

Santa Clara, CA

-----Original Message-----
From: Peter Griem [mailto:griem(--nospam--at)]=20
Sent: Friday, January 24, 2003 9:35 AM
To: 'seaint(--nospam--at)'
Subject: RE: Dual Systems

On a related note...

In order to satisfy the 25% requirement, do you have to proportion your
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? =20

Back to the original question, my take is that if you have say MF's and
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
count it in your analysis I think you technically have a "Shear Wall
System" which is only allowed in the lowest seismic zones and only
to concrete shear walls.

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


-----Original Message-----
From: Yousefi, Ben [mailto:Ben.Yousefi(--nospam--at)]
Sent: Friday, January 24, 2003 11:13 AM
To: 'seaint(--nospam--at)'
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
with moment frames. The rationale for that is that, the SMRF is a much
ductile system (or supposed to be!) so when you mix it with a system of
ductility you should use a smaller R value. However, if your dual system
comprised of EBF's and SMRF's you'll get the highest possible R, since
systems have substantial ductility.

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

Ben Yousefi, SE
San Jos=E9, CA

-----Original Message-----
From: J+R [mailto:jeffrandi(--nospam--at)]
Sent: Thursday, January 23, 2003 7:48 PM
To: seaint(--nospam--at)
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
but I could not find any explainations).  It seems odd that a more
system is penalized with higher design loads than if that axis had all
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
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
the lower R value of the moment frame/shear wall be used and the shear
distributed based on the classification of the diaphragm?

Jeff Morris

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