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RE: Steel Moment Frames in Seismic Zone 0

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Charlie,
I need a little help here!!
This is a more general question not specific to the project.  When I look at the 1997 UBC the R value for an OMF is 4.5. In the IBC the R value is 4.0, yet you are talking about an R of 3.  When looking at the seismic provisions 1997, 1999 Supplement 1 and 2000 Supplement 2 nowhere does it discuss using an R of three.  Section 11 also goes on to give some guidance with regards to notch tough wire, back-up bars and continuity plates, which is what I was possibly suggesting.  Where I get lost is the R of 3.  I think what you are saying is that if the local building department allows the use of a R=3 factor that you can forgo the backup bar removal, notch tough wire, and if by calculation the continuity plates, which may be cheaper then the other requirements?  

If what I think your saying is true, knowing what we know about the performance of moment frames I think that we are ignoring the root of the problem and saying that if we design it to a higher force it will be OK.  Yet the higher force is what causes the problem in the first place (besides the inherent problems in the joint).  

I guess the bottom line is weather  I design a SMF, OMF, or IMF it doesn't matter what part of the country it is in, the detailing should be the same for the chosen system.  If I am allowed to and choose to design to a R=3 then the detailing requirements of the AISC seismic provisions do not need to be followed.

Thanks for your input,

Mark


-----Original Message-----
From: Carter, Charlie [mailto:carter(--nospam--at)aisc.org]
Sent: Saturday, July 20, 2002 7:17 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Steel Moment Frames in Seismic Zone 0


>I am preparing to design a 4 story moment
>frame office building on the east coast
>(seismic zone 0) and was wondering what
>other folks have been doing.  
>What are some other engineers in this area
>requiring?  What is standard practice?

It all depends upon the R factor you select (or are permitted to select in
the applicable building code). If the soil is bad or the importance factor
drives you to a higher seismic performance category, you may have to use a
system from the AISC Seismic Provisions (SMF, IMF, OMF). Otherwise, you will
likely be permitted to choose R=3 and design and construct the building
according to the requirements in the AISC Specification for Structural Steel
Buildings without the additional requirements in the AISC Seismic
Provisions. If you choose to take R greater than 3, you must meet the
requirements that correspond to the higher R in the AISC Seismic Provisions.

R=3 systems are generally less expensive than systems with higher R factors.
R=3 as described above gets you a system of normal ductility that will
remain nominally elastic for the design seismic forces. For moment frames,
this can be achieved using the basic moment connections shown in the AISC
Manual, such as flange plates, end plates, and welded flanges. You can also
consider flexible moment connections and PR moment connections if you wish.

Charlie






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