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RE: Steel Moment Frames in Seismic Zone 0

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Mark,

Take a look at table I-C4-1 (Design Factors for Structural Steel Systems)
on page 62 of the 1997 AISC Seismic Provisions.  This table should explain
to you where the R=3 comes from.  It should be noted that this is the
commentary, so it very likely (depends on how the AISC committee ballots
it and if it is somehow written in mandatory language...commentaries are
usually not) have no "official, leagal" bearing from a code sense.  I am
guesses that since this table is in the commentary, it is the AISC's way
of providing input to those who define the R values (model building codes
under the older system and ASCE 7, if appears, in the future) as to what
the steel community thinks it should be.  Typically the break between what
should be material related (ACI 318, MSJC, AISC) vs. load related (ASCE 7)
is an easy determination, but it does get a little messy with the R
factors.  Some believe that the material folks should define them, while
others think the loads people should define them.  Currently, the load
people define them.

The ultimate point is that an R of 3 is being suggested by the AISC
committe for those moment frames that don't require any real ductility
(they will still have some).  In otherwords, if there likelihood of facing
a seismic event is small, then wind will likely always govern and having
the ductility that normally would be desired for a seismic event is not
required.  The intent is that the moment frame will behave in a much more
elastic range so the ductility details that are associated with OMFs,
IMFs, and SMFs won't be necessary.

HTH,

Scott
Ypsilanti, MI


On Thu, 25 Jul 2002, Harper, Mark wrote:

> 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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