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RE: OMRF (R value)

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>I'm confused of what R value to use.
>The UBC says the value of R = 4.5 while 
>the 1997 Seismic Provisions, Supplement
>No. 2 indicates R = 3.5.  So when can 
>we use R = 4.5 and when can we use R = 3.5?

Basically, the UBC provisions are out of date with the AISC Seismic
Provisions. As a result, the AISC Seismic Provisions and UBC have different
levels of detailing (energy dissipation capacity) associated with the
respective OMF connections that go with the two R values. 

SMF, IMF and OMF have evolved a bit over the past few years. During the SAC
Steel Project, it was conceived that an SMF should be good for an R of 8 and
based upon testing to achieve an interstory drift of 4 percent (of which
approximately 3 percent is inelastic). Similarly, an OMF should be good for
an R of 4 either in prescriptive form as given in the AISC Seismic
Provisions or based upon testing to achieve an interstory drift of 2 percent
(of which approximately 1 percent is inelastic). And to allow for an
intermediate condition that didn't make 4 percent but exhibited good
behavior, an IMF was included with an R of 6 and based upon testing to
achieve an interstory drift of 3 percent (of which approximately 2 percent
is inelastic). This system was incorporated into the AISC Seismic
Provisions, NEHRP Provisions, and IBC draft at the time.

Thereafter, it became obvious from the wealth of steel connection testing
that assemblies either performed very well (i.e., achieved SMF
qualification) or were only good for OMF status. Actually, the testing to
date has shown that, with proper design and fabrication/erection, its hard
to configure a moment frame that would not perform acceptably as an SMF --
and that OMF performance is also easily achieved with very basic connection
detailing. The IMF category thus seemed like a white elephant and it was
considered that the IMF should be removed.

Along the way to that conclusion, however, it came into favor that the IMF
and OMF should instead be recategorized. The OMF was recast as the
prescriptive form given in the AISC Seismic Provisions (note: this is NOT
just the pre-Northridge connection, it has significant improvements to
welding and configuration, backing bar treatments, web detailing, etc.). The
IMF was recast as a tested assembly like the old OMF with a higher R and
inelastic drift demand. This was incorporated into AISC Seismic Provisions
Supplement No. 2.

On the UBC side of things, the code is just behind the times relative to
what has happened already in the AISC Seismic Provisions. So the R factor
you see corresponds to connection detailing requirements that were loosely
consistent with older versions of the AISC Seismic Provisions.

Long term, the transition to the IBC or NFPA document will take place and
these annoying stutter steps in code progression will disappear. For now,
just make sure you properly match the selection of R to the corresponding
detailing requirements.


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