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Re: moment frame conn. per FEMA

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On 5/12, Martha Alic wrote:

> I have designed a one story steel-framed mezzanine with braced frames in one
> direction and monent frames on the other direction. These moment frames are
> not part of the primary building frame system. The moment frame member design
> is governed by seismic load and the Rw used was for OMRF, but the final member
> sizes are based on drift limitations (since a pinned base was used the drift
> is high, even under low seismic forces). 
> 
> Considering FEMA 267 moment connection design guidelines, the criteria apply
> to all SMRF structures designed for eathquake and OMRF structures located
> in...seismic zones 3 and 4. Light single story buildings, the design of which
> is governed by wind need not consider this interim guidelines. 
> 
> Do the moment connections need to comply with FEMA guidelines or can the
> connection be designed for 3Rw/8*the prescribed lateral forces. Is there
> another alternative to design this connection?
> thanks
> 
Recent testing by Subash Goel at the Univ. of Michigan Ann Arbor 
demonstrates that even connections employing light framing members, 
but using typical pre-Northridge detailing are vulnerable to 
premature fracture - at very low plastic rotation demands.  The 
connections should be designed per SAC FEMA-267 recommendations, 
unless they can remain elastic.  3/8Rw loads does not guarantee 
elastic response.  The real loads for elastic response are something 
like Rw/1.4/1.5 times the code specified forces.  The 1.4 is the 
inherent load factor on Rw.  The 1.5 is an estimate of material 
overstrength.Ronald O. Hamburger, SE
Regional Manager
EQE International, Inc.
San Francisco, California