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effective length factor for sway moment frames? K for frames wit h partially restrained connections?

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Hello everyone,

The March "Modern Steel Construction" steel interchange has a letter where a
fellow asks if effective length factors can be unity for sway moment frames
when second order analysis is performed.  The fellow who responded indicated
the effective length factor should still be calculated for determining the
critical buckling column capacity.

1997 UBC 2213.5.3 indicates K can be taken at unity when drift is kept in
code limits and the columns are fixed at each joint.  I have an old blue
book and the commentary indicates the code requires drift and P-delta
consideration so K>1 is a further unecessary complication.  The 2000 IBC
adopts the "AISC Seismic Provisions for Structural Steel Buildings" and it
does not appear to mention mention that the effective length factor can be
taken as 1 for moment frame columns.

How are other places that are using the IBC handling this?

For fully restrained moment frames typically 1<K<2 unless the beams are so
wimpy that there is no restraint.

What kind of effective length factor should be used for moment frames with
partially restrained connections when the frames do not have drift limits?  


Scott M Haan  P.E.
Plan Review Engineer
Building Safety Division, 
Development Services Department,
Municipality of Anchorage
phone: 907-343-8183   fax: 907-249-7399