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effective length factor for sway moment

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For frames restrained against sidesway (drift), K will always be <= 1.

For frames NOT restrained against sidesway (drift), regardless of whether or 
not you have fully or partially restrained joints, K will always be > 1 and 
can be much greater than 2, 3, or even 4.

(Simple sketches will show that this is true.)

While P*delta will give you the increased moments in the column due to 
sidesway, it will not relate to the stability of the column.

Hope this helps.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Scott Haan wrote:

. > 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?  

. > Thanks! 

. > 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
. > mailto:haansm(--nospam--at)