Subject: RE: effective length factor for sway moment frames? K for frames wit h partially restrained connections?
From: Charles Espenlaub <Cespenlaub(--nospam--at)martinaia.com>
Date: Fri, 30 Mar 2001 09:03:54 -0500
If it is a "theoretical" analysis, you may get "an" exact value. But since
theory is based on educated assumptions, the exact value you get may not,
and probably will not, be the "exact" value that occurs in nature. Just
because a computer can give you a four digit answer, does not mean,
necessarily, that it is the right answer.
Charles F. Espenlaub, III, P.E.
From: Waterman Drinkwater [mailto:Drinkwater(--nospam--at)EQUATION.COM]
Sent: Thursday, March 29, 2001 4:55 PM
Subject: RE: effective length factor for sway moment frames? K for
frames wit h partially restrained connections?
Try theoretical (FE) analysis. You may get the exact value.
> From: Haan, Scott M.[SMTP:HaanSM(--nospam--at)ci.anchorage.ak.us]
> Reply To: seaint(--nospam--at)seaint.org
> Sent: Thursday, March 29, 2001 4:02 PM
> To: IBC list (E-mail); Se-Practice List (E-mail); Seaint list (E-mail)
> Subject: effective length factor for sway moment frames? K for
> frames wit h partially restrained connections?
> Hello everyone,
> The March "Modern Steel Construction" steel interchange has a letter where
> fellow asks if effective length factors can be unity for sway moment
> when second order analysis is performed. The fellow who responded
> 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 http://www.muni.org/building,
> Development Services Department,
> Municipality of Anchorage
> phone: 907-343-8183 fax: 907-249-7399