Subject: Re: effective length factor for sway moment frames? K for frames with partially restrained connections?
From: "Mehdi M. Khabbazan" <MKhabbazan(--nospam--at)allott.co.uk>
Date: Mon, 02 Apr 2001 10:43:35 +0100
The second order analysis takes into account the beam-column action (P-delta
action), or in other words the axial deformation and its interaction with
bending and transverse deformations. Effectively, it is a step forward in more
accurate elastic analysis of frames, in particular tall multistorey frames with
heavy column loads as opposed to the more conventional stiffness method of
analysis. The results of the analysis can be used when calculating the moment
magnification factor in ACI and AISC codes, but stability limit states should
still be treated as usual.
Mehdi M Khabbazan
PhD, CEng, FASCE
"Haan, Scott M." 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?
> 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