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

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I must have had a hangover that day in "Structural Finite Elements".  How
does a finite element program determine K factors?  I thought all FEM did
for you was figure how much force to move nodes around and not figure the
shape between those nodes.  I believe it was called "rigid body motion" or
"rigid body displacement" but go ask a PhD.

If a modeling program is calculating K for a code check isn't it just using
alignment chart stuff?  I have RISA 3-D and all that does is plop in one of
the K's out of the figure in the back of the AISC manual.


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
mailto:haansm(--nospam--at)ci.anchorage.ak.us

> -----Original Message-----
> From:	Effland, Greg [SMTP:geeffland(--nospam--at)butlermfg.com]
> Sent:	Friday, March 30, 2001 6:51 AM
> To:	seaint(--nospam--at)seaint.org
> Subject:	RE: effective length factor for sway moment frames?  K for
> frames wit h partially restrained connections?
> 
> I agree that FEA might be more accurate than the old alignment charts
> (actually the use of K factors in general is an assumption) but I have to
> agree with Charles on this one.  The results of any computer program are
> only as good as the input information, assumptions made within the
> program,
> and how accurate they are for that particular case.  To my knowledge all
> FEA
> programs are based upon assumptions, otherwise all physical testing would
> be
> unnecessary.  The point I think Charles was making is that this is a
> complex
> system in general... it is not like looking at bending on a flat plate.
> How
> many times has anyone analyzed a pinned base plate with more than 2 anchor
> bolts/rods?  Even with 2 anchor bolts this is not a true pinned
> connection...  Besides, the numbers are statisically massaged after
> physical
> testing to achieve a best fit line or curve... this always leaves some
> cases
> that don't exactly agree with the reported findings.  Again, I agree with
> Charles the resulting answer whether reported in 4 or 6 decimal places is
> only as good as the assumptions which are never perfect.  Key here is to
> make as good of assumptions as possible and go on... unless you want to
> take
> a year or more to design 1 frame.
> 
> my 2 cents worth anyway,
> Greg Effland, P.E.
> 
> 
> -----Original Message-----
> From: Waterman Drinkwater [mailto:Drinkwater(--nospam--at)EQUATION.COM]
> Sent: Friday, March 30, 2001 9:16 AM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: effective length factor for sway moment frames? K for
> frames wit h partially restrained connections?
> 
> 
> Theoretical results satisfy all the governing conditions. The theoretical
> K
> factor is theoretical exact. They are more reliable than the ones from
> alignment charts. Those alignment charts were introduced before computer.
> For the time being, almost every structural engineer has a computer for
> analysis. Why should we keep applying those inaccurate charts?
> 
> When computers were not available for most structural engineers, engineers
> used some approximate methods, for example, two cycles, to analyze frame
> structures. No one uses two cycles since computer is everywhere. All the
> results are theoretically analyzed on computers. FEA of K factors is a
> must.
> 
> 
> 
> > ----------
> > From: 	Charles Espenlaub[SMTP:Cespenlaub(--nospam--at)martinaia.com]
> > Reply To: 	seaint(--nospam--at)seaint.org
> > Sent: 	Friday, March 30, 2001 9:03 AM
> > To: 	seaint(--nospam--at)seaint.org
> > Subject: 	RE: effective length factor for sway moment frames?  K for
> > frames wit h partially restrained connections?
> > 
> > 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.
> > Martin-Espenlaub Engineering
> > Tel  215-665-8570
> > Fax 215-561-5064
> > 
> > 
> > 
> > -----Original Message-----
> > From: Waterman Drinkwater [mailto:Drinkwater(--nospam--at)EQUATION.COM]
> > Sent: Thursday, March 29, 2001 4:55 PM
> > To: 'seaint(--nospam--at)seaint.org'
> > 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.
> > 
> > WD
> > 
> > 
> > 
> > > ----------
> > > 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
> > > 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 http://www.muni.org/building, 
> > > Development Services Department,
> > > Municipality of Anchorage
> > > phone: 907-343-8183   fax: 907-249-7399
> > > mailto:haansm(--nospam--at)ci.anchorage.ak.us
> > > 
> > > 
> > 
> > 
> > 
> 
>