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RE: effective length factor for sway moment frames? K for frames wit h partially restrained connections?
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- Subject: RE: effective length factor for sway moment frames? K for frames wit h partially restrained connections?
- From: "Effland, Greg" <geeffland(--nospam--at)butlermfg.com>
- Date: Fri, 30 Mar 2001 09:50:40 -0600
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 > > > > > > >
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