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RE: effective length factor for sway moment frames? K for frames wit h partially restrained connections?
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: effective length factor for sway moment frames? K for frames wit h partially restrained connections?
- From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
- Date: Fri, 30 Mar 2001 15:45:29 -0900
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 > > > > > > > > > > > > > >
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