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RE: effective length factor for sway moment frames? K for frames with 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 with partially restrained connections?
- From: Waterman Drinkwater <Drinkwater(--nospam--at)EQUATION.COM>
- Date: Mon, 2 Apr 2001 12:54:40 -0400
No. There are more than two types of FEA programs for structural analysis. K fector is governed by a PDE. Apply FEM to formulate it, and solve it. Output is K. No massage is necessary. No initial imperfection is required, either. No alignment charts. That is the one I mentioned for FEA of K factor. WD > ---------- > From: Effland, Greg[SMTP:geeffland(--nospam--at)butlermfg.com] > Reply To: seaint(--nospam--at)seaint.org > Sent: Monday, April 02, 2001 11:58 AM > To: seaint(--nospam--at)seaint.org > Subject: RE: effective length factor for sway moment frames? K for > frames with partially restrained connections? > > In clarification of my previous answer, there are two seperate programs > types that are called FEA. > > The first are programs like RISA, STAAD, STRUDL, etc. that I consider > "stiffness analysis" programs. These programs, use a matrix stiffness > analysis of one type or another and typically determine the results based > upon an elastic analysis. Through the use of a P-delta analysis you could > massage out factors similar to k factors. I think Scott had mentioned the > fact that most of these just pull their k-factor out of a table from AISC > (that's my understanding also). > > The second are programs like ADINA or others that typically model using > meshes etc. Programs of this type can typically take effects such as local > buckling, initial imperfections, etc. into account which physically > determine the actual in-place K factors (kind of an oxymoron statement > since > K-factors are approximations to adjust our imperfect equations to more > accurately model physical performance). These programs also typically > take > more time to model since the imperfections and etc. must be entered. How > do > you know what imperfections will exist in the field??? I never get > advanced > notice for fork lifts running into columns or somebody going crazy with a > cutting torch. > > Most of us use computer programs of some type to design our buildings, > bridges, etc... To a computer, looking up a K-factor in a table is as > quick > or quicker than trying to model inelastic or nonlinear behavior. Ever had > a > program tell you a member had 300" of deflection (typically because > something was entered wrong)... even if entered correctly, could that > member really deflect 300" without breaking, yielding, etc. and how would > this affect the other results... > > *** > My point is that there are tools or methods that can deliver more > *precise* > approximations. But more *precise* numbers are not always more > *accurate*. > *** > > As far as Scott's original question, it is my understanding that the use > of > a p-delta (2nd order) analysis does not completely eliminate the need for > the use of K-factors (definitly room for interpretation). I would tend to > agree with Mr. Geschwindner who responded in MSC. > > > Greg Effland, P.E. > > -----Original Message----- > From: Waterman Drinkwater [mailto:Drinkwater(--nospam--at)EQUATION.COM] > Sent: Friday, March 30, 2001 11:32 AM > To: 'seaint(--nospam--at)seaint.org' > Subject: RE: effective length factor for sway moment frames? K for > frames wit h partially restrained connections? > > > We are not discussing which field of computing can have assumption. In > fact, > all engineering computing has assumptions. > > What kind of assumption is for FEA of K factors? It is the ordinary > structural theory. If you can accept those ordinary theory for daily > structural analysis, there is nothing wrong to apply the same theory to > analyze K factors. Those analytic results are much more reliable and > accurate than those from alignment charts. > > FEA of K factors does not require any extra input data. The same set of > data > for "ordinary structural analysis" is sufficient for analysis of K > factors, > and you can get the output. The cost is much less than applying the > alignment charts. A pentium II 300MHZ can analyze 200 members' K in 1 > minute. If applying alignment chart, how much time is necessary? > > > > > > > ---------- > > From: Effland, Greg[SMTP:geeffland(--nospam--at)butlermfg.com] > > Reply To: seaint(--nospam--at)seaint.org > > Sent: Friday, March 30, 2001 10:50 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 > >
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