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Re: effective length factor for sway moment frames? K for frames with partially restrained connections?
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- 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: Wed, 04 Apr 2001 17:53:38 +0100
Hi Here are my views, I hope they are helpful: 1) The alignment charts or effective length graphs have been derived by making certain simplifications to the structure. As long as the user is familiar with these simplifications and is able to make suitable adjustments for a given situation the application of the charts normally yields at reasonably accurate results for design purposes. 2) The replacement of the codified stability approach for beam columns with p-delta analysis has been a very attractive and interesting subject for many years and still is. Some European codes have accepted this while the American codes are still reluctant in pursuing the method. The reason being the concern over the nonlinear stability flexure of the columns under large earthquake amplitudes that would result in higher forces compared to those predicted by a P-delta analysis. However, there are still a lot of research going on for this subject which would hopefully put more light as we carry on! Regards Mehdi M Khabbazan PhD, CEng, FASCE. ijr wrote: > One more argument in a different direction: > > 1)Do we agree that except for time consumption, > nothing is VERY wrong with alignment charts for SWAY > frames, and that alert designer can always take care > of factors not counted for in the charts? > > 2)If yes, do we agree that current codes do not yet > support replacement of K with PDelta?,though several > publications are available trying to support this > argument. > > Then > If there is a software that can verify that this > replacement will at least closely give results > compatible with those of current codes for beam-column > or columns, then please let us know. > > Just like others, I have dwelt on the subject, but > have yet to find a software that verifies this. > > Thanks , nice discussion > > --- Waterman Drinkwater <Drinkwater(--nospam--at)EQUATION.COM> > wrote: > > 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. > > > > > > > === message truncated === > > __________________________________________________ > Do You Yahoo!? > Get email at your own domain with Yahoo! Mail. > http://personal.mail.yahoo.com/ >
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