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RE: effective length factor for sway moment frames? K for frame s 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 frame s wit h partially restrained connections?
- From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
- Date: Fri, 30 Mar 2001 09:38:35 -0900
It is a capacity - limit state issue, not an analysis issue. The code used to say if you limit drift and use a P-Delta then you could use K=1. Theoretically for a column fixed at the top and the bottom and allowed to sway the buckled shape would have K=1. If the beams allow the ends of the columns to rotate then the buckled shape would have K>1. There are approximation formulas for K so that you would not have to use a AISC alignment chart: K=sqrt[(1.6*Ga*Gb+4*(Ga+Gb)+7.5)/(Ga+Gb+7.5)] <==== from Tamboli "steel design handbook" for unbraced frames. If K>1 then the columns are going to have a lower strength / capacity and larger columns are required. Sounds like a conspiracy. My point was mainly that the code used to allow K=1 and now the code does not appear to mention it, so for Anchorage at least get your alignment charts out. 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: Scott A. Dunham, PE [SMTP:sadunham(--nospam--at)gte.net] > Sent: Friday, March 30, 2001 7:56 AM > To: seaint(--nospam--at)seaint.org > Subject: Re: effective length factor for sway moment frames? K for > frames wit h partially restrained connections? > > Being from the old school (I did my homework using a slide rule back in > college), I guess I fail to appreciate that FEA (finite element analysis, > I > assume) as the "only" way to get the correct value. Bearing in mind that > most of the loads that we apply to the structures we design, and that the > true material properties we use are at best guesses, what's the big deal > about getting the "exact" number to arrive at a less that exact solution > to > the problem at hand. Having spent the bulk of my career working in the > production side rather that the design side of this business, I can tell > you > for a fact that there are a lot of Engineers that can't see the forest for > the trees. > > Sorry for the sermon, but I think Mark has a valid point. We as Engineers > need to spend a little more time getting it right rather than getting it > perfect. > > Scott A. Dunham, PE > Dunham Engineering Services > Dothan, AL > 334-678-6948 > > ----- Original Message ----- > From: "Mark Gilligan" <MarkKGilligan(--nospam--at)compuserve.com> > To: <seaint(--nospam--at)seaint.org> > Sent: Friday, March 30, 2001 10:00 AM > Subject: RE: effective length factor for sway moment frames? K for frames > wit h partially restrained connections? > > > The author of the following response implies that because we have computer > access we should compute the K factor using the computer. > > I would like to suggest that such practices often take longer than quicker > but less exact methods. On numerous ocassions I have seen engineers spend > considerable time creating spread sheets to perform a calculation that > could be done quicker by hand. > > Given limited design fees and a limited schedule how exact should our > calculations be? > > Similarly how much effort should we spend to reduce construction costs > when the fee is limited? > > Have computers in many instances actually increased our work load? > > > > >>>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.<<< > > > Mark Gilligan > > >
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