huangyhg

effective length
i am working on a 3 bay two story steel frame for a section of a building using etabs 9. for some of the columns, i am getting outrages columns sizes (w14x257). when i look at the print out, i see that my effective length factor is sometimes 50 and sometimes 70 for some of them. i know this is wrong because looking at the manual of steel construction, effective length cant go higher than 20. can someone who uses etabs, explain why this happens? i have made all my end connections hinged at the based and fixed at the beam supports. the columns are only 12 feet and what is control is the compression strength of the column which etabs says it 2 to 6 kips(this is low for a column since the manual says 2940 kips for a kl with respect to ry). obviously, the k factor is controlling this but i do not know why the program calculates a k so high.
the resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. there is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. eladio dieste
what kind of lateral system are you using? are these columns part of the lateral system or are they gravity only columns?
take this to etabs tech support.
hth
mj
this is part of the lateral system for the building designed as a ordinary moment frame. i am down the road for tech support but i have been waiting for awhile for them to get back to me (almost two days).
the resisant virtues of the structure that we seek depend on their form; it is through their form that they are stable, not because of an awkward accumulation of material. there is nothing more noble and elegant from an intellectual viewpoint than this: to resist through form. eladio dieste
how does etabs calculate the effective length? does it do the aisc alignment chart calcs or some kind of matrix eigenvalue calc? you really need to dig in and see what it's doing. the etabs manuals should have info on this, probably in teh steel design chapter.
k is unlimited for unbraced frames. there's no limit of 20, perhaps a *practical* limit exists, but no real limit. k = very large or infinity means that the column is unstable, like a flagpole pinned at the base with no moment restraint at the top.
i'm pretty sure etabs will let you override kx. if you can't figure out what it's doing then calc your own kx and over-ride it.
you could also use the direct analysis method which allows kx=1.0 for all columns. see the 13th ed. spec. app. 7.
really, the bottom line is that you must know how it's calculating everything.
hey, i'm going to hazard a wild guess at what's causing the problem. i betcha that you have the column base pinned and the girder ix somehow very small, either with a property modifier or by accidentally turning the beam on its side. other option is that the girder has the end pinned accidentally.
is the top of the column restrained against out of plane movement?
try as a test making one column pinned at the base and at the top, without sway and see if it gives you a k = 1.
look at the sway and see if it's in the ballpark. hand check the reactions and moment diagrams for busts.
maybe you can try it with a different analysis procedure.
etabs is notorious when it comes to calculating k factors. have you meshed the column element into finite pieces explicitly? i think etabs follows aisc alignment charts and uses the stiffness properties of the joint. i think you have a braced system because you are saying that your column bases are pinned. if that is the case, i would suggest use a k=1.0 conservatively and not spin your wheels too much unless you really have the time to.
what kind of deflections are you getting at the top. is it reasonable? if your base is pinned and every connection is a simple connection, maybe you have an instability.
a moment frame is a sway frame and has a minimum k of 1, it is likely higher than one. i agree with 271828 that it probably has something to do with the base conditions of the columns (being pinned) and the beams framing into them having a small ix (relatively speaking).

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