Effective Length
Effective Length
(OP)
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






RE: Effective Length
RE: Effective Length
HTH
MJ
RE: Effective Length
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
RE: Effective Length
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.
RE: Effective Length
RE: Effective Length
RE: Effective Length
RE: Effective Length
Maybe you can try it with a different analysis procedure.
RE: Effective Length
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.
RE: Effective Length
RE: Effective Length
If ETABS is using the alignment charts, then it should be checked thoroughly. It is a cakewalk to use those charts *wrong*, but it takes a little judgment (ETABS have this?!) to use them right. For example, look at the adjustments that are listed in the text on Page 16.1-242 and 243 of the 13th Ed. Manual. Some of those make a pretty big difference. What is ETABS doing with them?! There's also the tau business, but it's conservative to ignore that.
I look forward to the inevitable death of the Effective Length Method, LOL. Direct Analysis Method!!
RE: Effective Length
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
RE: Effective Length
Which K is coming out very high, Kx or Ky? If these frames only provide lateral resistance in the strong direction, then these columns are leaner columns in the other direction and Ky=1.0 and Kx>1.0. For buckling in the weak axis, the mode is like a gravity column. Sway buckling is the behavior in the strong direction, hence Kx>1.0.
I'm not sure I'd just calc Kx and over-ride in this case because honestly, it doesn't seem like there's an understanding of what the program is doing. There might be other issues also that will go undiscovered if you over-ride.
You're right: dig deeper into what it's doing. Can't type this enough times: One MUST be able to nearly exactly duplicate manually what the program is doing (at least on a simpler version of the problem) or don't use it.
The DAM is in the 13th Ed. Manual, Appendix 7. There will be a design guide out there soon, but it's not available yet. The AISC Manual comes with a CD with examples. There might be something in there on the DAM. Other than that, there might be some papers out there or a newly updated textbook.
RE: Effective Length
I know you've mentioned before that there should be a design guide for stability out shortly. Do you have any idea when that might be? I am anxious to get my hands on a copy.
Hopefully it will be available for free download if you're a member of AISC.
RE: Effective Length
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
RE: Effective Length
htt
I found that quite useful.
RE: Effective Length
So if you are looking for a factor to mulitply L with then take
Free to rotate on plan = 1
Lateral restraint only = 0.85
Lateral and torsional restraint = 0.7
I hope this helps.
Ryan