Elastic buckling stress of unsymmetric section

[Lion06]

I am trying to check the capacity of a small member per F-12 in AISC 360. For LTB it simply says to determine the elastic buckling stress by using a textbook. I have 2 mechanics of materials texts and an advanced mechanics of materials text, but can't find anything regarding the elastic buckling stress of an unsymmetric section. I have information regarding bending and determing the principal axes for an unsymmetric section, but nothing regarding the elastic buckling stress.
Does anyone have a referene they can point me to?

 

[rb1957]

d'ya mean crippling ?

you know the applied stress, from your bending calcs, the allowable is limited usually by short column failure of the remote flange.

 

[Lion06]

I'm not sure exactly what I mean, that is what I am trying to figure you. F12.2 in AISC 360 just calls out Fcr as the buckling stress for the section as determined by analysis. In the commentary it says to use a textbook, paper, or journal to determine Fcr.
I am trying to track down a reference.

 

[rb1957]

i think it is crippling that you're looking for, Bruhn or Michael Niu are good sources.

carefull what you look at tho'; most times they're considering the entire section loaded in compression. in your case it's probably sufficient to consider only the flange, or angle that's in compression. you're looking for the method that considers individual flanges. call if you need help !

 

[miecz]

The commentary to F12 refers you to the Structural Stability Council's Guide to Stability Design Criteria for Metal Structures. Before you shell out the money for the 10 year old guide that's due to be replaced in a couple of years, maybe someone here can see if it contains the solution. Judging by the Commentary's suggestion to avoid the problem, I'll bet the SSRC Guide doesn't have a solution for unsymmetrical shapes.

 

[Lion06]

I saw the reference to Galambos 1998, but I can't seem to get my hands on that reference.

 

[rb1957]

here's a rule of thumb of mine ...
if b/t of the compression flange is <5, then your allowable is fcy

if it is > 10, you're probably looking at a crippling limit.

b is the flange width (in your case sounds like the distance from the neutral axis to the extreme fiber)

if there is a lip on the end of the flange, then consider the flange to be stable (pretty much any practical lip will accomplish this) and the lip is the critical element.

becareful if the flange thickness is greater than the lip thickness ... much less likely that the lip will support the flange.

 

[Lion06]

I understand that, that is a local buckling effect that is dealt with in chapter B. What I am talking about is a lateral-torsional buckling failure mode of an unsymmetrical section.

 

[minorchord2000]

There are some examples of built-up unsymmetrical sections in the companion CD to the 13th Edition AISC Manual. All the examples for the 13th Ed are on the CD. It is worth a look see, maybe it can help.

 

[SWComposites]

Timoshenko and Gere, "Theory of Elastic Stability" is a good source.

 

[rb1957]

ok, in that case you might try NACA TN 733

 

[UcfSE]

My Galambos 1998 book does not have what you're looking for. Instead, there is a little discussion about the topic and some references to various research papers. TG notes that many methods are numerical. You may end up using a numerical analysis, or using a different section. Testing may be another alternative.

 

[himat42]

Structural EIT

Please look at a German Paper by Professor Dr. Ing. W. Mann on RC members , Beton und Stahlbetonbau, Feb 1976
or Professor Dr. Tekn- M.P.Neilsen, Bygningsttatiske Meddeleser, No. 4, 1997.
HTS

 

[JStephen]

rb1957- are those limits for hot-rolled steel?

Re: Theory of Elastic Stability- that is an old old reference- lots of work has been done since then- use with caution.