Lateral Torsional Buckling

[Qrs]

Will lateral torsional buckling ever control the stress in the tension flange of a singly symmetric member?

Lets say I have a wideflange with a channel on top. The top flange will only be in compression and the bottom flange will only be in tension. (uniform loading, single span.) I am checking the top "flange" stresses (beam flange+channel) independently of the bottom flange stresses.

Can the bottom flange be affected DIRECTLY from LTB? Obviously if it governs the top flange, that will be an indirect affect.I am under the impression that the bottom flange need only be checked for yielding .

 

[RFreund]

You might be correct, the section properties of the entire beam are used in the equation for LTB. Not sure if you consider that an effect. I'm not sure if the code/manual (AISC) explicitly states this. Either way, what are you getting at here? What is the application?

EIT

http://www.HowToEngineer.com

 

[Lion06]

AISC 360-05 doesn't limit stresses, it gives you a nominal moment strength. This moment strength is obviously for the entire section. The lateral torsional buckling strength is affected by what's going on at the compression side of the member.

 

[Qrs]

The application is for a crane beam without a channel cap. For the minor axis, I look at the total lateral load going into the top flange of the beam (due to the tractive forces being applied above the centroid.) In doing this, you look at the combined bending in the top flange, and single axis bending in the bottom flange (because the minor axis is assumed to go into the top flange).

I was following the example in Design Guide 7, where he considering yielding as the only limit state for the tension flange, and the top flange is controlled by combined bending between LTB and minor axis bending (although, his channel has a cap). I had never thought of it in this way before, and wanted to get some other opinions. It seems as if it LTB controlled bending in the top flange, the same would be true for the bottom flange, but perhaps if the beam is only symmetrical about one axis, that is not the case?

 

[Qrs]

perhaps this is just a vital difference between the old code and new? (I am not well acquainted with anything before AISC 360-05, excuse my ignorance.)

 

[BadgerPE]

Simply put, LTB affects a member in compression. A member that is compressed will eventually want to buckle. LTB is the act of the compression zone of the member wanting to buckle in a twist fashion. When a member is in tension, there is no buckling failure mechanism because the member is being stretched and therefore no out of plane buckling can be observed.

LTB is not directly related to tension flange capacity. A beam that passes yielding checks but fails LTB checks, can have the top flange braced in a manner to resist the out of plane buckling forces.

 

[BadgerPE]

Maybe my last post was a little unclear, but LTB affects the portion of a bending member that is in compression. Not simply a member that is in compression.

 

[Qrs]

I guess I am confused at DG7's example (18.1.1). Perhaps its simply due to a procedure that is based on stresses rather than nominal strength as Lion said.

While LTB controls the beam, the author does not consider the stresses from LTB when checking the bottom flange - the only stresses he considers here are from yielding.

 

[BadgerPE]

I don't have DG7 in front of me, but I am assuming the author is not considering the stresses from LTB on the bottom flange because the bottom flange is in tension and therefore, not subjected to LTB effects. So yielding checks would be appropriate.

I also assumed that this is a simply supported beam and not a continuous member. If it is continuous, then the bottom flange should be checked for LTB when in compression.

 

[Qrs]

I agree, Crackerjack. Thanks for all input.