Lateral Torsional Buckling 1

[RFreund]

When evaluating weak axis bending of a member of any shape, can Lateral Torsional Buckling occur?

EIT

http://www.HowToEngineer.com

 

[Trenno]

Quote from some textbook: "Weak axis bending is allowed a lower safety factor than strong axis bending since Lateral Torsional Buckling is not possible (since Ix is stronger than Iy LTB can't occur)"

So from what I gather, when a beam is bent about its minor axis, it cannot deflect laterally (lateral torsional buckling) because Ix >> Iy such that it will just keep buckling about it’s minor axis until section capacity is reached.

On the other hand, bending about the major axis, because Iy << Ix there becomes a vulnerability such that the section will want to deflect outwards and thus initiating LTB before sectional capacity.

 

[RFreund]

@Trenno -> that was my instinct as well however I struggled to find something to reference. Thanks for the input.

EIT

http://www.HowToEngineer.com

 

[SlideRuleEra]

According to the August 2004, issue of AISC's "Modern Steel Construction" magazine:

Normally, no. But, in certain cases it can. See this link:

http://msc.aisc.org/globalassets/modern-steel/steel-interchange/2004/2004v08_si_web.pdf

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[KootK]

I find it most illustrative to think of this from an energy standpoint. Every strong axis loaded beam, in its heart of hearts, wants to flip over to weak axis bending. This is because weak axis bending results in more deflection, the load moving closer to the earth, and a reduction in overall system potential energy. All things in nature naturally tend towards lower energy states. I think that's a consequence of entropy. Weak axis loaded beams don't laterally torsionally buckle because they're already where they want to be. For them, lateral torsional buckling to the strong axis position would result in an increase in potential energy. It would be akin to dropping a soccer ball on an inclined slope and having it roll uphill.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Sorry SR, I didn't mean to duplicate your post. We must have simul-posted. Great reference. Do you know what the "lipped" business is with respect to a lipped through girder? I assume it's some bridge engineering technology.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

From SRE reference:

KootK...check out the author of the AISC reply above. Just discussing him yesterday.

 

[SlideRuleEra]

KK - No, I have not encountered "trough girders"... but did find there are references to them. Here's a Google result:

http://aspirebridge.com/magazine/2010Summer/Project_South_Watt.pdf

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[SlideRuleEra]

Ingenuity - Thought that author's name looked familiar - had to be from your post.

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[Ingenuity]

Lipped steel-trough girders are (were?) used in Australia for large span bridges:

 

[RFreund]

Thanks, great info!

EIT

http://www.HowToEngineer.com

 

[KootK]

Ahh... thanks Ingenuity. I mistook trough for through.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

Looks close to an orthotropic steel girder.

Mike McCann, PE, SE (WA)

 

[Ron]

We call them trapezoidal box girders.