Check for lateral torsional buckling around the strong axis

HanStrulo · May 25, 2021

Hi Everyone.

I recently started designing with the CISC and I am loving the process of understanding a new code.

I have a small question regarding the check of beam columns for combined axial and bending.

In clause 13.8.2, in the third check for lateral torsional buckling strength, the code specifies that Cr should be based on the weak axis. In the beam i am designing, we provide enough lateral restraints in the weak axis that the KxLx/Rx is actually the one dominating the design and the buckling should be about the "strong axis".

Is there a reason for the code to always ask about the weak axis? does it mean the axis for which buckling is dominant? and how can i combine that with the lateral torsional buckling?

Thank you for your input.

KootK · May 25, 2021

Consider:

1) LTB may be envisioned as a combination of:

a) Lateral, weak axis sway and;

b) Torsional rotation.

2) Two possible buckling modes in columns include:

a) Lateral, weak axis sway and;

b) Torsional rotation.

So, clearly, whatever bracing condition applies to LTB also applies to Cr. That's basically all that code provision is saying. Designer's probably shouldn't need to be told that but better safe than sorry I suppose.

HanStrulo · May 25, 2021

Thanks for the explanation.

I mean that regardless of the number of restraints i provide in the weak axis, lateral torsional buckling in the beam will always take place in the weak axis (am I correct?) but sometime, the axial buckling will end up being more critical in the "strong axis" because of all the restraints on the weak axis. Should i just take Cr for strong axis and combine it with Mr?

Sorry if the question is too elementary. [bigears]

JoshPlumSE · May 26, 2021

HanStrulo -

I don't have the CISC code in front of me. But, I remember going through a similar process as you when I first encountered it. My belief is the CISC code is organized a little differently from AISC which always confuses me a little since I grew up with the US codes.

For item c) in your snippet, the primary concern is LTB and the Cr is calculated as the weak axis value because that's the axial mode that will affect LTB.

Similarly, there will be an item a) and b) where the primary concern will be different failure modes and the Cr and Mf values will be based on those primary concerns.

Of course, I'm going off of memory. So, take a closer look at those sections yourself to make sure that I've summarized it correctly.

Liam Nesson · May 29, 2021

You have essentially created a laterally supported member which will fail due to in plane bending and axial combined. Buckling failure mode is for laterally unsupported. The code commentary has more on this.

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Check for lateral torsional buckling around the strong axis

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