Lateral torsional buckling effective length?

[arash68]

Hi,

I wanted to ask for your opinion on the effective length for lateral torsional buckling in relation to the elastic critical moment used for determining the capacity of an element

AISC 2016:


The equation states that Lb is the length between points that are either braced against lateral displacement of the compression flange or braced against the twist of the cross-section. A similar equation exists in the Eurocode with more refined forms.

Eurocode:


Kz is the effective length factor related to the restraint against lateral bending.

I am a bit confused about choosing the right length in below and I was wondering about the value of Lb or KzL for the cantilever beams shown in cases (a) and (b). Is my interpretation correct?
a) Kz is 2 since the beam does not restrain the compression flange.
b) Kz is one since the HE400 is restraining the compression (through a bracket the HE400beam is connected to the bottom flange)


a b




Thank you in advance

 

[JoshPlumSE]

This is actually a more interesting question than I thought it would be.

I would say that Kz*Lz is the following (for the beam labeled H1000):

i) for the hot pink portion of the beam it is 1.0 * pink length. There are diagonal braces here. Therefore, I say this section is nicely braced at both ends.

ii) for the grey/light blue portion of the beam it is 2.0 * grey/light blue length. Since there is no brace, I think this is safest.... I don't know that the beam connected to the end of the cantilever provides either lateral or torsional restraint.

Caveat, I am somewhat familiar with the Eurocode for structural steel. However, my primary area of knowledge comes from understanding the AISC code.