Need help understanding the torsional unbraced length vs flexural unbraced length

[850R]

Hello everyone,

I am trying to understand the difference between the torsional unbraced length and the flexural unbraced length for an I-beam in order to calculate
𝐹e(elastic buckling stress) using equations E4-10 and E4-12 in AISC 360-22.
I would really appreciate it if someone could sketch it out and attach it here to help me understand.

It seems that the torsional unbraced length should be significantly larger than the flexural unbraced length for these equations to govern.

Thank you in advance.

 

[KootK]

I would really appreciate it if someone could sketch it out and attach it here to help me understand.

You might have to go first to give us something meaningful to react to. A sketch showing the details of the situation that you're evaluating would be a great help.

I am trying to understand the difference between the torsional unbraced length and the flexural unbraced length for an I-beam in order to calculate
𝐹e(elastic buckling stress) using equations E4-10 and E4-12 in AISC 360-22.

1) Your flexural unbraced length is the distance between points of lateral restraint with respect to the buckling axis being evaluated.

2) Your torsional unbraced length is the distance between points of cross sectional rotational restraint.

It seems that the torsional unbraced length should be significantly larger than the flexural unbraced length for these equations to govern.

That's right. Typical wide flange column sections posses pretty decent torsional stiffness, at least as far as open sections go. You'd need something like a W18x35 with 1/4 PT weak axis restraint and no torsional restraint to make torsional buckling the governing mode.

 

[850R]

KootK, thanks for your response. I wish I had a specific case to give out but unfortunately I am just trying to create a case so torsional unbraced length will govern and it will need to check equation E4-10 and E4-12 in AISC 360-22.

I've prepared a sketch to show you if I am getting this right.
Based upon this sketch, the torsional unbraced length for W18x35 (green) is 30ft (because W8 doesn't brace the whole web of W18x35) and the lateral unbraced length is 5ft.

For the W18x60 (blue) the torsional unbraced length and the lateral unbraced lengths are 10ft.

Is this correct?

 

[KootK]

I would say that is correct with one exception:

Based upon this sketch, the torsional unbraced length for W18x35 (green) is 30ft

I think you meant to say 15 ft there.

I'm curious why it is that you're studying this stuff in relation to a horizontal grillage. These provisions relate to compression loads which would normally speak to columns.

 

[850R]

Hi KootK,

That is correct, I meant to say 15 ft for the torsional unbraced length of W18x35.
That was just a model I sketched up to show you if I am getting this right, that's all.
I really appreciate your help.