Wood biaxial bending NDS ambiguity

[AaronMcD]

Wondering if anyone has an answer to this.

Biaxial bending is in chapter 3.9 of NDS. It has F_bE in the denominator of the last term. Go back to section 3.3.3 and this variable is a factor of some other variables:

1) It is a factor of the slenderness ratio, which is otherwise used to calculate the beam stability factor, which is not applicable if (a) the compression edge is fully restrained or (b) flat use. Both of these conditions are met in a skewed purlin (one for each direction of bending)

2) It is a factor of Emin' which in turn depends on flat use. Lets assume no flat use since the direction for F_bE is not specified. However, since the stability factor is not required, is F_bE still required? If so, in which direction of bending is it calculated?

 

[lexpatrie]

On #2 >
Isn't that factor only applicable for beams and stringers?

 

[ChorasDen]

For biaxial bending with any axial load, I generally default to section 15.4.1 of the NDS. If you truly have a situation where biaxial bending is present but no axial load exists, I suppose you could use section 3.9.2, but I and struggling to come up a situation where that would typically occur, maybe a window header?

1) The slenderness ratio, Rb, is used for the calculation of F_be, we are not using the beam stability factor to calculate F_be. Rb still needs to be calculated based on the dimensions and properties of the member. Note that F_be only applies to checking loads along the strong axis (fb2).

2) The direction of F_be is specified. note in equation 3.9-3 that F_be is applied only in the fb2 direction.

 

[AaronMcD]

If you truly have a situation where biaxial bending is present but no axial load exists, I suppose you could use section 3.9.2, but I and struggling to come up a situation where that would typically occur, maybe a window header?

A window header could be one. I mentioned a skewed purlin in my OP (ignoring diaphragm action to be conservative). 3.9.2 applies to bending and compression as well and looks a bit easier to plug into a spreadsheet than 15.4.1

1) The slenderness ratio, Rb, is used for the calculation of F_be, we are not using the beam stability factor to calculate F_be. Rb still needs to be calculated based on the dimensions and properties of the member. Note that F_be only applies to checking loads along the strong axis (fb2).

2) The direction of F_be is specified. note in equation 3.9-3 that F_be is applied only in the fb2 direction.

Strong axis = fb1
And it would make sense to calculate F_be for bending about the strong axis, plus it is grouped with fb1, it just never specifies in the NDS the way it does for fb1 & fb2.

 

[AaronMcD]

On #2 >
Isn't that factor only applicable for beams and stringers?

Where are you getting that?

 

[lexpatrie]

I think C_fu it's a factor on E_min but it's only for beams and stringers. Referring to your initial question.

 

[AaronMcD]

Oh you're right. I hate how convoluted the NDS is when trying to figure out which factors to use on which properties. for which sizes. It's like a giant 3D matrix scattered around multiple chapters of multiple documents.

 

[lexpatrie]

Each chapter has that table, but a lot of the factors depend on the particular piece (LVL, glulam, versus say dimensional lumber). It's gotten better since 1993 if you ask me. But all these new materials keep wedging their way into the mix.

 

[ChorasDen]

Strong axis = fb1

Good point, you're correct. Serves me right for answering before coffee...