Gable end frames

[rahulsharma3285]

Hi,

I want to know what unsupported length should be used on a portal frame? eave to eave? or eave to ridge?

Thanks,

Rahul

 

[JoshPlumSE]

For what type of buckling? Beam type lateral torsional buckling? Buckling of the overall frame? Weak axis column type buckling due to axial force? If its column type buckling are you concerned about the beams (due to axial force in the inclined member) or the columns?

 

[rahulsharma3285]

Hi Josh,

Thanks for the reply. I am concerned about the buckling in the beams due to axial force in the inclined member. Unsupported length for the minor axis will be the distance between the purlins or secondary members. What will the unsupported length in the major axis be?

Thanks,

Rahul

 

[JoshPlumSE]

Two thoughts:
1) If you're doing a simple hand calc, then there isn't a true right or wrong answer. I'd probably take the length from ridge to eave, but then use a K value greater than 1.0 to account for the fact that I'm in a sway frame and I don't have full support at the ends. What value to use for this K is pretty tricky though.

2) If I'm doing an FEM analysis of the frame (one which considered P-Delta effects) such as the Direct Analysis Method from AISC, then I would use the same unbraced length (eave to ridge). However, I would use a K = 1.0 and let my 2nd order analysis predict my member and frame buckling. Note: this may require introducing extra joints along the length of the member to capture the P-little delta effect.

 

[rahulsharma3285]

Thanks a lot Josh. I will use the distance from the eave to ridge as my unbraced length. My Ram elements model has calculated and assigned a value for K as 2.3 for beam. I will stick to that.

Rahul

 

[hokie66]

I think that major axis buckling is normally ignored, as bending is predominant. But as to minor axis buckling, the purlins don't necessary brace the bottom flange. You need some diagonal 'fly braces'.

 

[rahulsharma3285]

Hi Hokie,

Thanks for the reply. I have an odd case. I agree; the purlins only brace the top flange. For uplift case, the bottom flange will be in compression and I will have to provide fly braces.

Thanks,

Rahul