Portal Frame Design - software application vs buckling length in the major axis

[Karlos80]

Hi,

I am using Autodesk Robot to model a standard low-pitch portal frame. In the software the rafters are modelled as two separate elements (broken by a central node at the apex).
There are separate settings for lateral buckling to define the spacing for adjoining members. However, I was wondering about the buckling along the major axis.
The rafter should be a full span element for the bucking along the major axis but it is split into two members and therefore the length is only L/2.
I can not make this super-member because it will change all the loading orientations in the model (as the rafters are not horizontal elements).
My understanding is that the buckling along the major axis is not really a problem because of small axial forces in the rafters. The most important is the lateral buckling along the weak axis restrained by adjoining purlins, stays etc.

Do you agree with me?

Regards
Karlos

 

[JoshPlumSE]

Not necessarily.

However, if you have lateral and gravity loading applied at the same time, and the software is sufficiently capable of doing a P-Delta analysis (big and little) then this analysis should capture the global buckling of the frame in the strong axis.

Also, the "unbraced length" you want to use for SA buckling of the rafters may be more than L/2 in reality. But, that's one of the reasons why AISC switched to the Direct Analysis Method.... Because determining unbraced length for something like this is not necessarily straight forward.

 

[liam1369]

Hi Karlos,

In my experience the rafters are usually governed by wind uplift and you need intermediate stays connecting the bottom flange to the purlins to prevent LTB to the rafter as you mentioned. Make sure you consider this and not just the top flange. Owing to the significant wind uplift which is common for a portal frame.

As you mentioned, you have a low-pitch, therefore, the axial column buckling aspect of it will not be governing really at all.

Depending on the software package, it will likely be put in as a 'beam' element, therefore, the checks the software carries out will correspond to the member designation, therefore, it will likely only be carrying out the beam checks = LTB, Moment, Shear, deflection etc.

Liam