Cold Formed Section- Global Buckling

[SteynvW]

Hi All

We were recently assigned to evaluate an old existing warehouse and to
establish if solar panels can be added to the rough.

Now the purlins are an odd size Z-shape where our design code does
not have tables for, so we dived into the abyss that is cold formed steel.
Our code is based on the Australian Code and as I understand it is based on AISI.

We went through the process of doing a finite strip analysis with CUFSM and got the signature curve, now the
AISI DSM Design Guide gives guidance for global buckling over an unbraced length (Attaced).

2 Points have to be selected where global buckling occurs and the procedure followed to get the alpha
and beta values to calculate Mcre.

Now the problem we have is selecting the points, selecting different sets of points gives us different
Mcre values. Is there a guide over which points must be selected or am I missing something entirely.

 

[SteynvW]

Here is the Signature Graph

 

[HLE111]

I've always calculated CFS capacities via effective width method, the direct strength method is rather new and doesn't have much commentary.

There is a CFS design software created by RSG Software that allows you to design custom shapes. It has a direct strength method as well but I've never used it.

RSG software has a free version with limited capabilities if you want to check it out.

 

[SteynvW]

Hi HLE

Yes, the effective width method is just quite tedious. As I understand it CUFSM more or less
the same as the module produced by RSG.

I went through my spreadsheet and it was a formula error, and get good results now.