PURLINS- BUCKLING PARAMETERS

[TSOFA]

 Hi again!!

I have a typical portal frame structure with portal frames every 5,42m. The stability check of purlins makes the purlins to be huge (IPE 220!). From experience I think that IPE 140 purlins should be ok. The problem is that I am not sure what buckling parameters I should use. Is roof panel provides buckling restrain or only lateral restrain? I am also not sure what is the difference between buckling member length (actual or coefficient) and buckling coefficient (see picture attached). 

[StefanoPasquini6790]

Hi,

The most performing cross section for purlin is "omega".

Purlins are subject to bending moment, then the buckling is not requested. You have only to check the slanderness.

For flexural torsional buckling you have the upper flange restrained from the covering panels.

Let me know if I'm clear and if you need more help.

[TSOFA]

sorry, which is the omega section? What slender limit do you use? Do you think that is ok to assume that the panel will provide protection against buckling also?

[Artur.Kosakowski]

I am also not sure what is the difference between buckling member length (actual or coefficient) and buckling coefficient (see picture attached). 

I'd recommend to not change the prior (set it as 1) and use the latter instead. Altering a bar length may influence the verification of a bar in a way that you don't expect it. 

[TSOFA]

So, I will only change the buckling coefficient and not the buckling length. 

What I did was to set the buckling coefficient of  axis (y) as 1 and of axis (z) as "zero" ( X ) because I assumed that the panel will provent the purling to buckle along Mz. I also set the upper flange as not able to buckle for the same reason.  Do you think that my assumptions are OK?

[Artur.Kosakowski · Accepted solution]

Yrs, I think that for simply supported purlin they are correct. For the multispan one you may also consider setting LTB restrains for the lower flange at the points where bending moment changes sign. 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[TSOFA]

In reality I will have multispan purlin, but in order to model this I will have to model compatible nodes between the purlins and the rafters. I don't want to do that because it makes the model more complicated. Do you think that I am from the 'safe side' if I model the purlins as simple supported?

[Artur.Kosakowski]

I would make one of the purlins as continuous and compare the results.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Artur.Kosakowski · Accepted solution]

The stability check of purlins makes the purlins to be huge (IPE 220!). From experience I think that IPE 140 purlins should be ok.

By default in Robot you run check against axial force with bi-directional bending and I sort of suspect that you for "hand calcs" you consider only unidirectional bending. In case you assume that the surface of the roof takes both axial and secondary bending you may exclude them in the Configuration settings.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.