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aruser
Posts: 82
Registered: ‎08-12-2011
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Purlins design - Lateral buckling

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01-29-2013 04:32 AM

Hi everyone,

I would like to ask/share the community the way they set the lateral buckling parameters in purlin design; specifically the buckling lengths for both the upper and lower flange.

 

Do you only use real distances measured between elements that fix the flange against the lateral buckling??Independently the load case and its moment diagram?

 

In my opinion the program should also be capable to calculate automatically this length depending on the moment diagram; measuring from where the sign changes until the fix element.

 

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Artur.Kosakowski
Posts: 4,956
Registered: ‎12-17-2010

Re: Purlins design - Lateral buckling

01-29-2013 04:37 AM in reply to: aruser

Please see: http://forums.autodesk.com/t5/Autodesk-Robot-Structural/LTB-amp-Steel-Bracket/m-p/3385765 (message 3)

 

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Artur Kosakowski
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Valued Contributor
aruser
Posts: 82
Registered: ‎08-12-2011

Re: Purlins design - Lateral buckling

01-30-2013 01:25 AM in reply to: Artur.Kosakowski

Ok, I am using that option to define the lateral buckling length.

 

Defining a two span roof purlins, I'm definig them as you can see in the attached image and robot file.

-->Upper flange: No lateral buckling since it's fixed to the cladding and I assume that it is enough strong to completely brace the upper flange (despite the fact that I'm designing a thin walled section and I also have the option of "purlins stiffened with profiled sheet EN3-1-3 chapter 10)

-->Lower flange: Lenght from the point where bending moments equal zero to the support.

What do you think about this configuration to define the purlins for the typical industrial building??

 

But besides that, why robot takes different buckling lengths for each bar?

 

Thanks.

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Artur.Kosakowski
Posts: 4,956
Registered: ‎12-17-2010

Re: Purlins design - Lateral buckling

01-30-2013 02:34 AM in reply to: aruser

What do you think about this configuration to define the purlins for the typical industrial building??

 

Based on what  you wrote I think it is OK.

 

But besides that, why robot takes different buckling lengths for each bar?

We will need to investigate this. To determine the location of bending equal zero Root checks values in 13 points along a bar (bar is divided into 12 segments) with the linear approximation of the values among them. In your case the 0 value is exactly in one of the division points and in such case the detection of the zero bending location is not exact for bar 2. This is most likely due to some numerical issue (if you reverse the orientation of the bar 2 the result will be the same as for bar 1). Anyway the difference in LTB length will not be larger than 1/12 of a bar (250 mm).

 

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
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Valued Contributor
aruser
Posts: 82
Registered: ‎08-12-2011

Re: Purlins design - Lateral buckling

01-30-2013 03:02 AM in reply to: Artur.Kosakowski

Indeed the results are the same if reverse the orientation of the bar 2. For gamma 0.

 

But If you incline the sections (usually sloped roof) then you will get different results again.(besides the inconveniences of different gammas: bar 1 gamma positive and bar 2 gamma negative)

 

Looking forward to finding this issue solved soon.

 

Regards.

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Artur.Kosakowski
Posts: 4,956
Registered: ‎12-17-2010

Re: Purlins design - Lateral buckling

01-30-2013 03:20 AM in reply to: aruser

 

But If you incline the sections (usually sloped roof) then you will get different results again.(besides the inconveniences of different gammas: bar 1 gamma positive and bar 2 gamma negative)


Mind which flange is upper and which lower (check the orientation of the local Z of bars). The results are as far as I can see the same for both bars (provided you 'reverse' LTB parameters for bar 2 after you change its 'direction').



Artur Kosakowski
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Valued Contributor
aruser
Posts: 82
Registered: ‎08-12-2011

Re: Purlins design - Lateral buckling

01-30-2013 03:36 AM in reply to: Artur.Kosakowski

yes this way is ok to manage the problem temporally while waiting the software fix .

Thanks.

 

 

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Artur.Kosakowski
Posts: 4,956
Registered: ‎12-17-2010

Re: Purlins design - Lateral buckling

03-27-2013 07:13 AM in reply to: aruser

 

Corrected in SP4 for Robot 2103. Please check for the availability of this update at the beginning of the next week.

 

 



Artur Kosakowski
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