## Robot Structural Analysis

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# Bucking analysis of column

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I am testing buckling analysis on a simple model of a column and I am confused with some results.

I have two models with different supports.

For the first model, the bucking length is twice the actual length (K=2) and the critical force is close to the analytical solution.

However, for the second model, the buckling length is 3.63m rather than expected 4.0m. Same with the critical force. Also, why is *Critical coeff.* shown as N/A?

(I missed the second moment of area in the equations. The section is W100x19 and I=4.77 *10^6 mm4)

Solved! Go to Solution.

# Re: Bucking analysis of column

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However, for the second model, the buckling length is 3.63m rather than expected 4.0m. Same with the critical force.

You need to divide the bar into smaller parts for the better accuracy of the results. Applies also to dynamics (modal analysis).

Also, why is

Critical coeff.shown as N/A?

This is the limitation of the implementation of this analysis in Robot. The number of requested modes has to be larger than 1.

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**

# Re: Bucking analysis of column

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I divided the bar and now I am getting N/A everywhere :-/

# Re: Bucking analysis of column

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lso, why is *Critical coeff.*shown as N/A?

This is the limitation of the implementation of this analysis in Robot. The number of requested modes has to be larger than 1.

**Artur Kosakowski**

# Re: Bucking analysis of column

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Ok, so now I have Critical Coefficient, but Critical Force and Buckling Length are still N/A.

# Re: Bucking analysis of column

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These results are not available for superbars (bars divided into parts/calculation elements). If you divide the bar into ('real') smaller bars then they will be displayed.

Please check the 2nd (and perhaps the 7th) post from http://forums.autodesk.com/t5/Autodesk-Robot-Struc

**Artur Kosakowski**

# Re: Bucking analysis of column

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I am not sure if the referenced posts are relevant to this issue, but the first paragraph answers my question. Thanks.

# Re: Bucking analysis of column

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__As your questions were for buckling length and critical force I wanted to make sure their correct interpretation as well as the description of the buckling analysis in Robot is easy to find for any other person reading this topic. I assumed that he or she may be not that familiar with Robot as you are. I hope this is OK.__

**Artur Kosakowski**

# Re: Bucking analysis of column

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Artur.Kosakowski wrote:These results are not available for superbars (bars divided into parts/calculation elements). If you divide the bar into ('real') smaller bars then they will be displayed.

Artur,

can you please explain what is exactly the diference between the superbar and the"real" smaller bars?

i follow this thread and i made my own example but when i divide the bar into two with the divede tool i still have NA at the critical forces and the buckling lengths.

are these bars that came from the division of the original considered " real" or superbar?

Also , the Ncd,Rd and the Nb,Rd from the steel desighn calculations are the Critical forces at the both axes?

thanks

# Re: Bucking analysis of column

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can you please explain what is exactly the diference between the superbar and the"real" smaller bars?

The superbar is just the bar divided into smaller calculation 'bars' by Robot itself. This is due to the fact that there are some nodes along its length. For the 'calculation' point of view this is the same as a chain of 'standard' bars. The difference is in the way you access the results (for one bar vs. for many bars) e.g. in the Forces table.

i follow this thread and i made my own example but when i divide the bar into two with the divede tool i still have NA at the critical forces and the buckling lengths.

are these bars that came from the division of the original considered " real" or superbar?

Mind that by default this option does not splits bar into a smaller ''standard' bars (see the attached picture). In addition check if your bar is under compression.

Also , the Ncd,Rd and the Nb,Rd from the steel desighn calculations are the Critical forces at the both axes?

The description of the forces can be found on the Detailed tab of the steel design results dialog. Mind that the buckling analysis is not intended for determining the buckling lengths of elements of a model for the code checking. This was the reason I provided the link to its description in one of the earlier messages.

**Artur Kosakowski**