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Buckling analysis results for bars and shell elements

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Message 1 of 4
skilak
911 Views, 3 Replies

Buckling analysis results for bars and shell elements

Hi,

 

I have got an issue with reading the results, namely the table shows the results for bars while the first and next 10 buckling modes are related to roof plate/shell. Following some of the posts on the forum herein I know that the cc is for the whole structure, the critical force ? I do not know what it represents for the case as on the attached model, slenderness and effective lengths ? what do they represent? e.g. bar no 2 is a truss type and critical force of -159.16kN represents what? bar no 4 has got critical force, buckling lengths and slenderness. Please could it be explained once more. I would like to know how I can use these additional information.

Is there any chance to ignore shell in buckling mode analysis, I mean avoid in the results so I can get the cc for the bars structure only?

 

Best regards,

 

3 REPLIES 3
Message 2 of 4
Rafal.Gaweda
in reply to: skilak

Buckling analysis calculates global buckling of the structure. Of course some modes (especially higher ones) correspond to local buckling of some parts of the structure (for instance columns).
For each load case the basic results of this analysis are critical coefficients (eigenvalues) and eigenvectors for appropriate buckling modes. Each critical coefficient corresponds to the factor by which the loads of appropriate load case should be multiplied to obtain appropriate loss of stability (buckling mode).
Basing on these coefficients and results of static analysis for appropriate load case (normal forces in different bars) critical forces for each load case, mode and bar can be calculated. Note that such critical forces are calculated for all bars (except of these which are under tension) disregarding the shape of buckling - these forces may concern bars for which buckling does not occur - in such case they have rather mathematical than physical meaning. For instance critical forces may be are also calculated for not loaded columns - in such case static analysis gives very small compression so the critical force (compression force* critical coefficient but for different (loaded) part of the structure) will be also very small (abnormally small).
Basing on critical forces and Euler's formula the buckling lengths for appropriate load case, buckling mode and bars are calculated.
In steel design module user needs buckling length of bar which is a property of bar itself and does not depend on load case and mode. In such case buckling length from buckling analysis can be taken only for very simple models or when load is applied directly to considered member and the buckling mode has local character related to this considered member.


Rafal Gaweda
Message 3 of 4
Rafal.Gaweda
in reply to: skilak


Is there any chance to ignore shell in buckling mode analysis, I mean avoid in the results so I can get the cc for the bars structure only?

 

Delete panels or replace them by diaphramgs (in case of slabs)

 

 



Rafal Gaweda
Message 4 of 4
skilak
in reply to: Rafal.Gaweda

I have read it already a few times;)
So if bar no 4 has got average axial force (why on earth the tension is negative, where I can change that the compression is negative ) of 8.8kN and x CC=15.312 gives the force 134.715 kN (close to the value in the table). Is this value, called critical force, the value which being applied will cause bar's failure? that bar is IPE400 and NbRd is 1274.23kN. Sorry for pushing but I cannot see the point of that feature (static load * cc) in Robot unless I analyse only one bar where the mode an cc is relevant to that bar.

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