Buckling Analysis
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.
Rafal Gaweda
Accepted solution
First of all - check supports. It seems there are pinned supports in book, fixed in robot.
Then divide columns onto several pieces ie 10, then check results
If it is not the case please attach the model.
Rafal Gaweda
Please mind that buckling analysis in Robot is intended to check the overall stability of the structure rather than determining buckling length of each particular bar of the model in the sense you think while running code checking according to a selected steel design code. In this sense the value of the critical factor is what can be referred to as 100% accurate (again this indicates the critical load level that would cause the entire model /or its part/ to become unstable).
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.
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Artur Kosakowski
Rafal, the link points the topic we are discussing in 
I read that, Iwanted some more explanations.
Thx a lot
Program calculates critical coeff (for which bar - it is not known because it is global buckling analysis). To calculate critical force Robot uses Euler formula , force in member (whatever value it is) and critical coeff (so critical coeff may not correspond to real critical coeff for abr)
Rafal Gaweda
Rafal Gaweda
Bcuking shape of your structure is this, you were looking at the deformation of the load case not the buckling shape.
I'm not sure that the buckling length means something easely understandable for this kind of shape.
If you keep only two columns and one beam :
