It is the first time that I do a buckling nonlinear analysis and I have some questions:
1) what is the "critical load" that I can see in the buckling Analysis results?
2) Can I have a buckling lenght coefficient from this critical load?
As concerns calculating the buckling length using buckling analysis to use it later in steel design module it is possible but in most cases the calculated buckling lengths are not the values user expects.
Please find below the step by step explanation of how it works.
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.