This window is displayed when you select Elastic Buckling from the Compute menu. After you enter the appropriate values and choose the appropriate settings, press the OK button and the Elastic Buckling Results will be displayed when the analysis is complete.
The elastic buckling analysis uses the finite strip method to determine the magnitude of the forces at which elastic buckling occurs. These buckling forces are affected by the member length (half wave length) and the stress distribution. The analysis will determine forces corresponding to numerous member lengths, and identify the lengths that correspond to controlling buckling modes.
The time to complete the analysis can take from a few seconds to several minutes depending on the complexity of the crosssection, the number of member lengths to calculate, and the speed of the computer.
Start Length  The smallest member length to consider in the buckling analysis. To capture local buckling, this should generally be less than 1/2 the width of the largest flat element. 
End Length  The largest member length to consider in the buckling analysis. Intermediate lengths of 5 to 10 times the size of the section may exhibit distortional buckling. Greater lengths will reveal flexural and/or torsional buckling modes. 
Increment Size  The size of the length increments
to use between the start length and the end length. The choices are:

Use Alternate Method  CFS uses a very fast eigenvalue solution technique to determine buckling stresses and mode shapes. There are some rare cases where this method does not calculate the buckling stresses for all member lengths. If that occurs you may choose the alternate method which might be able to obtain more of the results. This alternate method takes much longer to run. 
Constrained Bending  Bending about a nonprincipal axis causes stress redistribution resulting from a rotated neutral axis. If you leave this option unchecked (recommended), the stresses will be adjusted but the buckling moments will correspond to the proportions you specify. If you check this option for constrained bending, the output buckling moments represent those which produce the stress distribution you specify. For sections oriented to the principal axes, this option has no impact. 
Repeating Connected Shape  Choose this option for sections which are rigidly connected in a repeating pattern, such as some panel installations. This will force the start of the first element and the end of the last element to translate and rotate together during buckling. This option is not available for closed sections. 
Stress Distribution  The vertical and horizontal slider bars are used to define the stress distribution you would like to apply to the crosssection. With both slider bars in the middle, uniform compression is applied. The vertical slider bar controls the moment about the X axis, where the top position produces a positive moment and the bottom position produces a negative moment. The horizontal slider bar controls the moment about the Y axis, where the right position produces a positive moment and the left position produces a negative moment. 
fbx, fby, fc  The relative stress level resulting from bending about the X axis, bending about the Y axis, and compression, respectively. Bending values may vary from 1 to 1, and the compression value may vary from 0 to 1. The sum of the absolute values is forced to be 1. 