#418 Some comments about EXCLUDE command in Linear Buckling Analysis using OptiStruct

The EXCLUDE command can be used to select a set of elements to be excluded from a Linear Buckling Analysis.

EXCLUDE = ESID
(ESID is Set identification number of an element set)

1. The excluded elements are only removed from the geometric stiffness matrix, resulting in a Buckling Analysis with elastic boundary conditions. This means that the excluded elements may still be showing movement in the buckling mode.
2. Extreme caution is advised when using the EXCLUDE command. In general, excluding any region from buckling analysis can, and usually will, result in a higher, overestimated critical load calculation, which then may produce false overconfidence in a structure’s load bearing capacity. The excluded region will have no effect on the calculated critical load only if the excluded modes are geometrically separated from, or orthogonal to, the actual critical buckling mode.
3. A descriptive engineering explanation is as follows: Buckling Analysis by design seeks the weakest, least stable configuration of the structure. Excluding any region from this search can cause you to miss the actual critical buckling mode that involves the respective region and has the least load bearing capacity.