Eigenvalue was found out of sequence

The purpose of this page is to explain the warning message "Eigenvalue was found out of sequence".

Modes are found in order of increasing distance of frequency from the shift. If lower frequency is found after a larger frequency, the program will display the "out of sequence" warning. Please note that this is a warning, not an error, and may occur if the structure is sensitive to instabilities.

It is usually an indication that the modes are very close together, and in that case it can be expected. It could also indicate that the model is numerically sensitive, with excessively large stiffnesses or poor connectivity or missing restraint. The user should check to make sure this is not the cause.

Mostly, it is just to let the user know that if they were expecting the modes in a particular order, this did not happen. Or if they are requesting a limited number of modes, they may want to rerun with more to make sure they are not missing any.

A structure that is unstable when unloaded will have some modes with zero frequency. These modes may correspond to rigid-body motion of an inadequately supported structure, or to mechanisms that may be present within the structure. It is not possible to compute the static response of such a structure. However, by using a small negative shift, the lowest-frequency vibration modes of the structure, including the zero-frequency in stability modes, can be found.