Question
Eigenvalue analysis stopped with an error message stating that the number of masses is insufficient. What is the cause of this error?
[Image] Insufficient Mass Count Error Message
Answer
The above error message appears when performing an eigenvalue analysis and the number of mass degrees of freedom (DOFs) in the structure is less than the number of modes considered by the eigenvalue analysis.
In the given model, the number of mass DOFs is 12, and the number of modes considered in the modal analysis is 15. Therefore, the error message appeared because there were 3 fewer mass DOFs.
In such cases, it is recommended to set the number of modes you want to consider in the modal analysis to be less than or equal to the number of mass DOFs in the model.
Generally, when modeling a building and considering a Floor Diaphragm, if the DOFs for a floor with a Floor Diaphragm are DX, DY, and RZ (3 DOFs), then for a 4-story building, the number of mass DOFs would be 12.
Starting from MODS2009(V760), an enhancement has been made to automatically adjust the number of modes considered in modal analysis to match the maximum number of mass DOFs when the number of modes considered is greater than the maximum number of mass DOFs.