Is there a way to run a modal analysis computing just eigenfrequencies with an associated mass fraction over a certain %? The aim is to safe computational time.
You would have to compute the mode before throwing it out anyways, so there is no benefit to the solution time. It would benefit result file size though.
In Real Eigenvalue Analysis (SOL103) you can use the MEFFMASS case control command to control the amount of modal content that is calculated. To do so, specify values for the MINT1, MINT2, and MINT3 describers (read the QRF for more info). NX NASTRAN solver continues to calculate modes until the total effective mass fractions in the translational X-, Y-, and Z-directions exceed the values you specify for the MINT1, MINT2, and MINT3 describers, respectively.