MSC.Nastran : Effective Modal Mass (MEFFMASS)

Modal effective mass indicates the mass participation of each mode in each of the 6 rigid body motions (T1< -> R3). It's only useful under constrained conditions (no Rigid Body modes).

Participation Factors can be negative, and the other outputs - Fraction, MEFF Wt, & MEFF Mass - are positive, related to the square of Participation Factors.

i) Fraction is generally the most useful output - Modal Effective Mass divided by total Rigid Body Active Mass - and adds to 1.0 when all modes are included. You're often looking for mode participations > 0.10 Fraction, and often want to calculate enough modes such that cumulative Modal Effective Mass Fraction > 0.9 in each direction. In your 30-mode example, T1 Fraction columns show mode 9 (look in your F06 file) as most significant in the X-translation direction, T2 shows mode 1 significant in Y-translation, and T3 does not show any significant mode > 0.10 in Z-translation, among the 1st 30 modes. That means you didn't calculate enough modes to show when Z-translation becomes distinctly active. Z-translation is the "stiffest" direction of your structure (under your current constraints), and higher-frequency modes participate in that direction.

ii) Modal participation factors are actually the "raw" cofficients of each mode which linearly combine to equal the rigid body vector in each direction. A high absolute value indicates a high participation in that direction. These values are squared & divided by modal mass to give the Modal Effective Mass values. Modal Effective Mass values are in turn divided by total Rigid body Mass in each respective direction to give Modal Effective Mass Fraction's.

iii) Modal Effective Weight & Mass columns show modal participations in wt or mass units, and add toward the total active Weight & Mass each direction. Total active mass is given by the diagonal terms on the 6x6 A-set Rigid Body Mass matrix, given at the top of the MEFFMASS output.

I generally only need Modal Effective Mass Fraction, and request the following:

MEFFMASS(FRACSUM,Grid=xxx)=YES

As related to my previous reply, selecting a reference grid other than Basic 0 may help give more meaningful and higher Rotation participation values. And RESVEC=Yes should give an approximate high frquency residual vector mode that participates in the Z-translation direction, which can help indicate at what modal frequency Z-translation becomes active.