For the "RF-/DYNAM Pro" add-on module, there are the parameters Modal mass, Participation factor, and Equivalent mass. Although the manual contains the formulas, the meaning and the explanation, it is also helpful to use them.

Answer

Modal mass

Each multi-mass system can usually be represented by a single-mass system. When performing this transformation, the modal mass of the system is necessary. This mass is required to generate the frequency of the equivalent oscillators with a single degree of freedom.
 
Participation factor

This factor can also be negative because it consists of the equivalent mass on a node and the corresponding displacement due to the mode shape. If the deflection is in the negative direction, the participation factor becomes negative. The replacement mass factor is then still positive, since the participation factor is squared. (see formula)


Equivalent mass

The equivalent mass of a system is a part of the total mass which is excited due to the vibration of the multi-mass oscillator. The equivalent mass of a system can be between zero and the total mass. The equivalent mass factor is only the quotient from the total mass to the equivalent mass. Thus, it is usually possible to check more quickly what the ratio of the excited mode mass of the respective mode shape is. Should it happen that the equivalent mass factor is greater than 1, you should check the discretization of the system and, if necessary, refine the division of the nodes or the FE mesh.
 
For an earthquake analysis, the equivalent mass factor and the equivalent mass are generally decisive because these values of the dynamic equivalent loads are calculated for the building.

Keywords

Modal mass Participation factor Equivalent mass

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