Learning Forum

[R12da]

Hello everyone,

I have a question about the difference between the cumulative mass fraction and the ratio effective mass to total mass

in the aim of finding the mode that verifies 90% of effective mass, i searched on the internet and saw that i have to see the value of the cumulative mass fraction

can you guys explain to me the difference between the two informations and how to answer the question of mode at 90%effective mass

Thank you so much

 

[peteroznewman]

Please insert images directly into posts because ANSYS staff are not permitted to open attachments.

You can see in the last column that the effective mass in mode 3 is 96.3% of the total mass for vibration in the X direction. You can see that modes 1 and 2 only add a tiny fraction more.

It's important to look at the Y and Z directions also. If you require over 90% participation, then 3 modes is enough for the X direction, but you have to check when the cumulative column exceeds 0.9 in the other two directions.

[R12da]

Hi, Thank you for your answer

for the pictures it's noted for futur threads

I have another example: 

 

can i say that the mode 27 verifies the 90%participation?  Cause i looked on the theorical ansys guide for the meaning of the cumulative mass fraction and that's what i understood

can i have more explanation?

Thank you again

[peteroznewman]

Yes, you need to include 27 modes to have 97% of the mass participating in a Harmonic Response or other analysis that uses the Modal analysis.