% masse modale

[Anonymous]

Bonjour,

dans une analyse modale, pour un mode de flexion anti symétrique à deux ventres par exemples

(1er mode d'une poutre sur 3 appuis),

dans le tableau de résultats de l'analyse modale, on trouve: masse modale UZ [%] = 0%

peux t on trouver la "vrai" masse modale du mode considéré dans Robot? par quel moyen?

merci

[Artur.Kosakowski]

Please check http://forums.autodesk.com/t5/Autodesk-Robot-Structural/General-torsion/m-p/3807626/

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

Ok merci,

dans le tableau d'analyse modale qu'est ce que le "facteur de participation" et "masses modales"

dans l'onglet "sommes de masses"?

[Pawel.Pulak]

Additional information to already provided by Artur.

Participation mass is zero for anti-symmetrical modes. The modal mass, which is calculated in different way is not zero in such case. It can be displayed in Robot - see below

Regards,

[Anonymous]

Hi, thank you!

What is the participation coefficient?

[Artur.Kosakowski]

Percentage of mass that participates in the vibration for given mode.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

so it should be mZ/tot mas UZ, but in all my examples it doesn't work...

how can this factor be >100? how can this be <0?

[t.sautierr]

Pawel,

to poursuie the explanation of the modal mass (mZ,mY,mX), why is this formula not verificated :

percentage of modal mass * total modal mass (= total mass actually) = mZ or mY or mX - for each mode

Thanks for your explanation.

Note : the formula is not verified even for symetrical mode.

[Pawel.Pulak]

As I have written before modal mass is calculated in different way than participation mass.

In case of participation mass it is cumulating for all possible modes (all dynamic degrees of freedom) to total vibrating mass ("Total mass"). On the screen capture below the participating mass for given mode and direction ("Cur.mas...") is expressed in percentage of total vibration so for all modes it is cumulating to 100%.

 

In case of modal mass it is calculated in completely different way and it is not cumulating to total mass.

A nodal modal mass for specific node and vibration mode on a specific direction is a product of node mass on this direction and of a square of value of the eigenvector for this mode on this direction normalized to 1.

A modal mass for specific vibration mode on a specific direction is a sum of nodal modal masses of this mode on this direction for all nodes.

Best regards,