Mass participation Rz

[Anonymous]

I am working on a 3d model for an industrial building and when I run a modal analysis, mass participation for torsional mode, Rz, appears with very low values (less than 1%). I am using consistent mass matrix and X,Y & Z directions of masses are active.

Why could this occur?

Any tip concerning this matter would be helpful.

Regards.  

[Romanich]

Hi @Anonymous ,

It's hard to say without the model. 

[Simau]

@Anonymous 

Could you attach dynamic results table with Ux Uy mobilzed mass

[risefire12]

hello mr roman

i have the same problem in RSA i get 0% for cur.mass.rz  for all modes even though my model have torsional irregularities... plz if you can look at my model

[Simau]

Hi @risefire12 

Looking at Dynamic results Table, it seems that there is no torsion in your building

[risefire12]

thank you for your reply!

if you view the deflection for mode 1 in robot you will realize that it is a torsional mode... 

in addition i did the same model in ETABS and almost same results in participation in X and Y (blue color) but in Rz you can see results in etabs (Red color) and the first mode is 53% in Rz (0,5388 green color )...

so there must be an explanation for that

[Simau]

Hi @risefire12 

I agree with you about Rz value.

In my side, i never use Rz for torsion verification, but only mass participation in X and Y direction.

So I did a test with a structure having an obvious twist and indeed the value of Rz is always close to zero.

Maybe the support team have explanation

[risefire12]

it is really weird that it doesnt give any real values for Rz...hope the dev team response quickly

[risefire12]

@Krzysztof_Wasik 

[Krzysztof_Wasik]

Hi @risefire12 

Similar topic was discussed before on Robot Forum

Please refer to

https://forums.autodesk.com/t5/robot-structural-analysis-forum/torsional-mass-participation-rz/m-p/6...

and linked

https://forums.autodesk.com/t5/robot-structural-analysis-forum/robot-modal-analysis-torsional-mass-p...

Rz mass participation is calculated for individual nodes torsion rather than for whole building torsion effect. Whole building torsion effect is represented mainly as UX and UY nodal displacement/mass participation.

In case of simple frame you can expect significant RZ mass participation for the following normal mode.

[risefire12]

Hi @Krzysztof_Wasik 
First of all thank you very much for you reply.
I saw the two links thank you again, but please can you explain more how the torsional effects is represented by Ux and Uy ?

[Krzysztof_Wasik]

Hi @risefire12 

You can decompose each node displacement into Ux and Uy component

[risefire12]

so in general the only way to get the Rz for the whole structure is by changing the calculation model to Rigid diaphragm (No Fe's) ?

[Krzysztof_Wasik]

Hi @risefire12 

This solution is proposed in previous forum threads.