Mass acoording to a local direction of a Wall

[Tuctas]

Are we able to neglect the dynamic mass obtained from the self-weight of a wall according to (only) one of its (local) direction?

[StefanoPasquini6790]

Yes you are, if I've right understood your question, you have to: ignore density in modal analysis, assign self weight to all the structural parts that globally partecipate to modal analysis. Create a load condition for the self weight of your wall and assign it separately self weight then, in the load to mass convertion, convert it only for the desired direction

It's easy.

[Rafal.Gaweda]

to (only) one of its (local) direction?

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Stefano,

You are right in case of having _local_ axes parallel to globals ones.

For other directions it will be harder - additionally you have to play with load to mass conversion coefficients to obtain resultant in correct direction.

[Tuctas]

Rafal, you pointed out exactly what i was trying to say. I was talking about the case that the local axes are not parallel to the global ones indeed.  

Can you explain what you meant by:"...you have to play with load to mass conversion coefficients to obtain resultant in correct direction" ? Did you mean that it is needed firstly to define a separate load case including only the self weight of the panel and then to assign proper coefficients by using (separately) mass directions X and Y in order to define manually the vector on which the masses will finally operate? 

[Tuctas]

Any further recommendation about this issue?

[Rafal.Gaweda]

Sorry I was mistaken - not possible for LCS.

[Tuctas]

On more question: When using "Nodal masses" we can orientate them in any direction we want, aren't we? So do you think that if we manually concetrate the mass of a wall at the upper an lower nodes of it by using the option of "nodal masses", then we can manage to do what i was initially asking? 

[Pawel.Pulak · Accepted solution]

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Tuctas wrote:

When using "Nodal masses" we can orientate them in any direction we want, aren't we? So do you think that if we manually concetrate the mass of a wall at the upper an lower nodes of it by using the option of "nodal masses", then we can manage to do what i was initially asking? 

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Unfortunetely it will not work. When defining rotated added nodal masses they are distribited in global directions like vectors:( Change request is logged for it.

The only workaround I have found to model masses active only in-plane of arbitrary oriented walls is using added nodal masses and rotated compatible nodes:

1. defining nodes compatible to nodes of the meshed wall in the direction perpendicular to the wall
2. defining added masses in the compatible nodes
3. defining supports in compatible nodes to fix displacements and rotations not used in the compatibility definition (to avoid instabilities but in the same time to do not add any additional constraints to the structure)
4. freezing FE mesh to avoid merging pairs of compatible nodes during re-meshing

I have attached a simple example illustrating it. It contains a rectangular wall rotated 59.036 deg about Z from XZ plane. In such case the compatible nodes, additional supports and added masses are defined as on the screen capture below:

I have defined compatible nodes in all nodes of FE mesh. I have defined added mass in them using tributary area - it is a regular rectangular mesh so the following factors are used: 1 for internal nodes, 0.5 for edge nodes, 0.25 for corner nodes.

It is also possible to use only some representative nodes.

When checking the vibration modes it can be noticed thet they are found only in plane of the wall (no out-of-plane modes). Moreover when adding any loads to the model it can be seen that reactions in "rotated but UX" supports are zero so they do not add any constraints and only prevent instabilities.

I know that the method is not easy to use but at least it exists:)

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Regards,

[Tuctas]

Hello Pawel,

If i define properly the panel through the option of "orthotropic" in the window of "new thickness" (by modifying the coefficient that concerns mass), am i also going to be able to achieve what i was asking for  ?

Regards.

[Pawel.Pulak]

Hello,

In the orthotropic thickness properties there is no possibility to control mass for various directions. The thickness setting (see below) will modify mass for all directions and not only for the selected local one. 

Regards,

[Tuctas]

Thank you for the response Pawel,

Actually i was referring to the type: "Constant thickness with reduced stiffness", see the attached screen capture.

[Pawel.Pulak]

It is the same. For these settings there is also a single factor for mass, used for all directions. So using this feature it is also not possible to set different mass for different directions.

Regards,

[Tuctas]

I see...

Thank you.