Modal Analysis Outputs

[Anonymous]

I am trying to obtain an angle of twist for each node under different mode shapes.  

I'm confused about the eigenvector values being given in the Dynamic Analysis Table.  I can obtain RX, RY and RZ, but these dimensions appear to be in mm, and not radians or degrees.  I know this as I can only obtain a value >0 when I increase the number of decimal places for displacements.  Doing the same for radians or degrees only does not increase the decimal places in the table.  Why is this?

The values shown  are not what I am expecting for the different mode shapes shown, and given the above I don’t know if they are actually giving me what I am looking for?

Help please!

[Pawel.Pulak]

The number format of components of eigenvectors is for both the translation components (UX, UY, UZ) and the rotation components (RX, RY, RZ) taken from Other>Displacement

As concerns the values of eigenvectors itself they are unitless but of course the relation between translation and rotation components is kept to give the correct shape of the vibration modes.

The rotation components are given as derivatives of translation components (like RY=dZ/dX) so they are also dimensionless.These derivatives correspond to tangents of appropriate angles.

It can be seen for instance defining simply supported beam with length of PI meters, centimeters, feet,... and defining dynamic modal analysis of it (mass matrix with rotation). When checking the1st eigenvector it can be seen that max UZ=1 and max RY=1 (improving with increased number of divisions) so it means that for unit displacement the rotation in beginning and end of beam is 45 degrees (RY=dZ/dX=1 or atan(1)=45degrees).

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