value of Displacement sum USUM on modal analyses in ANSYS Mechanical APDL 18.1

[andradesilva]

Hi!

I am doing several modal analyses in ANSYS Mechanical APDL 18.1 for my work as R&D engineer.

My element is a beam with section width b=0.2 [m] section height h=0.3 [m], and lenght L=1 [m]
I am getting USUM values from ANSYS. From what I can understand, USUM is the sum of the 3 displacement components: UX+UY+UZ
However, I am getting displacements of 2-3 meters for some modes for eigenvalues between 400-600 Hz. When I look to graphical results, the deformed displacements are not really close in the model to 2-3 meters. In fact, they look like 0.05-0.1 meters. However, there is like 2-3 meters on the scale below. I found out that 95% of the USUM value comes from the UX component, which is the direction of the section width.

I am using consistently S.I. units: [m] [N] [kg]
The values I am obtaining do not look acceptable to me. Do you know what is going on? If everything is fine, could you explain the reasons of these values?

Thanks in advance,
AndradeSilva

 

[rickfischer51]

First, you are plotting an eigenvector. It is not an actual modal displacement, but rather a mode shape. If you want to see actual deflections, you need to do an expansion pass. See the MODOPT command. Second, the deflection results are likely scaled for plotting. You can use the /DSCALE command to change the plot scale. /DSCALE,1,1 will produce a plot with the observed deflections matching the values in the contour legend.

Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[scheah]

This applies to analysis as well: [URL unfurl="true"]https://www.uml.edu/docs/feb00_tcm18-189824.pdf[/url]

p/s: USUM is vector sum magnitude (square root of the sum of the squares)


Kind regards,
Jason

 

[andradesilva]

Hi,

Thank you for the replies and for the help.

I've done a modal expansion and I've scaled the results to a factor of 1, as suggested. I don't understand how can a beam with 1 meter of Lenght and a section width of 0.2 meters can deflect 2 meters for most of the modes, not only in terms of displacement vector sum, but also in UX (displacement in the direction of the width).

Can these results be possible? Am I doing anything wrong? I am sure I am not understanding something. Could you please post a link to a resource where this is explained (book, thesis or website)?

I am using units in [m] and [kg] (S.I.)

Thanks again,
Best regards,
AndradeSilva

 

[rickfischer51]

Did you ably any loads or boundary conditions or is it just a free vibration?

Rick Fischer
Principal Engineer
Argonne National Laboratory