Displacement results from Dynamic Analysis, Modal Analysis

[Yoan]

Hello,

I'm in the process of doing a vibration analysis for a client, who wishes to derisk their design with respect to vibration.

I was able to successfully get the model to run, performed the modal analysis,and determined the modal shapes and natural frequencies. However, when looking at the displacements predicted by Pro-Mechanica thru the modal analysis, the displacements seem very large. My first question is, since I haven't put any loads or inputs into the system yet, what are these displacements that mechanica is predicting?

I subsequently ran a dynamic frequency analysis with a number of measure points (monitoring acceleration) at areas of interest per the modal analysis. I was able to generate acceleration vs. frequency curves which seemed reasonable, and showed amplification at some of the natural frequencies.

I then reran the dynamic frequency simulation with some measure points monitoring displacement. I set these to monitor the Z axis displacement, at a point, with Time/Frequency Eval set to on, with Dynamic Evaluation set to "At Each Step".

When I plot this measurement, I get a graph that at 0 Hz is at an impossibly large displacement of 400 m, falling to 0 m in a linear fashion at around 250 Hz, and remaining 0 m for the rest of the spectrum. At this point, I'm a little perplexed what the graph is telling me.

 

[Twoballcane]

When you do a modal analysis, there is no load that you put ont the model thus there are no displacments, really just a percentage. Not untill you add a load (random or sine) you will see the displacments.

Tobalcane
"If you avoid failure, you also avoid success."

 

[Yoan]

I believe I found the issue, and just wanted to post it here should anyone else run into the issue.

When I was doing my dynamic frequency analysis, I always had my input set relative to ground. This gave me good acceleration curves, but gibberish for displacement.

Changing the input to be relative to supports, I got displacement graphs which now made sense. Displacement magnitudes were reasonable, and the graph mimicked the acceleration graph. This seems reasonable to me as displacements should increase as we approach natural frequencies and amplification starts to occur.