need instructions for vibration (modal) analysis

[Skicat]

I have a simple 3D truss that will be used to support a small wind generator. My truss is subjected to a gravity joint dead load and a joint wind load due to the wind generator. I feel fairly confident that I am capable of performing a vibration analysis of my truss (the wind generator company wants to ensure the natural frequency of my truss does not match the natural frequency of their wind generator). I would appreciate it if someone could provide step by step directions detailing how to perform a vibration analysis and review the output. Thank you.

 

[sebivaq]

Dear skicat,
first of all make your model, with the proper stiffness and masses. If you want to consider same load as a mass for the dynamic analysis, go to define/mass source.

When you run the analysis, activate "MODAL CASE" (as default is activated).

Then you should go to display/show deformed shape/ pick the modal case and the mode number. On the display you can see the period of the structure.

Bets regards,

Sebastian

 

[IJR]

Exact like Sebastian mentioned

But you mentioned a 3D truss, and that makes correct modelling tricky. It all depends on how your mass is distributed on your truss and the presence of very weak elements.

Please check the modeshape to see that the first one you get moves the whole truss together, not just a few parts. The period you get for this modeshape will be the correct one.

If your first modeshape shows only a few elements moving while others are almost stationary, that will be a bad mode and wont really represent your frequency

Distribute your mass carefully, realistically.

respects
ijr

 

[Skicat]

Thank you both very much for your help.