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Harmonic v. Harmonic FRF

Message 1 of 6
925 Views, 5 Replies

Harmonic v. Harmonic FRF

Hi all,

I've wanted to check harmonic analysis in Robot using single supported beam and I've recived a bit odd results.

I have created simple supported beam with one nodal mass in the middle of the beam (the weight of the beam is disregarded) and I've assumped no damping. I've also created a simple checking calculations in Mathcad (attached with model to the post). The force is equal to 1000kN and frequency is 10Hz.


Harmonic analysis

I've obtained the following result. The bending moment in the middle of the span is equal to 478kN*m




Harmonic analysis in the frequency domain.

I've used a trick. I've assumed that initial frequency is the same as the final in order to obtain results for only one frequency (10Hz). The bending moment in the middle of the span is equal to 4955.28kNm. The same result I've obtained from my Mathcad calculations.

As you can see the results from harmonic and harmonic FRF analysis are different. Could you advise what is not OK?






@Anonymous Artur Kosakowski marked post as the solution. If it did not work as an answer, please post your reply in this thread so that I can unmark the solution. 

Message 2 of 6
in reply to: Anonymous

It seems added mass is not or wrongly considered in Harmonic case.
Need to be checked be development.
Thank You for reporting

Rafal Gaweda
Message 3 of 6
in reply to: Rafal.Gaweda

Correct way of definition Harmonic case for considering masses from modal case


Rafal Gaweda
Message 4 of 6
in reply to: Rafal.Gaweda

Starting from 2016 release both ways of defining harmonic cases are correct:

  1. using "New" case button in Analysis Type window and selecting Harmonic
  2. using "Change analysis type" button in Analysis Type window to change static case into harmonic one



Pawel Pulak
Technical Account Specialist
Message 5 of 6
in reply to: Anonymous


I have a question about damping in harmonic and FRF harmonic analysis :

Without damping, one frequency set up in both analysis ( 72.5Hz for harmonic and 72.5Hz to 72.5Hz for FRF), the same added load (10 daN) : I get the same results of internal forces as you did before.

A soon as I set a constant damping (0.004 for both) , the results are completely different. Could you explain why?



Message 6 of 6
in reply to: Anonymous

Hi @Anonymous 

Do you have a citation where did you get the dynamic numerical calculation for this simply supported beam?

Or evenmore could you answer these questions:

* I can't get the same result -0,755 m for displacement due to dynamic force, what parameters are you using inside equation?

* Why did you used only 1000 kg mass in the middle of the beam but not 1000 kN?

* On Autodesk Robot analysis how did you manage to disregard weigth on modal analysis?


Thank You,


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