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Robot Structural Analysis

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Registered: ‎06-23-2008

Re: Simple Time History problem - why no resonance

01-15-2013 06:34 AM in reply to: bjur

            In the case of an FRF analysis, if I use a different load value, will I get different FRF results?


Yes. Because it is linear dynamic analysis increasing the load for instance twice will result in increasing the results twice.



            I've been trying to understand this FRF analysis type better. Does the load direction matter?  Can I just use a unit load?


What do you mean by the load direction? Changing the values of  components of the load thus changing the value and direction of the resultant or only changing the signs of load?

In this first case of course it can completely change the response.

In this second case it will results in changing signs of results.

Generally the load defined in FRF analysis is as in harmonic analysis the extreme value (amplitude) defined for specific time moment. It is changing according to sine function with these values as amplitudes. The difference comparing to harmonic analysis is that in harmonic analysis single frequency or period of excitation is defined. In case of FRF analysis some range of frequencies is defined, solver is running a series of harmonic analysis with different frequencies and results of all of them are stored in single compound load case.

The interpretation of results from FRF and harmonic analysis is also the same - these are amplitudes of steady-state harmonic oscillations.


As concerns your first post in this thread: using FRF or harmonic analysis you will not be able to model a rotating mass on the top of cantilevered column what was possible in time history analysis using X and Y direction excitation forces with sine and cosine functions respectively. In case of FRF or harmonic analysis only sine function is used so the resultant is not rotating but oscillating in diagonal direction between X and Y.


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Pawel Pulak