Vibration analysis

[Menl2006]

Hi, I'm trying to model a simple beam supported by hinges. I want to give it small push and see how it vibrates from that.

What I did was create a beam with ca 51 BEAM3-elements. I defined two loadsteps, first a short step with a F-load on the midnode during .2 seconds and a second step with no load during 10 seconds.

I used the options:

ANTYPE,TRANS
TRNOPT,FULL,,NODAMP
ACEL,0

Obviously I want to model a beam with no gravitational influence and no damping.

Now the damping is giving me trouble. When I watch the POST26 displacements of the middle node, I see a beautifull sine with decreasing amplitude.

What to do to get the damping out of the model (if it's damping I see)

I also tried ALPHAD,0 and BETAD,0 -> no succes.

Any help is greatly appreaciated.

 

[cbrn]

Hi,
if you need undamped free vibration, I don't see clearly why you wouldn't perform a modal analysis, which is by definition the search of the free vibrations.
Anyway, in your analysis make sure you don't have any source of damping other than Rayleigh damping factors (which you already tried). From your graph, calculate the exponential decay rate and hence the structure damping factor, then search for any setting which match this value.
Good luck.
Regards