Modal analysis ANSYS Simply supported beam

[Xtina]

Hi all, i am performing a modal analysis of a beam in ANSYS. In the laboratory, the beam is supported on transversal steel bars, so vertical positive displacements are allowed when I hit it with the hammer, and the bars are fixed to the ground. If I model the supports as typical rigid supports the frequencies that I obtain are not similar to the real ones, but if I run the analysis without the constraints the results are very close to the correct values.Is it possible to assume that the beam's behavior is more likely to a free-free condition than a simply supported (UX=UY=UZ=0) condition since there is a loss of contact between the beam and the bars? I will be grateful for any suggestion. I tried to model the supports as two blocks of steel with contact surfaces between them and the beam to simulate the separation and then run a transient analysis, but the results (accelerations become in frequencies) are very strange, there is not a clear spectrum, what could be the problem? I will be grateful for any suggestion.

Thanks in advance.

 

[GregLocock]

Dunno, that is a bad experimental setup. Repeat it with proper pin joints, or free free, or on bungee cords. You should be able to see from the mode shapes from the experiment.

Cheers

Greg Locock


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[btrueblood]

What Greg said. Adding: impacts with the supports will create a lot of high frequency "noise", both in simulation and in reality, and yes, will make a mess of your spectra.

 

[Xtina]

Thank you for your respones. The fact is that the measured spectra in laboratory is very clear and eigen frequencies can be obtained using the accelerations without problems, but the model does not provide the same results... and then was when I tried to run the modal analysis without constraints...getting very good results. I read some papers that explain that for example, in the test of Charpy, the specimen may be considered as free-free and not simply supported because there is a loss of contact when it receives the impact... however, my beam is too heavy to experiment this loss of contact I think. What do you think?