Interpreting FFT results for vibration testing of concrete floor

[FE_struct1]

Hi all,

A team I'm with are conducting an FFT analysis on the results they got from an impact test on a concrete floor slab (basically they let a 5kg ball drop on the floor - no bounce). I've linked below the results they got:

Impact test:

http://files.engineering.com/getfil...5-447a-8ce9-fe7a0595ad7b&file=Impact-test.PNG

Raw FFT:

http://files.engineering.com/getfile.aspx?folder=e9b42839-0b20-4532-85ae-cd835ec836a1&file=FFT.jpg

Processed results:

http://files.engineering.com/getfil...a3f8-b682a75ab7ed&file=Spectrum-processed.PNG

What concerns me is that from these tests they've quoted the natural frequency of the slab as 32Hz (which sounds ludicrous since it's not a slab on the ground) and have said that this is since the maximum peak is at that frequency value. They also mentioned that the smaller peak noted at 16Hz (which is a much more reasonable value) is likely a sub-harmonic.

I've done some back of the envelope calculations which show that 16Hz is quite likely the correct value. I've never dealt with FFT before but it seems to me that they're going wrong somewhere. Thoughts ?

 

[WARose]

A note on the "natural frequency": As Strong noted above there are (for a multi-degree of freedom system) many natural frequencies. A lot of my fellow structural engineers (inaccurately) refer to the lowest frequency as the "natural frequency". (Even though very little mass may be participating in this mode.)

Even ASCE 7 is misleading in that regard (in referring to a building's "fundamental natural frequency")......the user may not realize he/she is (typically) calculating the first translational mode of the building.