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 ?

 

[gravityandinertia]

I'm not that familiar with the dynamics of concrete slabs, however 32 Hz feels high to me for the lowest natural frequency of a concrete slab. That's some quick vibration! My gut would lean towards the 16Hz as well, but with no basis to back that up other than your data and my feel of concrete having lower natural frequencies.

 

[scheah]

Any chance that's the natural frequency of the ball? Perhaps repeat the experiment with a different ball/calibrated hammer? My 2 cents.


Kind regards,
Jason

 

[IRstuff]

The time series seems to show some ringing after impact at around 16 Hz.

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[GregLocock]

There's no such thing as a subharmonic.

Cheers

Greg Locock


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[FE_struct1]

Thanks for the replies folks.

I'm not that familiar with the dynamics of concrete slabs, however 32 Hz feels high to me for the lowest natural frequency of a concrete slab. That's some quick vibration! My gut would lean towards the 16Hz as well, but with no basis to back that up other than your data and my feel of concrete having lower natural frequencies.

The same team got back to me today with some FE analysis results of the slab and they're getting a natural frequency around 14 Hz. More evidence in support of the 16Hz result I guess..

Any chance that's the natural frequency of the ball? Perhaps repeat the experiment with a different ball/calibrated hammer? My 2 cents.

Interesting thought. I did ask if they could repeat the experiment with a calibrated hammer but unfortunately they won't have access to the area for a while now so we're kinda stuck with the data we have.

The time series seems to show some ringing after impact at around 16 Hz.

Do you think this would be the reason for the peaking at 32Hz ?

There's no such thing as a subharmonic.

Could you explain a bit more on this ? I've no experience with sub-harmonics but I thought they did exist (albeit only in signal processing).

 

[GregLocock]

Sorry can't help. It isn't a real thing in a linear system. Please get whoever said it to define what they mean and provide a reference. That should be entertaining.

Cheers

Greg Locock


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[scheah]

Interesting thought. I did ask if they could repeat the experiment with a calibrated hammer but unfortunately they won't have access to the area for a while now so we're kinda stuck with the data we have.

You could repeat the test on any other concrete floor slab (the office?) with the same ball and instrumentation. See if that 32Hz appears.


Kind regards,
Jason

 

[Sofistioelevib]

Hy guys,
i'd like also to remember the importance of the phase relationship during this test.
Some time i found some vibration on the conrete (probably due to combination factor) not really a resonance problem due to no phase shift at the biggest frequencies.

Another considerations is related to the amplitude of the impact have to be no too much and to too less according with concrete structural weight.

Hoiping to help
regards

 

[Strong]

Put the cursor on the next peak (natural frequency) down from 32 Hz. It is about 16 Hz as you expected. So what is the real issue with the results? The test method and instrumentation could be better. You did not define the structures dimensions, materials and support/foundation.

Walt

 

[GregLocock]

@Sof... The test used a weight dropped onto the floor, so no load cell, so there's no phase information that is of much help.

Cheers

Greg Locock


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[WARose]

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.

What is supporting it at the edges? Heavy RC beams? How thick is the slab? What are its dimensions in plan?

If it is rigid enough (at the supports).....it may not be that far off.

 

[271828]

Can you provide an FFT magnitude plot with linear scales?

What do FFTs look like without the ball drop?

 

[FE_struct1]

Hi all,

Sorry for the late reply, I've been busy with other stuff.

I realise I didn't include details about the slab in the OP so here they are:

1. Slab : 140mm thk Concrete slab with 900mm deep prestressed band beams @ 4.3m spacing. Band Beam is 500mm wide.
2. Dimensions: 39m by 42m (band beams span along the longer dimension)
3. Support conditions: supported by columns @ 4.3m spacing on the two shorter sides and @7m spacing on the longer sides.
4. Material: 40 MPa concrete in slabs and beams (this is a 20 year old building so the concrete has likely gained some strength and stiffness over the years).

Put the cursor on the next peak (natural frequency) down from 32 Hz. It is about 16 Hz as you expected. So what is the real issue with the results? The test method and instrumentation could be better. You did not define the structures dimensions, materials and support/foundation.

So the largest peak is not the natural frequency then ?

What is supporting it at the edges? Heavy RC beams? How thick is the slab? What are its dimensions in plan?
If it is rigid enough (at the supports).....it may not be that far off.

See above

Can you provide an FFT magnitude plot with linear scales?
What do FFTs look like without the ball drop?

I'll post the data here once I get it from the team.

 

[WARose]

[blue] (FE_struct1) [/blue]

1. Slab : 140mm thk Concrete slab with 900mm deep prestressed band beams @ 4.3m spacing.
2. Dimensions: 39m by 42m (band beams span along the longer dimension)
3. Support conditions: supported by columns @ 4.3m spacing on the two shorter sides and @7m spacing on the longer sides.

You didn't give the width of the band beam....but making the assumption it's about half of the depth.....I see what you are saying as far as frequency goes. I didn't get anywhere near 32 Hz (or even 16) with that layout. And that is even with using uncracked section properties and 100% fixed supports. (We are talking no column support anywhere out in the 39x43 meter bay right?)

Almost all of my mass was participating (vertically) in modes at 6 Hz or less. (The majority at less than 2 Hz.) Unless I am misinterpreting something.......all I can say is: maybe their measurement is some sort of localized effect. (Although I am at a loss to explain their FE showing 14 Hz as the lowest vertical mode.)

 

[FE_struct1]

You didn't give the width of the band beam

We are talking no column support anywhere out in the 39x43 meter bay right?

I must've been half asleep when I added the data. The width of the beam is 500mm and there are internal column supports every 14m along the 42m span (See image)

 

[WARose]

Ok that makes a lot more sense. (I ran what you gave before under the static load and the deflection was outrageous.)

I still get frequencies much lower than anything they came up with. With fully fixed supports (and uncracked section properties) I get 85% of the mass participating at frequencies of 7.38 Hz or less. With pinned supports and a estimate of the cracked section (about 35% of gross), I came out with 90% of the mass participating in modes that had frequencies of 4.11 Hz or less.

Either the band beam is much stiffer than I am giving it credit for.....or they were accounting for some localized effect. (Or there is something else I am missing.)

You may want to contact them and see just how much of the floor they were considering (i.e. just between the band beam or somewhere else in their FE analysis)......if you don't already know.

 

[GregLocock]

Perhaps a happy morning with a sledgehammer, an appropriate load cell, and some accelerometers is warranted. Your original test people's comment about subharmonics suggests that they may not be entirely up to speed. Politely.

Cheers

Greg Locock


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[Strong]

"So the largest peak is not the natural frequency then?"

That is correct, since there can be many natural frequencies with peaks of different amplitudes. The amplitude has little value for your test, since impact force (hammer-load cell) was not used along with measuring Transfer Function, and using multiple measurements to define a Mode (single frequency) shape. Your data clearly indicates more than one natural frequency is present. Make measurements at different locations on floor and you should get different amplitudes (and relative amplitudes) between the different vibration Modes. I hope you used an impact pad (like a rubber sheet) for steel ball to land on!

Walt

 

[271828]

"So the largest peak is not the natural frequency then?"

Not necessarily. Another excitation might've caused a higher response. That's why I asked about the FFT for ambient conditions.