Vibration Conversion 2

[enigma2]

I've seen similar topics come up in this forum before, but I'm still confused.

I'm a structural engineer, looking at vibration of a floor that will support a vibration sensitive MRI unit. The supplier has specified a maximum allowable vibrational acceleration of -85dB g. For my design computations, I need the maximum vibrational velocity in ? in/s. I've tried the calculators from Wilcoxon and DLI off the internet, but since I don't know what conversions are actually being made, I have no confidence in the output I'm receiving. Can anyone help me with the conversion? How dependent is my conversion on the frequency of the floor where the unit is being placed?

As an aside, what does a negative dB value really mean?

 

[BobM3]

To get a velocity from acceleration, you would have to know the frequency:

Velocity = acceleraion/(2*pi*f)

So if you have 20 g's at 10 hertz, you would have:

Velocity = 20*386/(2*pi*10) = 122.86 in/sec.

For dB you need a reference value, then you have

dB = 20 *log (value/ref value).

 

[SylvestreW]

sorry to make things worse but you also need to know what the measure is (RMS, Peak, Peak-to-Peak). It's not always clearly stated.

Negative is just a result of the math. As Bob said, you need to know the reference value. Once the value is less than the ref value, the result of 20lg(value/ref. value) will be neagive (e.g. 20lg(.1/1) = -20.

However I suspect that isn't what the supplier means as -85 dB is ~20,000 times less than the ref. value.

and -85 dB g?

 

[sms]

Your db spec means nothing without a reference value. Did they specify one?

In practical terms, MRI machines require essentially 0 vibration.

-The future's so bright I gotta wear shades!
Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[enigma2]

Well, the information from the supplier states "vibrational acceleration transferred through building vibrations to the magnet (of the MRI) may not exceed -85dB g measuread as the max RMS value in the Fourier transform."

I do know that the frequency of the bay is in the 5-6 Hz range. It varies as I adjust the stiffness of the framing to try and attenuate vibrational effects.

I follow BobM3's equations, but I don't know what acceleration to use in the equation. Additionally, I don't understand value vs reference value. I do understand that in log scale, a ratio less than 1 will give a negative dB. I appreciate everyone's insight.

 

[enigma2]

So I was thinking that g is actually my reference value. If I follow the equations from BobM3, I can solve for the value (acceleration (?)) and then use this acceleration to convert to velocity. Am I on the right track? Is this reasonable? I was expecting a value in the range of 500 micro in/s. This calculation gives me 576. I'm hoping it's not just a fluke. Can anyone confirm my logic?

 

[271828]

mcsjrp, I've dealt with this a few times. Their stated limits are usually CYA and are so stringent that a slab-on-grade supported on solid limestone can't provide adequate resistance.

In practical terms, I think the best thing to do is satisfy the DG11 Chapter 6 limits for MRI, then gets the owner to pistol whip the MRI people into writing a letter that softens their CYA stance. That's worked 3-4 times in a row.

Don't get me going on surgical light folks too... They want NO deflection, LOL.

 

[GregLocock]

mcsjrp can you give the full spec as it pertains to that measurement? It needs to specify a bandwidth and the number of lines, or the frequency resolution, and should really specify a windowing method (eg Hanning) and the compensation factor (energy or peak). 99 times out of a hundred it won't!

That's not a bad attempt at a spec, but it is still meaningless, unless you are dealing with a purely tonal excitation.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[271828]

mcsjrp, you're designing a floor, not testing an existing one, right?

 

[enigma2]

It's a new floor--elevated. Not a slab on grade, which adds to the complication.

 

[271828]

Again, I think you should have the owner make the mfr sign off on your design, which I assume is based on AISC DG11 limits for MRI.

You could also request from the MRI mfr that he give you the info in units compatible with DG11. If he wants to be difficult, you could ask the arch or owner to make him give you the information that you need.

If you go this second route, the MRI mfr won't want to go through the hassle of figuring out his own stupid criterion (yeah, I'm jaded from dealing with these guys a few times, LOL) and will probably just sign off on your design based on DG11. It was almost funny the one time we did this. We got the MRI guy on the phone and it was obvious that he didn't know what it meant either.

Forget all the technical stuff in this situation! That's doing it the hard way. IF you succeed in converting it, you'll get something so severe that you can't satisfy it anyway.

Another thing, I don't know if you've used DG11 Chapter 6, but you will have an almost impossible time getting that to work also. If you satisfy that, you will not have problems with your MRI regardless of what CYA criterion the mfr gives.

 

[enigma2]

I did do some preliminary design using the vibrational velocity values for an MRI in DG11 and agree it's near impossible to satisfy the criteria.

I've spoken with the supplier's rep and got the impression that he didn't grasp the requirements either.

If I can't meet the criteria, how do I know what Target value is reasonable?

 

[271828]

People often use the DG11 Chapter 6 limit of 500 micro-in./sec. without even asking the mfr.

You could also recommend that the owner hire someone to vibe test the floor after it's built. Then have the mfr sign off on the measured vibe levels. We've done this before and it made the problem go away. The bottom line is that the MRI people set these stupid CYA criteria. Putting the ball back in their court is like calling their bluff. They deserve it, LOL!

Just in general: I do floor vibe research and I can tell you that the current state of the art is in its infancy. If you predict 500 micro-in./sec., you don't know if you'll really get 250 or 1000 in reality. I can run a vibe test on a simple lab specimen and then run my most refined FE model, using measured damping, and be lucky to get within 50% of the measured acceleration. Keep in mind that your loading function is somebody walking across the slab. This is A LOT different from trying to predict the accel from a known excitation (like say, an unbalanced rotor in a machine). Variability is walking gaits is extreme to say the least. Fourier amplitudes of walking force look like shotgun blasts when plotted against frequency.

As a designer in North America (I assume), the best option you have is AISC DG11 Chapter 6. I'd go with a design based on this and work "other channels" to put the onus back on the MRI people.