High level question regarding vibration

[atrizzy]

I was discussing an issue with a colleague yesterday and wanted to get a few opinions on the matter.

There exists an industrial steel structure on the fourth level of which are placed 3 pieces of equipment that vibrate quite vigorously. Not 30' from this equipment, on the same level, is a control room which vibrates like crazy due to the equipment.
This is obviously caused primarily by the fact that the control room shares the same structure as the equipment. Nobody bothered to separate the structures to prevent the vibration.

This is causing quite the uproar at the facility, and in response the engineer who designed the structure is adding steel to the structure, primarily (or so I hear) with the aim of adding mass to resist the vibration. Yes, you read that correctly.

My opinion is that adding mass to this floor is not likely to help the situation. As the vibration is constant, not transient (such as walking down a flight of bouncy stairs), I don't think that damping this structure will help. It seems, intuitively, that for a floor so high off the ground, and with such a constant vibration loading, the amount of mass added will be nearly meaningless, and the vibrations will be felt regardless.

I think providing a structural separation is in order, though admittedly, this would be a lot for the engineer to admit and he's likely trying to do everything but, at the moment, just in case it work.

Am I crazy in thinking that the added mass is likely to do nil?

Thanks!

 

[Ron]

Adding mass will provide minimal mitigation. Isolate the equipment by supporting on damping devices.

 

[GregLocock]

Depends how much mass he adds, but it is rarely an elegant solution. Options are (1) employ a vibration guy (2) isolate the machines (3) isolate the control centre (4) tuned absorbers (5) active absorbers

Unless you can quantify the issue(s) then its a bit hard to say which is likely to be effective.

Damping may help if resonances are involved, but if it is a forcing issue then it probably won't.

Cheers

Greg Locock


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[atrizzy]

Ron/Greg, thanks for the responses, however, for the purposes of an academic discussion:

Doesn't the damping ability of a structure (in this case improved by the extra weight added) really only responsible for how quickly the impact load is dissipated? What I'm getting at is that an increase in the damping of the structure may help a single impact load dissipate quickly, but in the case of a constant shaking and vibration, it will still always be felt, will it not?

 

[WARose]

If he's adding framing he is probably changing the stiffness far greater than the mass. (Which will have a bigger impact on the frequency of the system.)

Of course that is said without knowing the particulars of the situation. I myself have handled a number of vibration issues in my career.....and I can tell you there is nothing more irritating than taking some measurements, getting some drawings and a plant manager in your ear 5 minutes later asking: "Ok, what do we have to do? Add a brace? Change some floor framing?" And my answer is pure speculation. First you've got to model the situation.

For example for your control room: Is it vertical displacement? Horizontal? Both? What level of vibration are we talking and what is tolerable?

You see what I am saying? You've got to 100% understand the problem in vibration before you can even speculate on the solution. For all I know, your structural engineer could be changing the vertical frequency but having minimal impact on the horizontal. (And the horizontal could be at issue as well.)

Step one is to get someone who knows what he is doing.....step 2 is to be sure he understands your criteria.

 

[Retrograde]

What I'm getting at is that an increase in the damping of the structure may help a single impact load dissipate quickly, but in the case of a constant shaking and vibration, it will still always be felt, will it not?

It depends. If it is a resonant response then damping can have a big impact on the magnitudes.

 

[WARose]

For elevated framing.....most of the time damping isn't playing a critical factor. We are talking damping levels typically at 5% or less. (About 2% for steel.)

 

[IRstuff]

30' seems to be a pretty long distance to be rattling something; has anyone done a resonance search of the structure?

Seems to me that if you can identify specific structural elements that are resonating, there may be some possibility that judicious use of additional mass might alter the resonance away or reduce the transmissibility.

TTFN (ta ta for now)
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[WARose]

@atrizzy: Something I forgot to mention before.....don't get "sticker shock" if you go to a outside consultant. I've worked on a bunch of these jobs and a lot of people look at them like they are your typical job: X amount of dollars for Y amount of drawings. Doesn't work that way. I've poured as much as 200-300 hrs of calculation time into essentially producing 1-2 drawings. (Obviously for a static design it is much less.)

I just wanted you aware of that fact. Don't be afraid to shop around.....but at the same time, it's no conspiracy: these things really are calculation intensive.

 

[Archie264]

RamSteel used to come with a vibration module. It not made for this type of situation nor does it have the complexity of some of what's posted above but if you already have the software it might provide an additional tool for you to use.

 

[bootlegend]

atrizzy,

Could you elaborate on what type of machinery is causing the vibration?

I second the idea that the engineer is likely trying to change the stiffness of the structure with the additional steel.

I have also encountered situations where the structure is designed with the machinery vibrations in mind but the interaction between multiple pieces of machinery was not accounted for. This is called beat frequency I believe.
Link

 

[dik]

As Ron suggested, the common remedy/prevention is to use vibration isolation.

Dik

 

[Tmoose]

Are there folks directly under the equipment? What do they have to say?

Adding steel strategically might add/increase stiffness. That could change the tuning of the floor system and make the vibration either better or worse if resonance is involved, depending how close the forcing frequency is to floor system resonance now.

Even if the new steel is added non-stategiclly so the principal effect is mass increase, mass isn't damping.

For constant speed equipment mounted on well designed real soft isolators, damping can actually increase transmissiblity according to most isolation curves.

http://personal.cityu.edu.hk/~bsapplec/Fire/Image486.gif

Successfully Isolating equipment when it is not on ground floor slabs can be quite tricky.

https://www.mason-ind.com/ashrae-lecture/

Way more info needed.