Fan Vibration on Mezzanine 1

[Finn]

Hey everyone - I'm new to eng-tips so bear with me! I am an EIT, one year out of school.. Not a lot of experience around here w/ vibrations so please help me out.

I am currently tasked w/ strengthening and stiffening an existing mezzanine that is having vibration and flex (due to footstep) issues. The mezzanine is only 50 SF, has a deck w/ about 3" concrete, and has a fan pedestal bearing on a small portion of the mezz that is causing excessive vibration to the mezz. The mezz is not strong enough to make the span in the first place - no studs (from what I can tell from underneath) for composite action..

My initial plan was throw in additional support beams underneath, but I am worried that vibration will still occur as the overall weight will not be increased by much. I would throw these beams in the strong direction of the deck at around 12" oc, as desired for site construction. I checked this new natural frequency of the slab span, and it is much higher (over 3X) than the force frequency produced by the fan. Am I good, or is a weight increase still recommended? The existing slab natural frequency was near the actual force frequency.

Another safe and probably better idea is to tear up the mezz. and replace it w/ a 8" slab, which is the same as the adjacent mezz that primarily supports the fan and is currently performing well in vibration. We will most likely go with this option, but I am using this as a learning opportunity..

Would the support beams solve the problem, as it's natural frequency shows it's sufficient? Or is the lightweight nature of the mezz still a concern?

Thanks!

 

[canwesteng]

Extra weight will actually lower the natural frequency of the mezzanine back toward the operating frequency of the fan. Stiffness (the steel beams in this case) will raise the frequency. Added weight should reduce the amplitude of vibrations. Generally I would suggest following the recommendations of the manufacturer for foundation weight if you are designing a foundation, but for floors I believe you should design them to be high tuned by at least 2x and to keep the amplitude of vibrations to an acceptable level.

 

[Agent666]

Going to exactly 3 times the frequency of the fan causing vibration if you already have a resonance issue will still mean some some multiple of the natural frequency (a n'th order harmonic frequency) is likely to be excited and still cause resonance. If natural frequency is a multiple of the forcing loads frequency then you will get resonance (e.g. 1/2x, 1/3x, 2x, 3x etc)

Other options to look at could include isolation of the fan itself, or tuned mass dampers, or possibly replacement of the fan with a different model with different vibration frequency.

A stiffer floor can help, but you really need to be assessing the acceleration as this is the thing perceived by occupants. Even with a stiffer floor it still may be unacceptable. The only way to reduce acceleration after going the stiffer route is increasing mass or damping.

Suggest looking at SCI P354, and/or AISC Design Guide 11. The method in SCI in particular takes specific account of the harmonics, design guide 11 sort of wraps things up into one equation but I've found this to be a bit crude when trying to assess problem floors.

 

[JGard1985]

Before you start designing a solution, have you confirmed the forcing frequency matches expected operating frequencies of the fan? Make sure your forcing vibration isn't due to some imbalance, misalignment, loose connection or rubbing that could be corrected before you start.

Considering you are working on a Mezzanine I would use support steel beams to retrofit and strengthen Mezzanine. I would work around what you have. I use same philosophy when assessing vibrations in pipelines due to rotating machinery. Demolition and pouring of slab in an operating facility can be very cost prohibitive and could require equipment to be out of service for a long time. I wouldn't attack the mass aspect of the problem, you might have to add substantial mass to make an effect. This could unleash a host of other structural connections which must be evaluated.

Jeff
Pipe Stress Analysis Engineer

http://www.xceed-eng.com

 

[WARose]

In addition to what some of the others have said, I'd like to add (without knowing how much framing this is): you would probably want to model this situation (i.e. the vibrations from the fan) in some FEA package (like STAAD). I could be wrong here but it sounds like you are doing this with hand calcs (i.e. checking individual beams and so forth).....and it would be probably be faster in STAAD. It would also be more accurate & complete.

The good news is: you know from the outset what the issues are. A lot of times you design for such machinery and you find out something started shaking on another floor (i.e. outside of your scope/disclaimer).

 

[Finn]

Yes, I will be sure the frequency out of range of harmonic loading. After studying DG #11 a bit more I'm understanding the design issues a bit more. Thx all.

Agent, I am confused as to the equation to use in DG #11 for peak acceleration to compare to the limit acceleration? It seems they are only concerned about peak acceleration from footsteps, while this is a fan base? Kinda lost w/ this. Probably will read the DG again tomorrow

 

[dvd]

Making some assumptions about the fan and fan base, but some fan bases have a hole in them for the express purpose of allowing cement to be poured into the fan base. It depends on the size of fan and fan base on whether this would help with the vibration problem.

 

[mitgala]

I'd Like to agree with WArose. You should model this. You could use the DG#11 eq 2.1 F=P*a*cos(2*pi*i*Fstep*t) and apply it as a time history load with P being your weight of the fan and f your frequency and then compare your acceleration to the DG11 recommended values. You could do this in RISA. Hope this helps.

 

[WARose]

[blue](Finn)[/blue]

Agent, I am confused as to the equation to use in DG #11 for peak acceleration to compare to the limit acceleration? It seems they are only concerned about peak acceleration from footsteps, while this is a fan base?

If I understand you correctly, you have 2 different vibration issues. DG #11 is specifically for excitation from human activity.....a fan is a different animal. There will be a sinusoidal forcing function which will be a function of a guesstimate of the unbalanced force produced by the fan. (That is, if the manufacturer doesn't have one.)

A model (to assess the fan's response) could also assist you in confirming the results from hand calculations using DG #11. (You can input a time-history for the footfall loading.)

There are plenty of charts out there for machinery performance and human response to vibration levels. (See p.20-22 of ACI 351.3R-04 for starters.)

By the way (and no offense here) with this kind of a problem you may want to get a more senior guy involved. (At least looking over your shoulder.) There are a million [more] things here I'm not mentioning involved in a dynamic analysis. Somebody just a year out of school (sometimes even much older) shouldn't be taking on this type of vibration problem by themselves. You are going to need some coaching.