Fan Vibration on Mezzanine
Fan Vibration on Mezzanine
(OP)
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!
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!
RE: Fan Vibration on Mezzanine
RE: Fan Vibration on Mezzanine
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.
RE: Fan Vibration on Mezzanine
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
www.xceed-eng.com
RE: Fan Vibration on Mezzanine
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).
RE: Fan Vibration on Mezzanine
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
RE: Fan Vibration on Mezzanine
RE: Fan Vibration on Mezzanine
RE: Fan Vibration on Mezzanine
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.