Floor Vibration in Steel Mezzanine

[Stenbrook]

Running a steel vibration analysis to get the ap/g down to under 0.5% usually results in much heavier sizes which can be rather expensive. Most clients don't want to pay for that. How important is it to meet that criteria? For example, would there be a noticeable difference between 0.8% and 0.5%?

Thanks for any help.

 

[mike20793]

I believe that is the threshold at which people can start to sense the vibration. I never exceed it if I'm worried enough about vibration that I'm checking it.

 

[manstrom]

Pitch it to the owner.

Do you want me to design it for good vibration criteria, or not?

It is ultimately a serviceability issue. It's the same as asking for L/720 or code minimum deflections.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[KootK]

Vibration is a serviceability issue only. If your client wants to go with a cheaper floor and roll the dice with regard to vibration, they can. For the sake of your liability and sanity, if your client takes this route, you'll want some written record of:

1) You explaining the situation to your client.
2) Your client accepting the risk.

If your framing is such that it would make sense, your client can roll the dice and then possibly add a mass damper to the girders at some point in the future to remedy any vibration issues that arise. I know at least one marquis structural firm that will routinely include a mass damper on their feature stair designs even when those stairs have been designed for vibration. It's just so hard to predict vibrations issues that it's prudent to give yourself an easy way to fix them should problems arise.


The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jike]

Usually the most efficient way to minimize vibration, is to add concrete to the slab thickness. Is the mezzanine for offices, equipment or storage?

 

[Stenbrook]

The mezzanine is going to be for storage initially and for future offices. I have looked into increasing the slab thickness a half inch or so with only minimal decrease in vibration. I would agree that that is probably the best route to go, however increasing the slab thickness will include increasing the beam and joist sizes. Also, this mezzanine is inside a PEMB building so the client is already trying to go cheap on this building overall.

Kootk, I'm not too familiar with how a mass damper would be installed or what it even looks like. Could you explain how that would work?

 

[jayrod12]

So designing for the storage load doesn't give your system enough stiffness for the office loading? I would expect a structure stiff enough for 100psf of storage load (neglecting vibration as it's storage and who cares) would be stiff enough for vibration from a 50psf loading situation.

Elsewise, you could specify on your drawings that when renovated for office space they need to add an additional couple inches of concrete. The 50psf reduction in live load would afford you quite a bit of additional concrete thickness. But it's hard to enforce.

 

[KootK]

Be extra careful with OWSJ. Often, adding concrete mass is the only way to deal with vibration. It seems that there's not all that much that one can do to the joists to improve matters. 5" min slab thickness usually seems to be appropriate.

The detail below is the one that I was thinking of. Pretty low tech really. Still, if you pitch a mass tuned damper on an industrial mezzanine, expect to see some raised eyebrows.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Stenbrook]

The floor slab right now is a non-composite 1.0C deck with 2" of concrete on top. And the framing is made up of K-series joists @ 3'-0" o.c. A big reason the vibration is so large is because the bay is 30'-0" long with 25'-0" long girders. I am unable to shorten that span per the owner. I could increase my joists from a 22K6 up to a 30K5 or so and the vibration gets into the 0.65% range instead of the 0.8. I'm not convinced that increase in size is merited just for the vibration.

 

[Brad805]

We have found that over the years this has become more of a problem due to the trend of large open office spaces. Years ago everyone got walls around their offices, and this was less of a problem.

Did you consider the Ecospan system to develop composite action? Those studs that vulcraft came up with are a pretty good idea.

 

[Archie264]

1.0C deck with 2" of concrete on top. And the framing is made up of K-series joists @ 3'-0" o.c. A big reason the vibration is so large is because the bay is 30'-0" long with 25'-0" long girders.

Yeah, I imagine that would vibrate. Discuss it with the owner but remember that your name is attached to it as well. What happens when the owner's engineer finds a new job two years from now and his replacement, knowing nothing of the history and tired of hearing tenants' complaints, pulls your phone number off the drawings and calls you up to "fix" your design? A bit far-fetched, perhaps, but if the design is to be skinnied down for budgetary purposes make sure you protect yourself with documentation.

That said, since you mentioned it might be used for offices in the future my recommendation is to try to convince the owner to let you beef it up now. Office workers are particularly prone to noticing vibration and if they are to be used for offices someone will never hear the end of it. My $.02 worth.

 

[KootK]

@Brad805: have you used Ecospan in Canada? I might like to give it a go if availability isn't a problem.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Brad805]

Koot, we are about too. A contractor suggested this option and we went thru the analysis. It seems like a smart system. Vulcraft and Nucor went out of their way to explain the system. Their lead sales guy was an ex Hambro salesman. The crux of the system is their proprietary stud that can be installed in the field using what appears to be basically a 3/4hp drill with a stand. Email their engineer and he will send you their technical document.