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Re: Balcony Floor Vibration

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Sounds like a pretty  difficult situation with the backup span not greatly
exceeding the cantilever, and designing for rhythmic activity rather than just
walking.  It doesn't surprise me that you've had some experience with other
floor designs which don't meet the criteria.  In talking with Dr. Murray
(co-author of the Guide), he is seeing more and more problems with vibration
now than ever before.  I know he does a lot of consulting and repairs related
to these type of situations.  Generally, I try to meet the criteria for
servicability and vibration, unless there is some reason to justify a bit of
relaxation of them.  Dealing with a vibration problem in place is expensive,
difficult and potentially limits the use of the space.  I might try emailing
Dr. Murray and seeing if he can give you any help (he'll probably put in a
pitch for his software) at tmmurray(--nospam--at)floorvibe.com

Eric Ober
Cagley & Associates
Rockville, MD

Steve Kramer wrote:

> I have a balcony with a 28 foot cantilever. The backspan varies from 30 to
> 45 feet. I'm trying to design for floor vibrations per AISC design guide
> 11. The balcony is for a church, and my thinking is that my natural
> frequency should be at least 6 based on chapter 5 and my peak acceleration
> should be less than 0.5% g based on chapter 4.
>
> The problem is that this requires enormous beams, something like W36x230 at
> 5 feet on center just to get in the ballpark, way more than required for
> strength.  The problem seems to be the large deflection I calculate for the
> cantilever, that results in a very low natural frequency.
>
> Has anyone run into anything similar? Maybe I could use the deflection at
> some point other than the end of the cantilever. Maybe 3/4 or 2/3 the
> cantilever length, although even this doesn't help as much as you might
> expect. Past experience I have had with this design guide is that almost
> any floor I check doesn't work.
>
> Any advice or comments would be appreciated.
>
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