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Re: Annoying Floor Vibrations

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Daryl:
The AISC steel design guide series 11 " Floor
Vibrations Due to Human Activity" gives basic
principals and calc. procedures. 
I think the problem may be low vibrational frequency
of the upgraded steel beam. I've read that the
recommended combined (beam & joist) freq should be
about 15 Hz; your system frequency is less than 5 Hz.
Since frequency is proportional to length squared,
perhaps you can add a pier at the center of the steel
beam and increase the combined frequency. 

Hope this helps,
Irv

--- Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca> wrote:

> Fellow Engineers,
> 
>         I have been retained by an architect for the
> renovation of a
> multi million dollar house.  Part of the work
> required the removal of a
> number of adjustable columns in the basement of the
> residence and
> replacing them with longer spanning beams in order
> to provide larger
> clear open spaces.  One of the new beams has
> vibration characteristics
> which the owner finds annoying.  The particulars are
> described in the
> following paragraphs.  For those who are not
> interested in this problem,
> please excuse the fact that this may be rather
> lengthy.
> 
>         The annoying beam was originally three 2x10
> supported on posts.
> It was upgraded by applying two C10x25, one on
> either side, with a
> simple span of 23 feet and supported by new steel
> columns and footings
> on either end.
> 
>         The loading consists of 2x10 floor joists at
> 16" centres
> spanning 12 feet on one side and 14" on the other
> with specified live
> load of 40 psf.  In addition, there is an open
> hearth fire place (a
> masonry fire pit supported on the floor with a metal
> hood and chimney
> suspended from the roof) of unknown weight
> (conservatively estimated at
> 7,000 pounds) located  near the third point of the
> 23' steel beam.
> 
>         Although renovation work is ongoing the
> owner has been living in
> the house for most of the summer.  As I understand
> it he has only
> recently become annoyed by the vibrations which must
> have been occurring
> for a few months.  In reviewing the problem I also
> would find the
> vibrations somewhat undesirable.  I guess I would
> describe them as "not
> really bad but some improvement is desirable".  The
> vibrations can be
> noticed by a person standing anywhere in the beam's
> tributary loading
> area when another person is walking or jumping
> anywhere else in the
> beam's tributary area, even from opposite sides of
> the steel beam; so
> the problem appears not to be the with floor joists
> but rather to be
> with the steel beam.
> 
>         The steel beam which I designed has a live
> load deflection of
> L/445 and a natural frequency of approximately 5 Hz
> based on dead load
> only.  The supported floor joists (14' span) have a
> natural frequency of
> about 12 hz based on the joists acting alone, and
> about 17 Hz if the
> plywood sheathing is assumed glued to the joists to
> create full T-beam
> action.  Nails may secure the plywood sufficiently
> to increase the
> natural frequency of the joist system but the
> magnitude of the increase
> is questionable.
> 
>         I have consulted two references: Commentary
> A, from the
> Structural Commentaries to the National Building
> Code of Canada, 1995
> (Alberta Building Code references 1995 edition); and
> CSA S16.1 M94,
> Appendix G.  These references have suggested
> remedies including
> increasing the stiffness (which I can do by
> increasing the the beams),
> and increasing damping (which I don't know how to
> do, except by adding
> cover plates which are loosely bolted to the beams
> to allow slipping;
> and I'm not too confident with this).  They
> specifically recommend
> AGAINST adding mass to the system.
> 
>         I would appreciate anything anyone can
> suggest to help me solve
> this problem.
> 
> Regards,
> 
> H. Daryl Richardson
> 
> 
> 
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