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Re: Annoying Floor Vibrations

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Daryl:

The floor vibrations are also a function of  live load deflection and the
best way to reduce it is with more depth in the beam.  This may not be
desireable if the floor joists are fliush framed as it appears from your
description of the framing.  There also may be a reduction in vibration if
one or more walls are to be added from the renovation on the floor above
which helps with dampening.

Best Regards,
Ray Shreenan
----- Original Message ----- 
From: "Daryl Richardson" <h.d.richardson(--nospam--at)shaw.ca>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, October 04, 2004 1:26 PM
Subject: Annoying Floor Vibrations


> 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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