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Re: Floor Vibrations

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On Thu, 7 Aug 1997, Greg Varney wrote:
> > The building is steel frame and the problem area is steel joists on
> > concrete slab.  The joists are 24 feet long and are spaced at 30".  The
> > slab is 2.5" deep.
> We had a high school project where the architect wanted to have 
> exposed metal joists in a 30' x 30' bay.  We had vibration problems 
> with the conventional joist design because they never carried enough 
> dead load to overcome the person walking by.

The main reason for feeling the vibration while walking is because 
of the very flexible floor frequency tuning with the frequency 
of walking (rate of stepping per second).  This is the reason why
troops marching on suspension briges can cause it to vibrate pretty
noticeably.

The control strategy would attempt to separate the two freqeuncies.
Stiffening the support system (or making it more flexible) works.  In an
existing structure, this is usually not possible.  Another alternative
is to add a flexible flooring system (supported on ribs perpendicular
to the joists) on top of the concrete floor.  The flooring system
can be made of a combination of wood and steel and the spacing of
the ribs can be adjusted to make its fundamental frequency be 
very different from that of the earlier floor.  Conventional 
vibration isolation systems also use the same principles.

Hope this helps.

Ravi Sinha

------------------------------------------------------------------------------
Ravi Sinha, Ph.D.		email: rsinha(--nospam--at)gemini.civil.iitb.ernet.in
Assistant Professor		       rsinha(--nospam--at)civil.iitb.ernet.in
Department of Civil Engineering	       Phone: (91-22)-578-2545 Ext. 2336
IIT, Powai, Bombay - 400 076, India    Fax: (91-22)-578-3480