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RE: controlling vibration on existing steel joist floor

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> From: Pedro Khouri
> Sent: Wednesday, January 05, 2005 4:22 PM
> Subject: controlling vibration on existing steel joist floor
> 
> The contractor is subjecting the addition of a mid span support
> with some posts to the room bellow to reduce the joist’s span.
> I am concern that the added support will negatively affect the
> joist by inducing negative moments at mid span although the
> joist members (top / bottom chord & web members) appear to be
> constant thru the length of the joist.

This will probably work.  You're introducing negative moment, but it will
only be approximately 1/4 the max positive moment in the original joist
span.  You need to check:
 - The bottom chord at the columns to make sure they can handle the new
compression forces caused by the negative moments.
 - The bottom chords must be braced against out-of-plane buckling at regular
intervals until you get out of the negative moment zone.  The spacing of the
braces depends on the compression capacity of the bottom chord.
 - The web members near the center (and the connections to the chords) were
probably designed for a minimum shear.  They'll now be subject to
considerably more shear with a possible force reversal (compression in a web
member only sized for tension).
 - The contractor probably wants a bottom-chord bearing connection to make
his job easier.  If so, you'll have to transfer the reaction force up
through the joist with added web members.

The easiest construction method would be to simply snug-fit the new beam to
the new seats (top or bottom chord).  This way, the new
beam/columns/footings only have to be designed to carry the live load
reaction, the negative moment is greatly reduced or even eliminated
(depending on your live/dead load ratio), and you won't have to worry about
jacking the existing floor system up to force the new support to carry the
existing dead load.


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