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RE: Joist girder stabilizer plates in existing building

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If there isn't additional bottom chord bracing along the length of the joist girder (i.e. angles from the ends of the joist bottom chords to the joist girder bottom chord) then it is possible that the joist girder was designed based on the assumption that there was no net uplift and the bottom chord would never be a compression element.
 
While it generally shouldn't cause any secondary issues to add stabilizer plates, they may not be necessary based on you specific design considerations.
 
Josh
-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com]
Sent: Tuesday, September 25, 2007 2:54 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Joist girder stabilizer plates in existing building


Andrew,
You are correct.  When there is wind uplift the entire bottom chord is in compression requiring bracing in some fashion.  The truss diagonal elements will be in compression or tension in a gravity load or with a wind uplift load.  Since the web / truss diagonals will compression elements, they will require bracing.

Regards,
Harold Sprague



From: akester(--nospam--at)cfl.rr.com
To: seaint(--nospam--at)seaint.org
Subject: RE: Joist girder stabilizer plates in existing building
Date: Tue, 25 Sep 2007 12:17:30 -0400

Aren't roof joist girder stabilizers there for the same general reason that continuous bottom chord bracing for joists are supplied, for uplift wind pressure loading? Like Harold said, this puts the bottom chord into compression and unlike the top chord that is braced by deck the bottom chord needs to be braced. There would also tend to be rotation and movement at the ends, thus the stabilizer plate.Something intuitively says the plates also help in gravity loading to keep the bottom chords from "kicking out". I am not a joist designer so I stand to be corrected.. 
 
 
Andrew Kester, PE
Principal/Project Manager
ADK Structural Engineering, PLLC
1510 E Colonial Ave., Suite 301
Orlando, FL 32803



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