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Re: help: bowstring truss modification (unbraced length problem)

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     I would brace it from the panel point by welding to the existing gusset and maintaining coincident center lines.  You could extend the gusset if necesarry by double plates and then bolt the brace to that (double shear).  The welding for a brace shouldn't result in significant residual stresses.  You may not have a perfect X or N but there will be "closure" in terms of the forces.

Greg

-----Original Message-----
From: dennis pantazis <dpantazis(--nospam--at)yahoo.com>
To: Seaint <seaint(--nospam--at)seaint.org>
Date: Saturday, August 05, 2000 11:59 PM
Subject: help: bowstring truss modification (unbraced length problem)


>i am working on an industrial roof that was damaged (read: blew off) during
>a windstorm
>
>the structural system is 60' bowstring trusses, 16' o.c., made up of double
>angles gapped at 1/2", and 2x10 joists and 1 by decking. the trusses sit on
>a multi wythe brick/clay tile wall.
>the truss is spliced at the center with bolts instead of rivets to the
>gusset plate. otherwise
>riveted everywhere else. the center of the span is braced with 'x' bracing
>made up of other double angles.
>
>in my analysis, i have found during MWFRS design windload the bottom chord
>goes into compression (obviously) and the unbraced length is too long. the
>solution seems to be to brace the quarter points of the span. issue here is
>how. i am temped to have a 1/2 tab inserted between the chord members and
>bolt X's to the top and bottom of it so it is effectively mechanicaly
>connected to the bottom chord member. this would have to occur at about 16"
>from the theoretical panel point location. i am uneasy about bracing the
>truss at a location off the panel point.
>
>is this a bad detail? alternates?
>
>tia
>
>dennis pantazis, pe
>
>
>
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