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

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: help: bowstring truss modification (unbraced length problem)
• From: "Greg Smith" <strusup(--nospam--at)gte.net>
• Date: Sun, 6 Aug 2000 10:46:37 -0500
```     Next step.   Since the centerlines are coincident, the members would also have to be (which is not possible) so, use a different angle than the existing diagonal and connect symmetrically (like a peace sign)  ;>) at a point on the vertical members.  They are all in compression if the bottom chord is and you will have to check the slenderness on the section between the top chord and the new connection for the additional vertical component and vary the angle for that.
The double plate will only brace the length and will have no vertical forces on the lower chord.  The welding (bottom chord) is to the extended gusset and the "pinned" connection is through the triple plate with the diagonal brace welded to the double plate.
Question: The weak axis of the double angle lower chord is normal to the plane of the truss.  Would additional bracing in this form suffice instead of planar bracing?
It's my day off so I have time to think and answer something on the list.

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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