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Scissor Truss

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Chris,

I would be very hesitant to *remove* any bracing that a truss manufacturer 
puts on their shop drawings.  Frequently, I want more bracing than is shown.

Remember, the truss fabricator and truss designer is *not* going to provide 
any more of anything than is ***absolutely*** necessary and if they show 
bracing at the peak of the lower chord, then I would believe that it is 
absolutely necessary.

While we always think of buckling as the result of compression, buckling due 
to tension is also a consideration.  I call it tension flange (or tension 
chord) buckling and it is a part of the AISC specs.  If you visualize the 
tension chord or tension flange of a flexural member as a rubber band 
separated from the compression chord or compression flange by struts with 
frictionless pins, you know that the rubber band is going to want to shorten 
and one way for it to shorten is to flip sideway, i.e., tension 
flange/chord buckling.

It is vitally important to keep the top and bottom chord/flange of trusses 
and beams in the same plane.  Bracing the tension chord/flange is the only 
way to do it.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Chris Towne wrote:

. > I am reviewing shop drawings for glulam scissors trusses for a cabin.  The
. > manufacturer has bracing at the peak of the bottom chord.  Does anyone 
. > know if this is necessary?