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RE: Question on wood Roof Trusses

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Neil,

For the record, I never said to brace one slender web member against another 
slender web member. That is, of course, dangerous!  What I said was that it 
is that the bracing can be self-equilibrating.  There are many ways that this 
can be accomplished, including:

   double king or queen post the slender member

   providing X-bracing between the slender member and the top or bottom
      chords of the truss with horizontal struts between the adjacent 
      trusses

   any method that will keep the braced points of the slender member in
      alignment with the ends of the slender member

By failing to completely show how the bracing is attached, the truss designer 
is providing an incomplete design!  That is like us showing a slender steel 
column for one of our buildings and saying, "Bracing is by the steel 
fabricator."  That the plated truss industry has unilaterally decided to 
foist off the problem of stability bracing onto someone else is irrelevant.

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

Neil Moore wrote:

>>Tarek:

If I remember correctly, one of the TPI publications has an example of how
ALL of the webs can buckle at once.  If the web bracing isn't anchored to
something, then this can happen.  Most of the time, the real loads never
approach the design loads and any buckling problem has moved into the
factor of safety area. 

If people are designing trusses in snow areas, say 160 pcf, then our
discussions and the possible reprecussions become important.  My original
involvement in the web bracing responsiblity problem came about 6 years ago
in a court case.  That's were I found out that the responsibility is kinda
hidden in the TPI specification, which the UBC refers you too.  How many
people owned the TPI spec?  Further, it is also difficult to interpret the
results of the computer output.  That presentation can be, and may already
have been, improved upon.  I've read where some engineers are going to get
tough about this and make the truss company provide the web bracing
anchorage to the building or to the roof or somewhere.  Good luck!.

It still is important to review the truss company's layouts; you might be
surprised at what you will find.<<