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RE: Question on wood Roof Trusses

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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.

Neil Moore, S.E.

>Peter:
>
>I personally have it in my specs that bracing the truss members is the
>responsibility of the truss designer for reasons too numerous to
>recount.this thread was started a while back so check the archives and in
>particular excellent postings by Roger Turk .
>
>I think the bigger problem is getting the framer to actually brace
>the members that need bracing. and unless somebody shows him where these
>members are on the framing plan
>they aint gona walk the building with the little 81/2 x 11's from the
>truss engineer and do it( all they see is a little astrisk next to said
>members)
>incidently every framer that we worked with told me that they have never
>installed such braces and never knew they were even required .Generally
>engineers have some sort of a typical detail on their plans that refers
>back to the truss designer's calcs. good luck getting the framer to
>comprehend it.
>it is my opinion that unless the compression bracing is shown on the
>truss layout sheet with the appropriate details refernced to it nobody
>will follow
>my 2 cents
>Tarek Mokhatr
>strucural engineer