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Re: Question on wood Roof Trusses

[Subject Prev][Subject Next][Thread Prev][Thread Next] Keith:

It appears that you have discovered one of the problems with the truss industry.  These member are supposed to be braced and this usually appears on the truss company's drawings that come to the site.  Apparently these were not installed.  What many engineers that specify prefabricated trusses don't know is that THEY are responsible for the bracing of the web members.  The truss company design will indicate what web members are to be braced, but the ENGINEER (if there is one) has to complete the bracing design.  That is, possibly being braced off to the gable walls or strutted up or down to roof diaphragm or to walls below. 

>From an older commentary in  the TPI-85 publication:  "The need for and location of lateral bracing that may be required to reduce the buckling of individual truss members is determined as part of the wood truss design and is the only requirement for bracing that will be shown on the truss design.  Lateral bracing details, including method of connection and transfer of member buckling forces to the structure, are to be determined by the building designer."  ...... there's more.

In your case, you can probably add another 2x4 to the compression webs to form a tee section.  The installation should be done with care to prevent spliting.

Neil Moore, S.E.

I'm having a problem with a 40' span Howe wood roof truss.  The web members and bottom chords are 2x4's and the top chord is 2x6.  I modeled it in RISA-3D. The interior diagonals are showing compressive forces, yet there le/d > 50 using the 1 1/2" depth. It's member length is about 130".  So I guess my guest ion is how do I analyze this truss?  The slender member can't takes compressive loads, yet my model shows it in compression. Was the truss built incorrectly or am I not understanding how it works?

Ultimately I have to "piggy-back" this truss with another truss to extend the roof line. Essentially turning a gable into a triangle.

Keith Hyndman
Exton, PA