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Question on wood Roof Trusses

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Keith,

With a Howe truss, all of the diagonals are in compression under a uniform 
gravity load.  Any compression member with L/d > 50, i.e., 75 inches for 2X 
nominal members, has to have intermediate bracing *detailed by the truss 
designer* as this is a condition of design.  Note that in order to carry the 
required forces, L/d may have to be considerably less than 50.

With regard to piggy-back trusses, make sure that the top chord of the lower 
truss (the Howe truss) is also horizontally braced and detailed by the truss 
designer as this also is a condition of the design.  With panel point 
blocking between the lower truss and the upper truss, a complex support 
condition exists for the upper truss as it is no longer a simply supported 
truss, but a continuous truss over a series of spring-like supports.  Members 
which would be in tension in a simply supported upper truss may now be in 
compression when analyzing the truss as being continuous over intermediate 
supports.  Likewise, these spring-like connections to the upper truss subject 
the lower truss to non-uniform loading.

Hope this helps.

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

Keith Hyndman wrote:

>>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.<<