From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Fri, 12 Jan 2001 10:30:11 -0500
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)
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.<<