I have an existing roof truss with double angles. Most of the double angles are SLBB. The truss is modeled in the X-Y plane and all of the double angles have the local 3-3 axis perpendicular to the truss. The top and bottom chords are modeled as continuous members. All of my loads are gravity loads -Y.
Why does the Ram Analysis and Design reports show values other than zero for M22 and show zero for M33 moments? Isn't this backward? There should be no out-of-plane moments.
Answer Verified By: SVGregory
Seth's response is correct.
The member forces are referenced with respect to the principal axes and not the local member axes. For double angles with the short leg back-to-back, the moment of inertia about the local 2-2 member axis is larger than the moment of inertia about the local 3-3 member axis. As a result, the principal axis is rotated degrees from the local member axes. The local 3-3 axis coincides with the principal 2'-2' axis. The local 2-2 axis coincides with the principal 3'-3' axis.
The following web page may also have helpful information:
http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/23793.ram-elements-local-versus-principal-axis-in-unsymmetrical-shapes