Rotation of Angle Members

I have a couple of questions concerning the rotation of singe angle members.  Which command should I use to rotate these angle members?  "Beta Angle" or "Beta 45" (or 135 or 225 or whatever your required angle is)?  I pose this question because I would have thought the results would have been the same but they are not.  The results for "Beta Angle" actually match those of an angle member modeled along its principal axis with no rotation or "Beta 0".  I also would have thought the properties for the weld design, namely Sz, Sy, CH & CV, would have changed with this "Beta Angle" command since the member end forces are no longer being applied in the same manner to the weld.  Any thoughts?

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  • Local Y is the principal strong axis of an angle - the one that passes through the vertex of an equal leg angle, and intersects one of the legs at the "Apha" angle for an unequal leg angle. Alpha is published in the property catalogues.

    For a member which is horizontal, and an "ST" angle is assigned, and BETA=0, the local Y axis is parallel to global Y. So, applying a load along local Y will automatically ensure that the load is along global Y too. Take a look at figures 1-9 and 1-10 in the Technical Reference manual.



  • I have noticed that after changing "Beta angle" to 45 degree to make legs a of equal angle parallel to global axis es, sectional properties of angle sections like Iy, Iz remain same and I think, it is because by specifying angle beta, we have rotated the locale co-ordinate system "as a whole" . However since my loading is parallel to vertical global Y axis, I have to use sectional properties (Iz etc.) of the angle in a local co-ordinate system whose Y axis is parallel to global Y when legs of the angle are parallel to global axis es. How to proceed in this case?
  • Your understanding of the beta angle is correct. However for analysis, you do not need to find out Iy or Iz about global axis. All you need to do is specify the loads along the global axes and NOT along member local axes. The software would automatically account for the correct stiffness in each global direction based on the beta angle specification.



  • Thank you. Now, since STAAD gives member force results (bending moments etc) about its local axis es, those obtained results for members with beta=45 degree will be about a co-ordinate axis es tilted to global axis es . But if I want to design the member manually from STAAD post processing results it will not work because I would like to design the member with sectional properties about global direction. So is not it possible to obtain member forces detail in global direction for members (specially steel angle sections) with angle beta?
  • You can get member forces printed in terms of global directions by using PRINT MEMBER FORCES GLOBAL LIST ... in the command file.

    The same can also be set using the GUI as shown next



  • I found this discussion that matches somewhat my problem and questions:
    I have to investigate a structure that uses also some single angles with equal legs. All the L members have one leg in a vertical plane, so I used a beta angle of 45 deg. I have to design the L members manually, using AISC 9th edition, based on forces extracted from STAAD (forces relative to the geometric axes of the cross section, not relative to the principal axes). Is there a way to extract the forces and moments relative to the geometric axes of the L members?
    Thank you.
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  • I found this discussion that matches somewhat my problem and questions:
    I have to investigate a structure that uses also some single angles with equal legs. All the L members have one leg in a vertical plane, so I used a beta angle of 45 deg. I have to design the L members manually, using AISC 9th edition, based on forces extracted from STAAD (forces relative to the geometric axes of the cross section, not relative to the principal axes). Is there a way to extract the forces and moments relative to the geometric axes of the L members?
    Thank you.
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