While designing single angle per IS code in STAAD pro software, the output report shows design based on principle axis for which the angle design qualifies. However when the same design is performed in STAAD per AISC 360-10 code, the output report shows design based on geometric axis for which the angle design does not qualify. Please clarify if geometric design needs to be considered for AISC/IS codes and why the STAAD design report based on IS code does not design as per geometric axis and designs only considering principle axis.
The AISC 360-10 allows single angles to be designed as per geometric axis only under certain conditions like if the angle has a continuous lateral restraint or the angle is an equal angle, bending only about one of the geometric axis with no compression and having either no lateral restraints or restraints at the point of maximum moment only. These conditions are laid out in the section F10 of the code and these are implemented in STAAD.Pro. So you will only get a geometric axis design for the angle as per the AISC 360-10 when these conditions are satisfied. You can declare the lateral torsional restraint for your angle through the design parameter FLX while designing as per AISC 360-10. Another thing that I would definitely recommend is that, please use the latest version of the software 20.07.11.82 because I know there has been many fixes/improvements made to design of angles in recent versions of the software.