I was once involved in litigation (1994) on K factors for pre-manufactured
metal bulidings with tapered columns. The metal building industry standard
is to use a K factor of 1.5. During the litigation, it was pointed out that
17 out of 20 metal building manufactures contacted used a value of 1.5.
During the litigation process, I also learned of Appendix F in tha AISC
manual. Since learning of this appendix, and having checked a number of
tapered rigid frames, a value of 1.5 is usually conservative.