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There is nothing wrong with AISC K-factors for tapered members in Appendix F
(including Commentary), however, those are of limited use to metal building
systems manufacturers because of three assumptions/requirements stated at
the beginning of Appendix F. 

While rigid frame web-tapered members always have at least one axis of
symmetry, the second assumption that flanges must have equal and constant
area is rarely satisfied. Flange size could vary along the member length,
and some manufacturers use different flanges for inside/outside. The third
requirement for linearly varying depth is typically satisfied for columns
but not for rafters.

Consequently, most MBS manufacturers use simplified approach with K=1.5 for
columns (that is quite adequate), and few use more sophisticated methods to
calculate it. Whatever the result is minor axis bending is often of