aisc manual clarification of chapter f

huangyhg

aisc manual: clarification of chapter f
hi all,
looking at chapter f in the aisc steel manual (ninith edition), the allowable bendig stress may be calculated by equations (f1-6), (f1-7) or (f1-8). is anyone aware of the rationale behind equation (f1-8)? it would seem that equation (f1-8) could be used for a rolled mill shape with larger l/rt values and would give much larger allowable stresses than equation (f1-7) would allow.
the commentary seems only to speak about equation (f1-8) in relation to girders and hybrid sections but gives no guidance as to it's applicability with rolled shape mill  sections.
thanks in advance for your thoughts and comments.
js

it's not clear from your post but you can use the higher value of the two equations.  
it's based on testing of lateral torsional buckling.  check "design of steel structures" by gaylord and gaylord, chapter 5.