just printed a copy of AISC 341-10. havnt got a chance to read it yet. can anyone explain to me when we are required to use highly ductile members or moderatedly ductile members? thanks a lot.
You're talking about the element slenderness / local buckling requirements for the members. Right?
The simplified answer is that highly ductile frames (SCBF and SMF) will require highly ductile members, moderately ductile frames (IMF) will require moderately ductile members.
That being said, there are cases (braces and columns, I think) where the code requirements do not always follow that simplified rule.