For channels under axial compression the limit states of flexural buckling and flexural torsional buckling need to be checked.
Flexural buckling (Fey) about the weak axis can be checked using equation A-E3-11 in the appendix of the 1999 AISC LRFD.
Flexural torsional buckling (Fexz) can be checked using equation A-E3-6.
The minimum of Fey and Fexz should then be used to calculate lamda for use in the appropriate equation (A-E3-2 or A-E3-3) depending on the local buckling properties.
Lamda is then used in equation A-E3-1 to calculate the final strength.