Stresses are not computed on rigid elements because in general the cross-section is not known and usually not uniform. CAESAR II uses 3D-Beam elements for analysis - think of an infinitely thin stick. The represents all elements. Areas and inertias are determined based on the diameter and wall thickness entered, and are assumed constant through the length of the element.

If you want to perform a Section VIII Div 1 Appendix 2 flanges (stress) evaluation, you can find a module to do that from the "Analysis" ribbon.