I am doing non linear buckling analysis to determine the buckling co efficient of flange buckling of I beam. For doing eigen value buckling analysis , and from there I do non linear analysis to determine the max compressive stress , and then use that stress analytically to get the co efficient. My worries are related to modelling. Please go through the attached images to see which type of modelling should be preferred for conducting the analysis . (Loading and boundary conditions in APDL)