Despite years of studies on critical bucking stresses, choosing the perfect boundary conditions at the junction between the web plate and flange & stiffener is still a question of engineering judgment. A conservative design criterion is adopted by all current codes of practice and standards at this juncture, based on the simply supported boundary conditions. However, in this study, we have proved Two theoretical methods were employed to investigate such situation that the boundary conditions along the junction between the webs and the flanges as built-in and the other boundary conditions along the position of the vertical stiffeners are assumed to be simply supported due to the relatively small stiffness of the stiffeners could be reached. The first method using the minimum potential energy concept to obtain a semi-exact equation from which the interaction equation can be deduced. The second method is performed by using a finite element technique (ANSYS software) to investigate the effects of the geometric parameters on the boundary conditions at the juncture.