Longitudinally stiffened I-girders are commonly used to build incrementally launched steel bridges. During the bridge launching, the girders are subjected to concentrated loading at the support points. These loads are generally larger than those the structure would sustain once located in its final operating position. Numerical studies have considered girder segments subjected to concentrated loading where the boundaries are simulated by kinematic constraints. It represents rigid body conditions where elements at the supported end are allowed only to rotate about an axis perpendicular to the girder web. The load is applied on the girder flange along a small length compared to the total width of the evaluated segment. This paper is aimed at investigating the validity of this hypothesis for longer lengths of the applied load. Herein, the ultimate load of a stiffened girder is determined through finite element modeling, considering the boundaries conditions in two different ways: (1) considering kinematic constraints; and (2) simulating a transverse stiffener. In addition, the used model considers the plastic behavior of the material, the existence of imperfections in the web of the girder and the effects of large deformations. The results show that for short length loadings both hypotheses yield similar results. However, as the loading length increases, significant differences in the results are observed.