The master thesis aim is to investigate, through a finite element model, how the presence of the bottom flange affects the stability of the steel section, during the process of the incremental launching of steel bridges. In particular, two types of constraints have been analyzed (one rigid and the other elastomeric) on two different types of launching bearings (one with a spherical knot and the other with a simple discharge). Additionally, this thesis investigates the effect of a possible eccentricity of the load, due to the imperfect alignment of the constraint with respect to the section. The realization of the model, and therefore the application of the boundary conditions, are based on laboratory tests carried out at the Chair of Metal Structures of the TU of Munich during the period January/February 2018.