This paper studies a new launching method for steel bridges based on a self-supporting deck system. This new and patent-protected procedure is able to launch bridges of a span length up to 150 m, in an economical and sustainable way. The use of the last span segments to reinforce the critical sections during launching replaces other conventional temporary means applied nowadays. The main objective of this research paper is to numerically analyse the best double deck configuration as well as to define an approach to stiffener distribution in order to avoid the patch loading phenomenon in the slender webs. With this in mind, the pre-design of a triangular cell along the low flange of each web is presented. A three dimensional finite element model (FEM) is built and the design of experiments technique (DOE) is applied to obtain the best bridge configuration. This new construction method can be used together with a continuous launching system in order to increase the velocity of the whole operation and to improve safety during launching. Very good results have been obtained, in terms of deflection, patch loading resistance and vertical load on the pushing device. The comparison with other different construction systems and the application to a real case allows us to ensure the viability of the method described.