Bridges are an important part of any country’s infrastructure. Welded stiffened plates are widely used as box girders for bridges. This article presents a design optimization problem for a single steel box with stiffeners on webs. The design is intended to be formulated based only on the current version of Eurocode 3 Part 1-5 ”Plated structural elements”. Eurocode 3 will definitely be introduced in all the Member States of the European Union. This optimization resource represents an engineering compromise, not only for society, but also for avoiding negative impacts on the environment.