In many steel structures like buildings, industrial halls and bridges, standardized hot-rolled sections are used. These sections are divided into specific types in Europe and similar profiles in the USA. The range of hot-rolled sections is limited and therefore fabricated plate girders are used when the standard hot-rolled sections do not meet the requirements for stiffness, strength, stability and economy. Such a plate girder is built up from steel plates for the top and bottom flange and for the web, welded together to an I-shape cross-section, single or double symmetric. Using this type of plate girders, a high degree of optimisation of material use is achieved by using different plate thickness and width for the flanges, and thickness and height for the web over the span of the girder adapted to the distribution of bending moments and the shear forces. A model is developed for the distribution of a certain amount of steel over the flanges and the web of a plate girder to achieve the maximum bending moment resistance. This model is based on a parametric study using the FEM method. The results of FEM models are validated with an test performed at the Lehigh University and with ten tests on plate girders performed at the Delft University of Technology. The model predicts the strength and local stability behaviour of laterally supported plate girders under pure bending. The following can be concluded: • The maximum web slenderness related to the steel grade S355 is at least • Material savings obtained by using high strength steel grades and by optimising the cross sections of plate girders are possible up to approximately 50%. This is an important finding for reducing the carbon footprint of steel structures.