In this work, the study of the evolutive behaviour of composite steel-concrete sections used in bridge decks was performed. A new method is proposed for the annex of EC3 part 2, in order to study the evolution effective steel area in a composite section, for serviceability and ultimate states. A coherent theoretical framework is presented, based on several relevant codes, namely the new Eurocodes. With this framework, a computer program is developed taking into account several important aspects in the behaviour of steel-concrete composite sections, such as nonlinear stress-strain relationships of the materials, local buckling of steel plates, time-dependent effects in concrete behaviour and load history. Also a new algorithm is proposed in order to compute strain fields at the section level from a known set of internal forces in the section, in nonlinear constitutive relations. Results obtained with the developed computer program are presented for several examples of typical sections found in bridge decks. Those results include moment-curvature plots, evolution of the strain field, evolution of the neutral axis, evolution of the stress in some points of the section. The influence of the concrete ultimate resistance and the staged construction sequence in the behaviour of the section is also evaluated.