• bridges
• composite cross-section
• plastic moment resistance

In order to calculate the ultimate bending moment resistance of composite beams, the Eurocode EN1994 provides two different methods: strain limited design, where the stress depends on the strain of every fibre of the section and full plastic design, where the section is considered entirely plastic.
However, due to approximation of the stress diagram in full plastic design where there is a uniform stress for every fibre, a coefficient called reduction factor β has been introduced in EN1994-1-1 and EN1994-2 precisely to avoid an overstatement of resistant moment.
It is desirable to state specifically that the reduction factor β has been calibrated only for building’s cross – sections: the same relationships, without any validation, has also been adopted for bridge’s cross – sections, in EN1994-2
The objective of the study is to verify if the relationship of the reduction factor β found for building’s cross – sections is valid for the bridge’s ones and, if not, to recalibrate this coefficient in order to be proposed for the new draft, which is expected to be finalized in 2020.
The main focus of the research is the choice of all the studied cross – sections: the composite cross – sections are real, selected from real road and railway bridges in sagging and hogging areas with double composite action; the reason is to avoid the study of cross – sections obtained by nonsensical combination of materials and geometry. The calculation and the final relationship for the reduction factor shall have not only a mathematical but also a structural sense.