Limits of plastic design for composite beams

Due to the demand for sustainable constructions, composite structures become more important and
lead to slim and economic solutions. The determination of moment resistance for composite beams
follows the rules for composite structures according to EN 1994-1-1.
For the plastic design it is still assumed, that each cross-sectional fibre can plastify without any
limitation of strains. For normal composite beams in case of sagging moments and a high-lying
plastic neutral axis, the real moment resistance is slightly higher than the plastic resistance. This is
based on the large strains at the bottom side of the section, so that the lower steel flange reaches the
solidification range. In case of sections with a large compression zone height xpl, a concrete failure
in the compression zone can happen before reaching the plastic moment resistance. In these cases a
strain limit design, based on the strain-stress relationships of concrete and steel, becomes decisive.
EN 1994-1-1 provides only for cross-sections of classes 1 and 2 in steel grades S420 and S460 a
limitation of plastic design. It is to point out, that the rotation capacity of a composite section is not
only conditioned by the b/t ratio of the steel section, but also the concrete part has a significant
impact. This applies not only for high steel grades. Though, in Eurocode 4 there are no further
regulations given for sections with lower steel grades. Especially, the more and more upcoming
integrated and compact composite sections underline the importance of such requirements.
Therefore, the objective of the recent research is concentrated on the development of additional
regulations, to consider the influence of concrete compression zone height onto the moment
resistance of composite beams.