Limitations of plastic bending resistance for composite beams deviated from strain-limitation

Due to the demand for sustainable constructions, composite structures have become more important and lead to slim and economic solutions with a low structural self-weight. EN 1994-1-1 distinguishes between four different cross-section classes for the determination of moment resistance. For the plastic moment resistance, it is assumed, that each cross-sectional fibre can plastify without any limit on the strain. For standard composite beams with sagging moments and a high plastic neutral axis, plastic resistance and strain-limited resistance give similar results. For sections with a large compression zone height xpl and limited rotation capacity, concrete failure in the compression zone can occur before the plastic moment resistance Mpl,Rd, is reached. The strain limit design becomes critical. This paper points out the impact of the concrete part on the design of composite beams. A parametric study comparing plastic and strain limited moment resistance of a considerable variety of cross-sections has been carried out to develop the beam design methods considering compression zone height for the next generation of Eurocodes.