Assessment of the plastic capacity of I-shaped cross-sections according to the partial internal forces method

Abstract This paper presents the partial internal forces method – a method for the assessment of the cross section capacity based on the plastic theory. The method is based on mechanical principles, promotes a fundamental understanding and is therefore very popular in education and engineering practice. A main advantage of the method is that it can easily take into account all internal forces and moments of steel members of arbitrary cross-sections. The basics of the method are presented for double symmetric I-sections without fillets. In principle, the allocation of the internal forces and moments to the individual partial elements, i.e. flanges and web, of the cross-section takes place, taking into account the equilibrium conditions. This results in partial internal forces and moments. The capacities of rectangular shaped partial elements define the limits of the partial internal forces and moments. Additionally, the paper compares different cross section verification methods (plastic linear interaction, interaction according to EN 1993-1-1 and a strain based approach) in form of the yield surfaces using different combinations of internal forces and moments. The results of the strain-based method are almost identical to those according to the partial internal forces method. Compared to the linear plastic interaction, the partial internal forces method facilitates more economic design results. EN 1993-1-1 provides simplified interaction formulae for specific internal force combinations. The approach may lead to non-conservative results compared to the partial internal forces method and the strain based approach.