Influence of Shear and Axial Forces upon the Plastic Moment of Beam

Abstract

In plastic design of steel structures, it is generally assumed that the influence of shear forces on the plastic moment is negligible. However, in some structures, for instance built-up members with cutouts or Vierendeel girders, high shear forces may be accompanied by high axial forces, causing an appreciable reduction of the full plastic moment of the cross-section. Therefore this influence on the plastic moment should be studied and if necessary taken into account.Although there are several studies on the influence of shear or axial forces upon the plastic moment, little seems to be known about the combined effect of shear and axial forces. It appears that plastic design of built-up members requires the solution of two problems, one to establish the interaction curves for the plastic moment under the influence of both shear and axial forces and second to develop a design procedure based on these interaction curves. In this paper the combined effect of shear and axial forces on the plastic moment was studied by using lower bound method in the plastic analysis. Interaction curves are presented using non-dimensional parameters. Tests on the influence of shear and axial forces were carried out which confirm the theoretical analysis. Finally a design procedure of a built-up beam with a cut-out was developed using an iteration method.