Role of Strain Hardening in Plastic Design

The collapse mechanism theory of plastic design requires that hinges form at the maximum moment locations in a loaded beam. Rotation occurs at these hinges until the structure under consideration becomes a mechanism. The behavior variations between this mathematical idealization and a real structure are studied. Previous plastic design studies have implied or stated that a rigid-plastic or an elasto-plastic stress-strain diagram is adequate for plastic design purposes. However, the paper illustrates the importance of the material possessing a strain-hardening as well as a plastic range. It is shown that a collapse mechanism cannot form in a beam that is composed of a purely elasto-plastic material. The real behavior of inelastic beams with strain-hardening capabilities is studied. As particular cases the fixed-ended beam and the Stussi beam are examined. It is shown that Stussi's results are explained and predicted by considering strain-hardening and local buckling.