Plastic coupled instabilities of I-shaped steel beams

• Plastic coupled instabilities of I-shaped beams are discussed.
• In-plane and out-of plane local mechanisms are defined.
• Using the theory of plastic collapse mechanism a parametrical analysis performed.
• Proper web to flange thickness ratio prevents out-of-plane local buckling.
• IPE sections are more sensitive to out-of-plane mechanisms than HEA and HEB one.

The paper presents the case of plastic instability of I-shaped beams where the rotation capacity is a very important characteristic in order to assure the required ductility in plastic or seismic design. The failure of such beams can be due to the local plastic plate buckling of compression flange and local plate buckling of the web in flexural compression, produced in-plane or out-of-plane. In the same time the failure can occurs by coupling of these two local buckling modes. In order to study the plastic buckling of beams and to determine the available rotation capacity which is required by the plastic or seismic design, the collapse plastic mechanism theory was developed and a specialized computer program DUCTROT-M was elaborated. The parametrical analysis reveals the paramount importance of the web to thickness ratio and furthermore the sensitivity in out-of-plane buckling of the European hot-rolled sections.