Shear strength of empty and infilled cassettes

This paper considers the shear buckling of light gauge steel cassette sections both with and without an infilling of relatively rigid thermal insulation. In cassette construction, in-plane shear stresses usually arise as a consequence of stressed skin (diaphragm) action and, in this context, it is local buckling of the wide flange that usually governs the design. Although there are some rudimentary equations for the shear strength of plain cassettes in Part 1.3 of Eurocode 3 (EC3) (Stahlbau 7 (1987) 197), this is a subject that is not fully understood and which raises some fundamental questions such as: What are the boundary conditions for plate buckling—is it sufficient to consider individual plate elements or is it necessary to consider the whole section? Is it sufficient to neglect the post-buckling strength or should this be incorporated in the design equations, as it is for plate elements in compression? How can the favourable interaction with a relatively rigid, thermally-insulating infill be incorporated in the design equations? This paper addresses these fundamental questions and illustrates the principles involved in designing for local buckling in shear.