AISC Orthotropic Plate Design Manual

The structural system with which we are concerned is orthogonal anisotropic plate (popularly termed "orthotropic"). This means orthogonal—at right angles—andanisotropic—different properties. Thus, we are talking about a steel plate that has different physical properties in mutually perpendicular directions. Can this be right? For all practical purposes, steel has the same modulus of elasticity, the same elastic limit, the same Poissons ratio, the same ductility, and the same ultimate strength in all directions. Considering these facts, there cannot be such a thing as orthogonal anisotropic steel plate. There cannot be because steel is an isotropic material. On the other hand, an assemblage of isotropic steel elements consisting of a steel plate stiffened and supported on a system of ribs, beams and girders may be thought of as having anisotropic properties. In other words, anisotropy is due not to different elastic properties of the material but to the physical dimensions and arrangement of the components of the assemblage. When we use the term "orthotropic steel deck" we are talking about bridges whose steel decks are idealized as orthotropic plates—all parts working together as a unit rather than individual pieces designed separately on the basis of their individual functions.