The use of plates in industrial applications often requires the use of stiffeners, ribbs, corrugated forming or pressed shapes for increasing rigidity. Isotropic classical plate theory cannot be used directly for vibration and power flow analysis. It can however be described analytically by the thoery of orthotropic plates for a plate rib combination or corrugated plates with an equivalent constant thickness plate using the theory of elastic equivalence. Different elastic rigidities of the actual orthotropic plate, such as a plate-rib combination for example, as obtained from elastc equivalence are reviewed. The determinations of elastic rigidity constants for out of plane and in plane load behaviour are presented in this paper. The solution involved replacing actual orthotropic plate with an equivalent orthotropic plate of constant thickness for whcih appropriate properties are derived. Shape properties of the plates are treated separately since material or rigidity properties depend on the specfic shape of the orthotropic plate. Since it is not always possibe to determine the required elastic properties experimentally, analytical determinations are encouraged.