You define orthotropic thickness to account for the structural orthotropy of floors.
Structural orthotropy refers to the nonuniformity of stiffness of a surface element in different directions, due to, for instance, floor ribbing. Orthotropy recognizes differences in stiffness in the mutually perpendicular directions, but it does not recognize the nonuniformity of the material. All the geometrical heterogeneities are recognized only in the stiffness matrixes of elements. A surface element with a thickness defined in this way should be treated as a structure with equivalent thickness but with different stiffnesses in perpendicular directions.