Analytical expressions for the orthotropic rigidities of concrete waffle-type slab structures are presented; the expressions are valid for both the precracking and postcracking stages and are therefore applicable to reinforced and prestressed concrete structures. Formulae for the rigidities of orthotropic slab structures of uniform thickness are also given. Flexural and twisting tests on several concrete waffle-slab elements were conducted; the experimental results verify and substantiate the proposed analytical expressions. The use of realistic estimates for the orthotropic rigidities will lead to economy as well as better design of concrete waffle-type slab structures.