In this paper the problems connected to the effective width evaluation for reinforced concrete beam with variable inertia support one-way slabs are analyzed. Three-dimensional non-linear finite element analysis till failure is performed on fifty five simply supported concrete beams. The finite elements adopted by ANSYS are used for this study. For the purpose of validation of the finite element model developed, the numerical study is first carried out on a simply supported concrete beam that was experimentally tested and reported in the literature. The most important parameters that influence the effective width are analyzed. Some preliminary criteria for an adequate design are presented.