Encased beam composite construction employs structural members that are composed of two materials: structural steel (rolled or built-up) and reinforced concrete, is an example of composite members. Encased beams have been used as rigid reinforcement in deck bridges for several decades. Nowadays they are used mainly in railway reconstruction with limited building heights. In this paper the effect of the upper steel section flange position of encased beam on the beam capacity and beam ductility is analyzed. Three-dimensional non-linear finite element analysis adopted by ANSYS till failure is performed on twenty one simply supported encased concrete beams. For the purpose of validation of the finite element model developed, the numerical study is carried out on a simply supported concrete beam that was experimentally tested and reported in the literature; good agreement with the experimental results is observed. Specimens were tested under lateral loading. The test results indicate that the behavior of the beam is greatly affected by the steel beam upper flange position. Upper flange width the most important parameter influences the beam capacity and ductility is analyzed, some preliminary criteria for an adequate design are presented.