Given the fact that earthquake regulations increasingly apply more serious criteria for seismic design, this increases the dimension of elements of the beam and column in reinforced concrete structures, which in turn has a negative effect on the structure of the building architecture Will have. Although the increase in compressive strength of concrete reduces these dimensions to some extent, but there are some economical limitations. In this research, three concrete structures of three, seven and twelve floors were modeled, and also for each model The compressive strength of the concrete has been changed from 250, 300, 350, 400, 500 MPa and, according to the fourth edition of 2800th regulation, Aba Beton regulation and the National building rules, have been analyzed and designed and optimized. Then, the cost of constructing each model is based on Column reinforcement base, Column longitudinal reinforcement, Cramped beam reinforcement, Armature reinforcement beams, Longitudinal reinforcement beams, Volume Finally, the sensitivity analysis for each of the mentioned cases is investigated and compared. The research shows that concrete with 250-strong resistance to a three-story building, 350 for 7-storey buildings and 300 for 12-story buildings make concrete building with Flexible Frame system more economical. So, obtaining to the mentioned point, needs researchers and authorites attention in the field of concrete technology