Abstarct: Today, with advances in science and technology needs and new demands in the field of structural engineering has occurred . Architects and engineers are always trying to find new solutions to the problem have been enclosed spaces . With the industrialization and development of the modern world demand for structures with large spans increase. Steel moment frxame buildings with increasingly low height and high openings are located in areas with high earthquake risk , are used . The result of such frxame design, large sections of beams . According to the laws , regulations , weak beam - strong column must be established to lead the implementation of very large columns and sections in these structures are more than needed , resulting in increased weight and non-economic structures to be designed . In this regard, the seismic performance of the three buildings moment frxames of steel with openings 5 , 7 and 9 m by ignoring this principle, the analysis of nonlinear static and nonlinear dynamic , standards-baxsed instruction seismic rehabilitation assessment were compared . capacity spectrum method is used to determine the performance . The analysis was carried out and they demonstrate the relative displacement and relocation of classes in these structures increases with increasing span length . Also, the indicators show that the span length increases in these covers life safety structural performance level dropped and the possible collapse of the increasing amount will greatly reduce the members Formability