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This paper presents a methodology for optimal design of steel frames considering semi-rigid beam-column connections. The optimization model aims structures of minimum cost by determining the optimal stiffness degree of beam-column connections concomitantly with the classical optimization problem that considers dimensional optimization of beams and columns. Initially, the Structural Analysis module was developed, allowing automatic evaluation of structure’s mechanical behavior for different configurations during the optimization process. The Finite Element Method is applied through ANSYS Workbench software to calculate mechanical responses. The structural analysis model considers two distinct finite elements: the classical beam element (representing beams and columns) and the rotational spring element, which enables simulation of semi-rigid connection stiffness. ModeFRONTIER software is used to obtain the structure's minimum fabrication cost. The optimization process is conducted through Genetic Algorithms (MOGAII). Continuous and discrete design variables are used. The design variables are beam-column connections stiffness (continuous variable) and geometry of beams and columns cross sections (discrete variables). Databases of Brazilian commercial structural frames were created using Excel integrated with ANSYS. The design constraints are critical forces and maximum allowed displacements of the structure, according to NBR8800/2008. The validation of proposed methodology is made by comparing results with applications available in literature.