The main purpose of the present paper is to describe an investigation made on the optimization of the flexural capacity of steel structures by varying the stiffness and resistance of its associate semi-rigid joints. This optimisation process is made using a genetic algorithm developed and implemented in a structural analysis program, FTOOL/SRC, [1,2]. This algorithm was inspired in the optimum-use algorithm, incorporating the problem domain's knowledge and leading to the development of a user-friendly system with consistent solutions. This paper describes in detail the formulation steps and procedures, and presents examples of semi-rigid steel portal frames. The purpose of the examples was to validate and prove the genetic algorithm efficiency towards an efficient bending moment distribution within the investigated semi-rigid steel structure. This purpose was fulfilled by varying the stiffness and the capacity of the adopted semi-rigid joints. © Civil-Comp Press, 2005.