The beam-to-column connections play fundamental role in the determination of the real behavior of steel structures. Therefore it is imperative to proper evaluate and consider the real geometric and mechanical properties of these connections, substituting the traditional idealized considerations, rigid and flexible, for the semi-rigid modelling approach. Nowadays one of the most used methods in the semi-rigid connection design is named the components method, described in the Eurocode 3. This method is based on the determination of the connection bending capacity and initial stiffness from the most relevant structural element parts. The main objective of this investigation was to evaluate the semi-rigid connection structural response. This was made by the development of a computer system capable to properly evaluate the behaviour of these connections. This program used the end plate beam-to-column connection material and geometric properties to fulfil this objective, producing a comprehensive database later to be used for more reliable structural project. Additionally an optimisation module was also implemented on the semi-rigid connection program, using genetic algorithms, where through the variation of certain geometric parameters, chosen in accordance to the user's requirements, the ideal connection structural behaviour could be determined from a range of pertinent solutions. Concluding the present study an alternative procedure is proposed for header plate connections where traditionally only shear was assumed to be transmitted. These connections were analysed according to the component method, to determine their bending and shear resistance, initial stiffness and rotation capacity, through modifications made on the relevant structural components presented in Eurocode 3 standard.