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Cold-formed steel portal frames are a popular form of construction for low-rise commercial, light industrial and agricultural buildings with spans of up to 20 m. In this article, a real-coded genetic algorithm is described that is used to minimize the cost of the main frame of such buildings. The key decision variables considered in this proposed algorithm consist of both the spacing and pitch of the frame as continuous variables, as well as the discrete section sizes. A routine taking the structural analysis and frame design for cold-formed steel sections is embedded into a genetic algorithm. The results show that the real-coded genetic algorithm handles effectively the mixture of design variables, with high robustness and consistency in achieving the optimum solution. All wind load combinations according to Australian code are considered in this research. Results for frames with knee braces are also included, for which the optimization achieved even larger savings in cost

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Pitched-roof Steel Portal Frames (SPFs) are common structures used in single storey buildings. It is necessary for this popular steelwork to pass through an optimisation process to minimise the total cost of the frame. Optimisation in terms of weight is well documented with different optimisation techniques. However, this approach to displacement maximisation is somehow rare. In this paper, an attempt is made to perform optimisation in terms of lateral displacements. To achieve this, a modified distributed genetic algorithm (DGA) is used to maximise lateral displacements of the SPF while the constraints meet the requirements of BS 5950. Furthermore, software 'DO-DGA' (Design Optimisation using DGA), coded by Visual Basic 6.0, has been developed by the authors to loop the optimisation process. Although the appearance and form of SPFs are simple, according to BS 5950, there are more limitations to be checked than in complex structures. A stiffness matrix has been derived for the haunched part of the rafter using a column analogy and a virtual work method to involve this part in the analysis process. Through two benchmark examples, the comparison is made between the results of weight minimisation and displacement maximisation.