Keywords: optimum design, genetic algorithm, elastic planar steel frames, non-linear analysis, semi-rigid connections, component method, Eurocode 3.

Traditional design methods which used simple and idealized models to represent structural behaviour are being substituted, nowadays, by modern methods that are able to represent their behaviour in a more realistic manner. A great effort has been made in the last twenty years in the development of efficient structural analysis methods. It can be said that at present, these methods are fully established, with the investigators orienting work more to their improvement than in the proposal of other new ones. During this period of time, the development of new design techniques has not proceeded at the same pace as this development in more powerful analysis techniques. However, in recent years, a great effort is being made to establish automatic design techniques which include advanced analysis tools since the calculating power of modern computers permits us to obtain complex designs in a reasonable time. The evolutionary methods are one example of the methods that are providing a solution to complex structural design problems in a practical manner.

This article presents a numerical method, based on evolutionary computation techniques, for the optimum design of non-linear elastic planar steel frames. A genetic algorithm provides the optimum steel shape for each member of the structure and the optimum semi-rigid connections between members taking into account discrete variables related to several sets of steel shapes, bolt diameters and steel plate thicknesses. An objective function is used which includes the combined member cost as well as the semi-rigid connection cost. Constraints related to both service and strength load levels are included. In the analysis, second order effects are taken into account for the presence of large axial loads (P-delta effect) and for the presence of large displacements (P-Delta effect). The behaviour of the semi-rigid connections between the columns and beams is obtained by means of the component method detailed in Eurocode 3. Designing a structure permits us to demonstrate that the proposed design method is a robust procedure for the minimum cost automated design of non-linear elastic planar steel frames.

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