Study On Optimized Plastic Design Of Steel Portal Frames With Genetic Algorithm

Steel gets good ductility after yielding. Plastic design can take advantage of the ductility to reduce the structural steel usage. For steel portal frames, plastic design method is more economic and convenient than the traditional elastic design method.Genetic Algorithm is a widely used optimization algorithm which is based on evolution theory. Recently, Genetic Algorithm attracts more attention of researchers because of its potential in complex structure optimization and successful application in structure design.In this paper, theory of plastic design is applied to steel portal frames, along with discrete Genetic Algorithm to achieve more optimum structural design. Mathmatic model is set up for steel portal frames based on the plastic design provisions in Code for Design of Steel Structures (GB50017-2003) and the foundamantal theory of Genetic Algorithm . The program for optimized plastic design of steel portal frames is generatored by FORTRAN. Two most commonly used steel potal frame configurations are designed with the optimized plastic design method and more economical structural design is achived, which shows that the method presented in this paper is effective and suitable for steel potal frames.As the basis of plastic design method, the simple plastic hinge theory often ignores the second order effects of structures. However in practice, the second order effects will always reduce the ultimate load-carry capacity of structures. In this paper, finite element models are set up for the two most commonly used steel portal frame configurations, using general finite element analysis software ANSYS. Structural analysis is done considering both geometric and material nonlinearities and the effects of the second order effects and strain hardening property of the material are compared. It is shown that strain hardening property of the commonly used steel (Q235) can compensate the bad effects of the second order effects on the ultimate load-carry capacity of the single-story steel portal frames, which ensure the safty and reliability of the plastic design method.