Optimization On The Bearing Capacity And Ductility Of Cold-formed Steel Flexural Member

Cold-formed steel(CFS)has the advantages of high regeneration rate and convenient construction.Compared with hot rolled steel,it is more economical and efficient,and has been widely used in medium and low rise buildings all over the world.With the continuous development of cold-formed steel frame system from lowto mid-rise,the requirements for the flexural capacity of CFS members are higher and higher,at the same time,the requirements for the ductility of members are also increasing.The production process of cold-formed steel is simple so it has great potential for development and optimization,many scholars have used optimization algorithm to find new or more effective cross sections.However,all these works were to optimize a single member.According to the principle of "less member,more combination",it is a key remaining problem to classify and sort out the members and form a basic member family with fewer members.With the material cost constant,a two-level optimization framework was proposed in this thesis in view of the above situation.The primary optimization focuses on improving the flexural capacity and ductility of the section.The second optimization aims to simplify the number of cross-sections.Firstly,the finite element analysis model which has been verified by experiments was established by ABAQUS to study the flexural capacity and ductility of cold-formed steel sections in different dimensions.Then,the relationships between the section dimensions and optimization criterions(flexural capacity and ductility)were analyzed respectively,section dimensions were optimized by BP(Back Propagation)neural networks and GA(genetic algorithm).Finally,the optimal section dimensions were obtained by TOPSIS(technique for ordering preferences by similarity to ideal solution)and the formulas for designing optimally section dimensions were presented;The second optimization focuses on the simplification of the number of individual cross-sections and these sections can still meet the requirements of flexural capacity and ductility in engineering.The main research contents are as follows:(1)According to the development history of cold-formed steel section,the back-to-back double lipped channel section,channel with web and flange stiffeners and configuration with web stiffener and folded flanges were selected for further study.Under monotonic and cyclic loading,the flexural capacity,rotational capacity,initial stiffness and stiffness degradation,deformation and failure modes,ductility and energy dissipation capacity of these sections were studied.Then,the energy dissipation mechanism of cold-formed steel members was analyzed.(2)The flexural capacity and ductility of cold-formed steel sections can be improved by changing the cross-section dimensions.The optimization results of cross-section dimensions under different weight of flexural capacity and ductility were discussed and the ideal section form with the optimal dimension was determined.The influence of steel strength grade and adding stiffener on member performance were also analyzed.(3)When the amount of material is constant,the specific trends of the flexural capacity and ductility of members with the variation of section dimensions were analyzed,so as to clarify how the section size should be designed to obtain the larger flexural capacity or ductility.(4)When the amount of material is not limited,the specific trends of flexural capacity and ductility of the members with the variation of section dimensions were also analyzed.The requirements of flexural capacity and ductility was put forward in this thesis,then the optimal design formulas of lipped channel,channel with web and flange stiffeners and configuration with web stiffener and folded flanges were given under the proposed flexural capacity and ductility conditions.(5)The range of flexural capacity and ductility of commercial lipped channels under bending was discussed.The flexural capacity and ductility of sections were optimized by using ANNs and GA.Then the fitness function was proposed to determine the minimum number of optimal cross-sections(family size)that can still meet the current engineering design demands.The efficiency of replacing commercial lipped channels with new member family was also analyzed.The results show that the prediction error of the neural network and genetic algorithm model is less than 2%,which has high accuracy and can be used to predict the flexural capacity and ductility of cold-formed steel.The flexural capacity and ductility of the optimized configuration with web stiffener and folded flanges are23.01% and 216.67% higher than the commercial lipped channel section respectively,which significantly improve the ductility and flexural capacity of the member.Families of optimal cold-formed steel lipped channel with 15 members can meet the overall flexural capacity and ductility performance of 48 commercial lipped channels.