Study On Optimal Design Of Semi-rigid Steel Frame Based On System Reliability

Structural optimization is the eternal goal in engineering; it is not a simple mathematical problem but an engineering problem with practical senses. It is a key research on structural optimization how to make those designs both economical and safe. Because of a series of particularity such as the large number of variables, discrete and highlighted nonlinearity, there is a serious gap between theoretical methods and their applications in practical design.This thesis presented researches on optimal design of steel frames in three areas, structural system reliability, semi-rigid connection and bracing.Firstly, structural system reliability was considered in optimal design, because it could reflect structural safe perfectly, which could be better to reflect the true meaning of structural optimization. Existing methods on structural system reliability were discussed, and some new methods--advanced Monte Carlo method and advanced PNET were proposed, which were suitable for complex structure analyzing using finite element method based on characteristics of reliability methods. Numerical examples showed that advanced Monte Carlo method was high accurate, east to implement and efficient; it could be reflected relationship between different failure modes obviously using advanced PNET. That method was more efficient then Monte Carlo method, and was suitable for evaluating to a structure at preliminary design.Secondly, in order to reflect characteristics of the frame and get a pratical optimal design, semi-rigid connection and system reliability were considered in optimization. The mathematical model of reliability based on finite element method for semi-rigid frame and reliability system theory were presented. An example of frame optimal design for semi-rigid frame using genetic algorithm (GA) was showed through a frame structure. It was shown that without reliability constraint, the optimal design obtained had a very low reliability, even would not reach the required level of structural safety of Chinese criterions. It may lead to potential safety problems dangers when optimization focus on economy only without due consideration of reliability. In the end of this part, it was shown that effects of semi-rigid connection on frame were coupled with other factors. A new design method, active semi-rigid connection method was suggested, and the optimization model of active semi-rigid frame was given in this thesis. At last, some conclusions on optimal design were drawn.And then, the optimization model of active semi-rigid frame bracing system based on reliability and discrete topology method was showed. And some issues were discussed through an example of frame. Considering the topology of frame, connection styles and reliability, optimization derived better engineering significance and reference value usually.And through a real example,some viewpoints about partial factor method were proposed.Finally, summarises and conclusions of whole thesis, and some prospects and further research of this thesis were showed.