Anti-Fatigue Optimization Design Of Cast Steel Nodes

With modern engineering structures showing trend to large, long-span, complicated and various, requirements on connection is more strictly. As a new style of connection method, cast steel nodes appeal great popularity to the domestic and foreign construction engineering, due to the characteristics of excellent mechanical property, flexible modeling, construction convenient, ect, compared with the traditional connection methods. Recently, engineering designers and relevant scholars recognized that the greatest advantage of cast steel nodes is the improvement on anti-fatigue property of structures, and enjoy increasing popularity in structures under fatigue loading. The anti-fatigue property of cast steel nodes can be divided into two parts:the anti-fatigue property of cast steel material and anti-fatigue property of girth butt weld. The anti-fatigue property of cast steel material is better than the girth butt weld, so the anti-fatigue property of cast steel nodes is determined by the anti-fatigue property of girth butt weld.Compared with traditional connection methods, although the cast steel nodes have many advantages, cast steel material will contain defects by nature inevitably. The research of fatigue property of cast steel material containing defects is insufficient, the fatigue data of girth butt weld is short of, so the method of magnifying the margin of safety is mainly adopted in engineering. Thus, current design methods of cast steel nodes are rather conservative, resulting in large redundancy of anti-fatigue property, wasting materials, increasing construction costs. Researching the anti-fatigue property of cast steel nodes, and the optimized design method of cast steel nodes based on anti-fatigue property is of great value in the engineering application of cast steel nodes.Faced with the problem above, this article did experiment research on the fatigue property of girth butt weld of cast steel nodes; analyzed the fatigue property of cast steel containing defects by finite element method; based on the research of fatigue property of cast steel nodes, combining with the engineering optimization method, did anti-fatigue optimization design of cast steel nodes, firstly, did optimization design on cast steel of cast steel nodes, getting the optimal lightweights size of cast steel nodes, then checked the fatigue strength of girth butt weld, achieved the optimization object. The main contents are as follows:1. Researching the domestic application of cast steel nodes, this article designed small scaled arc plate specimens of two girth butt weld style, tested the anti-fatigue property of specimens, processed the experiment statics, fitted the average S-N curve and p-S-N curve, and analyzed the experiment results with the data of similar styles of girth butt weld. This article recommended the S-N curve of girth butt weld of domestic.2. Recommending the parametrical design method, detailing the parametrical modeling process of cast steel node, this article modeling the cast steel node of the lower cast steel nodes of tour tower structure on the platform of Hangzhou bay bridge by parametrical design method. Then, based on the theory of fatigue damage cumulative, detailed the calculation of fatigue damage cumulative of cast steel node containing defects.3. Based on the project's research result of fatigue experiment of girth butt weld, and coupling with Chen Da's paramatical research result of hot-spot method of girth butt weld, this article shows the hot-spot S-N curve of usual weld details of domestic, providing the basis of checking the fatigue life of girth butt weld in the process of optimization of cast steel nodes.4. According to the characteristics of anti-fatigue property of cast steel nodes, and researches on the anti-fatigue property of cast steel nodes, this article proposed the optimized design method of cast steel nodes based on the anti-fatigue property:1) establish parametric model; 2) screen out parameters by sensitivity analysis method; 3) take the minimum mass of cast steel nodes as optimization objective, take fatigue damage limit of cast steel material containing defects as constraint conditions, combinatorial optimization strategy is adopted to optimize the cast steel material; 4) calculate the fatigue life of girth butt welds of the optimized cast steel model by the method of hot-spot stress approach, if the fatigue life of girth butt weld is lower than the design value, then amended the local parameter of the girth butt weld until the fatigue life is satisfied; finally, the optimization object is achieved. Taking the lower cast steel nodes of tour tower structure on the platform of Hangzhou bay bridge as example, this article optimize this node with the method of optimization design of cast steel nodes based on the fatigue property. Finally, the mass of cast steel node is reduced by 23.3%, satisfying the anti-fatigue property of cast steel nodes, compared with the original design.