Study On Fatigue Load Spectrum And Fatigue Life Of The Plate-type Grid Structure With Suspension Cranes

The plate-type grid structure has been widely used in industrial buildings with suspended cranes.As the rapid developing of construction industry and industry in China,the number,tonnage and operating frequency of suspended cranes are gradually increasing as well.In the meantime,the fatigue problem of the grid structure is getting more and more serious.The research was supported by National Natural Science Foundation of China(“Dynamic fatigue reliability analysis and fatigue life assessment of the plate-type grid structure based on health monitoring”,51578357).In this paper,the fatigue load spectrum and fatigue life of the grid structure with suspension cranes were studied in depth.The main research contents and conclusions of this paper are as follow:(1)The field measurement of the stress state of an in-service grid structure under the load of suspended cranes were carried out.The stress distribution of the grid structure and the fatigue load effect characteristics of the suspended crane were analyzed.The results indicated that the stress of the grid structure under the load of suspended cranes showed obvious periodic change,and the suspension crane load effect was strongly region-dependent.The stress simulation of the grid structure under the crane load was conducted using finite element software,and the analysis results were in good agreement with the measured values.(2)Based on the measured load data of the in-service grid structure with suspended cranes,the fatigue load spectrum of the grid structure was compiled by combining the data processing,rain-flow counting and mathematical statistics.Furthermore,the theoretical compilation method of fatigue stress frequency spectrum of grid structure was discussed,and the fatigue stress frequency spectrum of the grid structure under different load distribution parameters was obtained.These achievements will provide references for fatigue life analysis of the grid structure with suspended cranes.(3)Constant and variable amplitude fatigue tests of M30 high strength bolt for the bolted spherical joint grid structure were carried out.It was found that the fatigue failure always occurred at the first engaged thread between the bolt and sphere.In addition,the constant amplitude and variable amplitude fatigue S-N curves were established.Through the morphology analysis of bolt fatigue fracture surface and numerical simulation of bolt stress,the fatigue failure mechanism of high strength bolt in the bolted spherical joint was revealed.Moreover,the constant amplitude fatigue tests of M30 high strength bolt with insufficient screwing depth was studied,and the corresponding S-N curve was obtained.The results showed that the fatigue strength of M30 high strength bolt reduced greatly when only three bolted threads were screwed into the sphere.(4)The numerical simulation of the stress concentration problem of high strength bolt for bolted spherical joints was carried out,and the influences of two different modeling methods and multiple different mesh sizes on the stress calculation results of the high strength bolt were discussed.The appropriate finite element model was selected to calculate the stress concentration factor and fatigue notch coefficient of high strength bolt.Meanwhile,the stress concentration factor of the high strength bolt connection in the bolted spherical joints was analyzed parametrically,which further revealed the fatigue failure mechanism of high strength bolt for bolted spherical joints.(5)S-N curve method,local strain method and damage tolerance design method were adapted to evaluate the fatigue life of M30 high strength bolt in the bolted spherical joint.Combining with the existing fatigue test data and theoretical analysis,the three fatigue life assessment methods had proposed amendments in terms of their calculation parameters.The results showed that the method after parameter correction has higher evaluation accuracy and is suitable for fatigue life analysis of high strength bolt.(6)Based on Palmgren-Miner's linear damage accumulation theory and the fatigue strength S-N curve,the fatigue life of the two types of joint in key components of the measured grid was evaluated.Then,the fatigue failure limit state equation of the grid structure based on the linear damage accumulation theory was established,and the probability distribution characteristics and parameter values of each parameter in the equation were discussed.The Monte-Carlo's simulation method was used to calculate the reliability index of the measured key components.Additionally,the influence law of the growth rate of fatigue load effect and crane load increase on fatigue reliability index were discussed.The results showed that whether to consider fatigue damage reduction caused by low stress range has a great influence on the calculation results of fatigue reliability index.After the low stress range damage reduction treatment,the reliability index is significantly improved.With the extension of the structural service time,the larger the fatigue load effect growth rate,the lower the fatigue reliability index.As the crane load increases,the fatigue reliability index decreases significantly.