Research On Plastic Fatigue Damage Mechanism Of Transmission Tower Under Strong Wind Excitations

Wind load is a very important designing load of high-rise transmission tower, which is the important component of large scale lifeline system. The collapse accidents of transmission towers occur now and then by wind excited, and high-rise transmission towers' safety is directly influenced national productivity and people's life. For a severe wind would be prone to bring out easy to result in elasto-plasticity behavior of the element, then induce the accumulated low-cycle fatigue damage and the collapse of the tower, studying on the structure's failure or collapse mechanism, realizing the structure's working status and insuring the security of service transmission symtem are very significant questions for discussion. Taking the Hanjiang long span transmission tower as the engineering background, the simulation method of downburst wind loads, analysis method of nonlinear transmission system's wind-induced response, spatial steel element's low-cycle fatigue failure model, the failure process of transmission tower under the severe wind load and long-standing wind induced crack initiation fatigue life evaluation technology are studied in this paper.In this thesis, the finite element model establishment method of transmission line's cable-structure mixed system is studied. In order to simulate transmission line, a prestressed four-node isoparametric curved element is proposed and a series of its finite element formulas are derivated. Then cable-structure mixed system's dynamic characteristics are analyzed and cable's influence on the dynamic characteristics of the tower is studied. Cable-structure mixed system's nonlinear static and dynamic analysis method are studied, and calculating code is compile for the establishing the model of the system, static and dynamic analysis using Matlab. The method and its code conquered many problems such as complicated calculating, difficult to converge and unable to consider the user-defined cable element and make the calculating on the efficient and accurate way. Then, the influence of the cable on the wind-induced response of the structure is discussed.Based on multidimensional stochastic vibration theory, modified spectral representation method are studied to simulate multidimensional cross-correlation fluctuating wind loads acting on the transmission tower structure. After widely investigating the characteristic of the downburst, the mechanism model of downburst is studied, a deterministic-stochastic hybrid downburst model and the loads simulating method of this model are proposed to provide the practicable loads for elastic-plastic dynamic analysis of the transmission tower under the action of hurricane.A number of steel specimens were tested to get the hysteretic curve under different plastic strain amplitude in order to calculate respective amplitude's ultimate cyclic number and ultimate plastic strain energy. The descending rule of specimen' stiffness during cyclic process is also studied. For several drawbacks of the widely used damage models, steel component's failure model based on low-cycle fatigue s-n curve is proposed and then verified by low-cycle fatigue tests of many specimens. The experimented results of cyclic tension-compression of steel shows that the proposed models can accurately describe the failure process of the member and several drawbacks of the widely used damage models are improved.Combining with the failure criterion based on low-cycle fatigue s-n curve, the elastic-plastic dynamic analysis method of the cable-structure mixed system under the action of downburst is studied, and the collapse process of tower is displayed under the action of downburst. All of this laid a solid foundation for the research of damage mechanism and failure process of transmission tower.Based on the fatigue cumulative damage theory, the real time crack initiation fatigue life evaluation method in wind-induced transmission tower's safe warning is proposed. By "identifying real time wind loading acted on the transmission tower by structure's safe warning system--obtaining the element's stress response time history by analyzing the nonlinear model--calculating each element's wind-induced real time cumulative damage" and "classifying the wind loading of the structure on site into many levels by its intensity--calculating probability characteristic of wind's appearance--evaluating crack initiation fatigue life", the real time residual crack initiation fatigue life is evaluated.