Multi-scale Simulation Method And Collapse Analysis Of Transmission Tower Structures

Transmissiont towers are vital components of transmission lines. Once the tower collapses, the power supply will be interrupted, and the economy and society will be deeply impacted. However, according to the statictical analysis, there are many collapses of towers in our country each year. The structural characteristics of the transmission towers are large flexib le, and small damping. Thus the towers are typical structures sensitive to wind loads. In order to improve the analysis and design of transmission tower structures, the strucutural multi-scale simulation method is introduced for analysis and design of transmission towers. Structural multi-scale simulation method is the frontier and hot issue in structural analysis. For a multi-scale simulation method, the concerned local compenents are modelled in detail with solid or shell emements, while the others are modeled with beam elements. Hence, the multi-scale model can consider the local detail in the global model of tower, and the structural multi-scale analysis can not only capture the global structure responses but also the local responses, including the stress of local gusset plate. In order to achieve the strcutural multi-scale analysis and design of transmission towers, five parts of research works are carried out in this thesis and listed as follows:(1) Study on the interface coupling of structural multi-scale modelling. The interface coupling method has a great influence on the accuracy of the multi-scale analysis. To achieve the displacement compatiblity and stress equalibrium at the interface, a new interface coupling method is presented based on constraint equation. The relationship between the nodal force and dispalcement constraint is derived from virtual work equation at interface. The numerical method for constructing constraint equation are presented in condsidering the interface coupling of nonlinear multi-scale method. The programs are coded based on FE software ANSYS and ABAQUS. Some numerical examples are carried out to compare the presented method and the existing method. The comparison shows that the presented method has better accuracy, especially for the stress results at the region of interface.(2) Experimental works on the phsical model of transmission tower. A model with a scale of 1:10 is bult of transmissiont tower. The model is built according to geometric similarity, including the gusset plate and bolts. There are 930 angle members with 23 different cross-sections, 402 gusset plate and 3649 bolts. Both the static and dynamic test are carried out. In static test, a concentrated force is applied at the middle of the cross arm of the tower. The global and local responses are measured, including the global displacement and strain of gusset plate of concerned local joint. The dynamic test is implemented by means of hammer test to extract the frequencies and mode shapes of the transmission tower. The test results are used to verify the structural multi-scale simulation model and the model updating method.(3) Study on the multi-scale modeling of the transmission tower. The multi-scale model is built for the phisical model of th transmission tower. The concerned local joint between lower crank arm and tower body is selected to build fine model with three dimensional solid elements. The solid element SOLID95 with 20 node is used for local joint model. Also, the contact problem in bolt connnection is taken into account. The other components are built with beam element BEAM188. The interface coupling is achieved by using the presented interface coupling method. Finally, the static analysis and modal analysis are carried out on the multi-scale model transmission tower. The results from multi-scale model are comparied with those form test to verify the accuracy of the multi-scale model.(4) Study on multi-scale model updating of transmission tower. The method for multi-scale model updating is presented based on the Kriging method. The multi-objective optimal model is constructed for multi-scale model updating. Both the global and local models of multi-scale model are updated by using the global and local response and dynamic characteristics. The multi-objective optimal problem is solved by means of multi-objective genetic algorithm to accomplish the model updating. In order to improve the computational efficiency of model updating, the Kriging method is used to built meta-model which replaces the multi-scale analysis during the model updating. The comparison between Kriging method and several common meta-models are carried out by numerical exmples. The results show that the Kriging method has better accuracy, especially for the highly nonlinear functions. Finally, the model updating method is applied to the multi-scale model of the transmission tower by using the test results of static displacement, static strain, frequencies and modes. The results show that the presented multi-scale model updating method can improve the accuracy of both the global and local responses.(5) The collapse analysis of the transmission tower under downburst by means of structural multi-scale method. The wind speed history is generated in considering both mean wind and fluctuation wind. Multi-scale model is built for transmission tower aiming at collapse analysis. The members with large deformation during tower collapse are modeled with shell element in detail, while the others are modeled by using beam element. Both beam model and multi-scale model of a transmission tower are used for collapse analysis of transmission tower to downburst. The results from beam model and multi-scale model are compared. The comparison between displacement time histories at top of the tower shows that the multi-scale model buckles earlier than the beam model. This means that the bearing capacity of the multi-scale model is smaller than the bearing capacity of beam model. By analysis of the deformation and equivalent plastic strain of the local shell model of multi-scale model, it shows that the multi-scale method considers the local nonlinear problems, such as stress concentration due to bolt connection and section collapse of main member, which lead to decrease of bearing capacity. Compared with beam model, multi-scale model can include more structural local nonlinear characteristics. Hence, the muli-scale method can obtain results with more accuracy.