Identification On Instabitity State Of The Compressed Members In Transmission Tower Structure

The vertical main members in tower body are the major force members of transmission tower structure. The vertical main members are prone to instability under the action of strong wind load, which may lead to collapses of transmission tower. So it is extremely necessary to identify the working condition and instabitity damage of the vertical main members in time and prevent collapses of transmission tower which caused by instabitity damage of the vertical main members.Study indicates that the vertical main members of transmission tower structure are not only withstand axial force, but also subjected to bending moment and are the typical compression-bending members. By considering the effects of initial geometric imperfection, geometric nonlinear and material nonlinear, such compression-bending members are subjected to elastic-plastic instability. The authors analyze the process of the vertical main member of transmission tower that is instable in elastic-plastic state by studying the degradation laws of axial compression stiffness in this paper. The finite element model of the vertical main member is firstly established with Finite Element Analysis Software ANSYS. The buckling analysis of the vertical main bar bearing axial force is firstly conducted in this study. And the relationship between the axial force and the axial compression stiffness is obtained. And then the application of buckling analysis to the vertical main bar that bear axial pressure at end and bending moment at end is carried out. The relationship surface among the axial pressure at end, bending moment at end and the axial compression stiffness is determined.The authors simulate instability damage by studying stiffness reduction of the vertical main member in this paper and a fractional-step detection method of the vertical main member damage is presented. Firstly, tower body of transmission tower structure is divided into several sub-regions along longitudinal direction. A impulse excitation is applied on the top of transmission tower’s head along the x direction, and acceleration time history response in every node layer of sub-regions that damage before and after is obtained. The acceleration time history response signal is decomposed based on bior6.8wavelet function. And then to identify the sub-regions that the main members damage may occur by building three factorial wavelet packet and the first band of energy curvature change rate index that damage before and after. Secondly, the change value of modal strain energy on each member that appear in the possible damage sub-regions is determined based on the theory of modal strain energy and interval estimation. And the estimated value of interval upper limit of modal strain energy change on all members that appear in the possible damage sub-regions is determined based on probability. The difference value between them is used as damage index to determine specific location of the instable members. Finally, the damage degree of the main members are detected by neural network method. The authors make numerical simulation analysis to verify the feasibility of fractional-step detection method. The results indicate that the fractional-step detection method proposed in this paper can accurately identify instability damage of the vertical main members in transmission tower structure.