| Electric transmission towers are the vital components of overhead transmission lines which play an important role in the operation of electric systems. Accurate prediction of the behavior of electric transmission towers is very important for design and accurate reliability assessment of the transmission lines and the power grids. The equal-leg angle steel is widely used in lattice transmission towers because of its light weight with L-shaped section making it easy for storage, transmission and fabrication. The angle steel members in lattice structure are normally bolted through only one of their legs and hence the forces transfering in the members are eccentric. The bolts within the holes are oversized1.5-2mm in lattice structure in order to provide an erection tolerence. For overhead transmission line structures, bolt slippage is likely to occur as relatively the bolt diameters are small, the leg plates are thin, the bearing type joints with lower clamping force are used, and the galvanized faying surfaces have a low coefficient of friction. Conventional analysis softwares for such structures assume the bolted connections as rigid ones and ignore the effect of joint eccentricity. Thus, the calculated deformations are smaller than the experimental ones under the same load. In some instances for structure still functioning well with large non-uniform settlement, failure was predicted by structure analysis softwares. The main reasons for the discrepancy between the experimental results and the analytical solutions are bolt slippage and joint eccentricity after years of ressearch, but most of the existing design softwares have not yet treated them properly. Therefore, it is important to model the joint of the lattice transmission tower properly in the design and safety assessment of the tower.This dissertation is a part of research project Effect of Bolted Joint on the Safty of Transmission Tower with Non-uniform Settlement, which is focused on some mechanics issues of the transmission tower, including the bolt torque coefficient, the slippage mechanism of bolted connection, the accurate modeling of the lattice transmission towers, and the safety assessment of transmission tower in the coal-mining area. The proposed algorithm and simulation results would provide good reference for further engineering applications. The main contents are as follows.1. Research on the torque coefficient of bolts in transmission towers. The torque coefficients of bolts in transmission towers are obtained by experimental measurement. The influence factors on the torque coefficient are analyzed, such as the strength grade of bolt connection and whether washer and lubrication are adopted. Furthermore, how to control the torque coefficient well and some other key issues on the bolt connection are discussed, which would provide the basis for the parameters control of the bolted joints in this study and provide good reference for other engineering applications.2. Test measurement and simulation of bolt slippage. For the representative bolted joints in the lattice structures used in this work, the load-displacement curves, the slippage mechanism and the failure modes of the bolted joints were investigated by tests. Using the finite element software ANSYS, the model of overlap angle steel bolt was established to simulate the bolt sliding process, and compared with the measured results.3. A slipping beam model is established to study the effect of bolted joint on the static characteristics of transmission tower with non-uniform settlement. The slipping beam model was developed based on the ANSYS, to which the deformation-load relationships of representative bolted joints were introduced and the load eccentricity in the structure component was also considered. This model was applied to analyze the deformation characteristics of a designed test tower and the calculated results are compared with those obtained from the traditional rigid-bolt treatment and the experimental measurement, verifying the validity of the proposed model. Observations have also been made for the ultimate load-carrying capacity and deformation properties of the test tower with different non-uniform settlements.4. A horizontal movable lifting device and displacement sensor fixing device are invented served for the test of the designed test tower and the ZM11transmission tower with non-uniform settlements, the displacement sensor fixing device has been invented for the slippage measurement of bolted joint online, and a horizontal movable lifting device has also been invented, which has low horizontal sliding resistance and digital pressure display.5. The effect of bolted joint on the dynamic characteristics of transmission tower was studied for the first time. The modal analysis of the transmission tower was performed by using the slipping beam model, into which the deformation-load relationships of the representative bolted joints were introduced and the load eccentricity in the structure components was also considered. The frequencies and mode shapes of transmission tower under natural excitation with four legs clamped and three legs clamped, the fourth leg under different loads were measured by tests, and compared well with the simulation results.6. The ultimate settlement displacements of the transmission tower with different non-uniform settlements were simulated by the slipping beam model, developed based on the ANSYS. When the transmission tower is subject to non-uniform settlements, we can judged the safety situation of the transmission tower through monitoring tower leg’s subsidence, which would provide good reference for future engineering applications.Through the research in this dissertation, an accurate modeling algorithm for the lattice structure has been established. It has the ability to analyze of the bolted connection effect on the static and dynamic characteristics of the lattice transmission tower. This would promote the higher accuracy of simulation for the transmission towers. |
PDF Full Text Request