Nonlinear performance analysis of steel lattice energy distribution towers

2022-2
Köşker, Yunus Anıl
Steel lattice tower structures play a vital role in overhead energy transmission and distribution networks. The safety of the towers has a great significance in order to keep power systems functioning. Despite their crucial function, these structures are susceptible to damage and sometimes total collapse as a result of environmental overloading. The collapse of tower structures in varying magnitude has been reported due to changing global weather patterns in the past few years. On January 16th, 2019, strong wind and heavy precipitation were predominant in Tufanbeyli, Turkey, and resulted in the collapse of 45 steel lattice distribution towers. In this thesis, some of these collapsed towers were taken as a case study and an investigation was carried out to evaluate the structural performance, estimate the load-carrying capacity, and determine the collapse mechanism of the structures under different loading conditions and modeling assumptions. On this basis, linear analyses were performed on eighteen suspension towers and three tension towers under the design loads and the failure condition loads to identify the exact cause of tower failures. Failure condition analyses of the towers were conducted by considering the loading cases of conductor break and ice with wind. Measured material capacities for tower steel members and conductors were incorporated in these analyses in an attempt to accurately simulate the actual conditions. The realistic conditions that the towers were likely to experience on the day of the incident were predicted based on meteorological data. Numerical results reveal that under design level loads and the specified loading cases the towers possess the safety level intended by the related design documents. However, at the time of the incident, ice accumulation around the conductors was multiple in size compared to the specified value by the design code. According to site inspections and field reports, the tower members experiencing failure were predicted accurately by failure condition analyses. Further studies were performed on the selected towers, focusing on the nonlinear properties to reveal the full collapse mechanism of the towers and failure sequence of the structural members. The nonlinear static pushover analysis with lumped plasticity approach was employed by modeling axial hinges with piece-wise linear force-deformation characteristics. The capacity curves of the towers were investigated and the most vulnerable parts of the towers were demonstrated. Results obtained from nonlinear analyses indicate that the tower response is governed by buckling of leg members in the tower body under ice and wind interaction condition, while for conductor break loading case the response is governed mostly by bolt bearing deformation of horizontal and brace members located near crossarms. It was determined that bolt bearing capacity provides a more ductile failure mechanism compared to sudden collapse due to member buckling.

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Citation Formats
Y. A. Köşker, “Nonlinear performance analysis of steel lattice energy distribution towers,” M.S. - Master of Science, Middle East Technical University, 2022.