Steel transmission structures have been in service for more than 100 years. As they age, these structures are experiencing metal loss both above and below ground level due to corrosion. Significant questions have arisen regarding how much corrosion wastage (section loss) is acceptable before some type of remedial action is required. Assessing the remaining capacity of a structure due to corrosion wastage and the associated loss of member thickness is a complex process due to the different failure modes that must be considered.
This project’s objective was to develop guidelines for acceptable metal loss which, in turn, could be used by utilities to make decisions on the suitability of structures: whether a structure should remain in service, whether it requires remedial action to ensure safety and reliability, and whether the life of a structure can be extended. The project began with a literature search and review of current and suggested practices from a variety of sources. Sources included utilities, industry organizations (including IEEE and NACE), and other industry practices like the highway and pipeline industries. A survey of participating utilities was added to gain knowledge of current industry practice that could later be used in developing guidelines.
To establish a process for developing guidelines, example structures were selected from widely used structure examples provided by participating utilities – a transmission tower as well as a steel pole structure. A review of the design for the selected structure was conducted focusing on the design details, critical load cases, and percent capacity utilization for different member types. The design codes applied to the structures were reviewed and compared to current design standards to identify any applicable code changes.
The selected example structures were then analyzed with simulated thickness losses to determine acceptable strength loss and developing corresponding metal loss thresholds. Computerized modeling software, like PLS-Pole and PLS-Tower, were used to model example structure and to simulate thickness losses. The analysis focuses on the near ground components as they are most heavily utilized as well as likely to see thickness loss from corrosion. However, the approach is applicable to any member or location on the structure. Uniform thickness loss on the entire cross-section of the pole was conservatively used in the modeling. For the lattice tower members, the entire angle member was replaced with thinner members to simulate thickness loss. This approach will result in conservative guidelines which will potentially be beneficial for establishing guidelines for several structures in the transmission lines.
Transmission structures, Poles, Lattice Towers, Corrosion assessment, Structure assessment, Strength assessment