Steel Foundations Design Optimization of a 138 kV Transmission Line

7 October, 2019 | White paper

Richard Maranda


Structural Engineer

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This paper summarizes the optimization and design of the post-tensioned steel foundations for a 138 kV transmission line project located in northern British Columbia, Canada. The paper specifically addresses how the foundation design methodology was adapted to meet the challenges presented during the various phases of the project, from design to fabrication and construction stages.

The foundations were designed to withstand severe loads on towers and conductors, including wind gusts of up to 320 km/hour and radial rime ice of up to 120 mm. Another important factor was the snow thickness (up to 10 m), which led to heavy snow creep and avalanche loads on the towers.

Optimization of the foundation type was critical to the project; the material had to be transported on site by helicopter and lifting capacity was limited. For this reason, the preferred solution for the project was to design a prefabricated steel foundation, mainly consisting of galvanized steel plates and steel HSS tubes. The design included post-tensioned rock anchors to reduce the weight of the foundations and efficiently resist the important overturning loads coming from the transmission line towers. Considering the significant transportation and installation costs, this was the most cost-effective solution compared to a heavy weight concrete foundation.

Additionally, the paper presents the fabrication and construction aspect of this project, which has brought many more challenges. The fabrication of steel assemblies included various tolerance requirements which had to be communicated overseas under tight schedules. Verification tests were conducted at the Manufacturer’s site to ensure fitting between the steel foundation and the structure.

Several anchors at each foundation were using load cells to monitor the long-term potential tension loss. The minimum tension requirement was determined and a thorough maintenance plan was established to ensure the design integrity and efficiency throughout the life of the asset.

Furthermore, the paper covers the advantages of the prefabricated steel foundation design with regard to the specific project site conditions and to overcome constructability issues.

Note that this white paper was written in collaboration with Maxime Sansfaçon and Guillaume Vachon and it was the subject of a presentation at the CIGRE Canada 2019 Conference.

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