Transformation of a Large Diameter Monopole to a Pinned Support Guyed Tower

7 October, 2019 | White paper

Alireza Aboutalebi


Geotechnical Engineer

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Richard Maranda


Structural Engineer

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Pierre-Luc Massé


Team Leader, structure

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This paper presents the engineering challenges encountered during the design of a large steel pin foundation as part of the detailed engineering of a new 138 kV transmission line near Stewart, British Columbia. The purpose of this project was to modify the foundation type of a monopole steel tower (LL-32) located in an area of poor rock condition from a fully rigid connection to a pin connection. This new 138 kV transmission line project is located in a remote area presenting site access challenges.

At the beginning of the project, a site investigation was performed to evaluate the different rock classes and to determine the line routing and structure locations. The evaluation of the rock was based on visual observations of the area and eight boreholes performed nearby.

Originally designed as a rigid foundation, the LL-32 monopole strain tower is a 40 m high, 2.6 m in diameter at its base, which supports significant loads. This tower was moved from its initial position to decrease the loads acting on the existing LL-33 adjacent structure. Blasting and excavation of the new LL-32 site uncovered a rock surface with a rock capacity inferior to what was expected (poor rock mass rating). Fracturing and shear zones were encountered and the LL-32 site’s rock class was therefore assessed as an inferior category. The location of the LL-32 structure foundation was weakened due to shear, fracture zones and weathering. There was no better rock condition in the vicinity of the structure. A concrete pad reinforcement solution was first proposed, but too costly since all work was done using helicopters. This solution had too great of an impact on the schedule and was ultimately rejected. The foundation of LL-32 had to be adapted to reduce the amount of rock anchors, thus decreasing its structural capacity. Since the original exterior steel foundation was already shipped to site at the time of the site blasting and the tower was already fabricated, a guyed approach was requested.

The decision was made to move forward and limit the impact on the schedule. The first step was to locally modify the tower structure to install guys. The second step was to modify the tower base connection to the foundation to limit the loads transferred. This entailed modifying a fully rigid base connection to a pinned connection of almost three metres in diameter, limiting the loads transferred to the main foundation of the tower and meeting the requirements of the decreased foundation and rock capacity.

BBA has developed a large pinned connection at the base of the tower. Since all the guys intersected at the same point in the tower, this assembly resisted the torsion loads of the tower while supporting vertical and lateral loads. The modification had to match the existing stub and foundation flanges. To facilitate construction, the pinned connection had to rigidly support the tower during construction. In order to limit reactions under the various line loads, the pinned connection was released after the guys were installed. It will remain in this condition throughout the service life of the line.

Note that this white paper was the subject of a presentation at the CIGRE Canada 2019 Conference.

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