Transmission Tower Seismic Risk Mitigation for British Columbia

British Columbia Transmission Corporation [BCTC] plans, manages and operates 18,000 km of transmission lines, 286 substations and 124 telecommunication sites on behalf of BC Hydro. Much of the existing transmission system was constructed in the 1960s and 1970s, although some portions date back almost a century. The original system was designed and built with minimal or no specific consideration of seismic events. The British Columbia Transmission System is expected to withstand and readily recover from high probability but low impact events and perform in a safe and acceptable manner in low probability but high impact events such as earthquakes, ice storms etc. Transmission assets have typically performed well historically during earthquakes around the world. In the case of steel lattice towers in particular, the importance of these assets and the rigor of their design for climatic loadings provides added robustness against induced dynamic loading from earthquakes. Tower failures from ground motions during seismic events have largely been due to liquefaction, slope instability, lateral movements, and differential movements leading to localized tower footing failure. This paper demonstrates a utility's approach for analysing seismic risks in order to prioritize, plan, fund and execute risk mitigation. Discussions on the analysis, design and evaluation carried out in order to make consistent decisions on the most cost effective engineering upgrade options are also included.