Groundwork for Developing Comprehensive Transmission Line Foundation Design Guidelines

There are presently no comprehensive guidelines for foundation design of electrical system and power delivery structures. In recognizing this deficiency, industry professionals formed the Electric Power Systems Foundation Working Group in 2012 under the auspices of the Deep Foundations Institute (DFI) with the intent to lay the groundwork for future guidelines and to identify the state-of-the-practice. This paper provides a synopsis of findings and recommendations identified by the working group on four topics: (1) foundation design models, methodologies, and practices, (2) structure loads and reactions, (3) subsurface conditions and geotechnical parameters, and (4) limits state and performance expectations. The lack of a consistent analysis and design methodology has resulted in transmission line foundation designs that vary significantly from utility to utility. The goal of the Electric Power Systems Foundation Working Group is to quantify the variability in the current design practice and recommend modifications in an effort to generate a more unified approach to foundation design. Various electric power industry codes exist for defining structure loads; however, there is a lack of guidance for the selection of the governing case for foundation design, how to handle extreme loading and appropriate factors related to foundations, and overall chain of failure of the system (component strength). The state of the practice for limit state methods use target reliability indexes that minimize risk; however, there is variability in indexes used for each structure type. Standards on the compatibility of loads and resistance are needed and should also include performance parameters that are consistent with non-linear load-deformation responses observed with transmission line structure foundations.