Standardizing Foundation Design for Electric Transmission Structures FEATURE ARTICLE SPECIAL ISSUE :SPECIAL INFRASTRUCTURE Unlike the commercial building and transportation industries, the electric transmission industry does not have a unified code that explicitly covers design and construction of the various foundation types used to support electrical structures; there is no overarching professional group that leads this effort. Guideline documents developed by other industries describe general design methodology for foundation types used in the electric power industry, but practices vary significantly from utility to utility. For this reason, DFI established the Electric Power Systems Foundations Work- ing Group in 2013, and upgraded the work- ing group to technical committee status in 2018 after significant growth in member- ship and plans for continual activity. Previous Standardization Attempts Nearly 40 years ago, the Electric Power Research Institute (EPRI) embarked on a major research effort to optimize reliability and economics of electric transmission structure foundation design. Strategies focused on improvements in design models, site characterization, exploration strategies, and testing to reduce uncertainty and variance in analysis. Transmission line engineers realized that safety and reliability advancements in design based on probabilistic approaches, with a focus on the unique nature of transmission lines, could better control risk and cost. Some transmission line professionals took advantage of these advancements, particularly where research led to improved software and models. Yet by 1995, after two decades of extensive research, the industry had made little progress in changing design habits. Most designers were still charac- terizing sites via deterministic methods, using loads that may or may not relate to a consistent probability of occurrence, and applying a traditional global factor of safety approach (Phoon, et al., 1995). The follow- ing two decades have seen even greater developments in software and reliability- based analysis methods, but old habits seem to die hard as recent surveys show less than Transmission line in protected wetland half of design professionals quantitatively assess risk in their design process. State of the Practice Recent industry surveys performed by EPRI (DiGioia, 2010) and by DFI (Kandaris and Davidow, 2015) attempt to quantify the state of the practice in terms of design methodology. These surveys examined the current design practices of electric transmission and overhead line design engineers working for U.S. engineering firms and utilities. Among the broader survey results, the EPRI study found that two-thirds of responders were still using traditional deterministic (i.e., safety factor) approaches for design of transmission line foundations, with safety factors ranging upwards of 4.0, depending on the structure type, foundation type and design model. The DFI survey a few years later showed about 50% of electric transmission foundation professionals using traditional methods, and just under 40% using probabilistic design approaches. AUTHORS Peter Kandaris, P.E., DiGioia Gray & Associates, and Steve Davidow, P.E., S.E., P.Eng., Quanta Subsurface DEEP FOUNDATIONS • JULY/AUG 2018 • 73
