Hardening Transmission Line Components to Optimize Performance Under Extreme Loading Events

Anthony DiGioia will present an informational session describing a reliability-based design approach to hardening the components (structures, foundations, wire systems, insulators and hardware) of transmission lines for optimizing performance under extreme design events. The session, Hardening Transmission Line Components to Optimize Performance Under Extreme Loading Events, will be held on Wednesday, May 9, in Room 207C of the Orange County Convention Center.

American Electric Power worked with DiGioia Gray & Associates, LLC, (Booth 1127) on an article in T&D World's January 2012. The article, “Revealing Foundation Design Approaches," discusses the many challenges faced in the design of foundations for transmission line structures. A transmission line is a complex system of interacting components: structures, foundations, conductors, insulators, shield wires and hardware. A line may consist of many structure types, which can be supported by an equally wide variety of foundation types. Additionally, a line may traverse terrain that changes from flat to rolling hills and mountains. A line can also traverse different geologic zones, which can present unique geologic challenges, such as karst areas, unstable slopes, mined and unmined areas. Thus, the foundation design process involves the application of both technology and art in each step of the process. And, this is especially true in the development of the geotechnical design parameters needed for foundation design.

As in the case with all technology-based design processes, research and development are performed on a continuous basis to bring new and innovative concepts and improvements to the design process. This is exactly what happened to the process for designing foundations for bridges, and what is happening to the process for designing foundations for transmission line structures. This closely parallels similar transitions in the concrete and steel building industries.

Over the past 30 years, the Electric Power Research Institute (EPRI) sponsored many research projects to advance the foundation design process. Prior to EPRI’s research efforts, the most common foundation design method, used in practice, was the Allowable Stress Design (ASD) approach. In the ASD approach, foundation designers assume a foundation size, compute the nominal design capacity (Rn) of the foundation and then computes the design capacity (RD) for the assumed foundation size by dividing RD by a safety factor. If the design capacity (RD) is less or greater than the applied load, the designer assumes a new foundation size and then repeats the process until he achieves an acceptable design. The biggest challenge facing the ASD designer is the uncertainty in establishing a safety factor; in that there is no systematic method for their establishment. Thus, the selection of factors of safety is dependent on the foundation design engineer’s background and experience.

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