Why upgrading the grid doesn’t mean replacing everything old.

Would you be surprised to learn that early lattice steel structures were not designed according to national standards? In the early days, each engineering firm followed its own unique design methodology, similar to how phone manufacturers once used different charging ports—until the USB-C emerged as universal standard. This lack of consistency resulted in structures built under varied criteria, creating significant challenges in their analysis that we continue to confront today.

The majority of the transmission infrastructure in the U.S. was built before 1960s and continues to support the electrical grid today. As electrical demand continues to rise, the grid must be modernized. This typically involves installing larger conductors or tightening conductor tensions to improve electrical clearances, leading to higher structural loads, sometimes exceeding the capacities the original structures were designed to handle.

The primary challenge in assessing aging structures is determining whether they still meet current code regulated strength requirements. Let’s explore the challenges of analyzing older transmission structures and summarize the various methods used for the analysis of these historical structures.

The Origins: Unique Design Approaches of the Past

To understand how these aging structures were originally built, it is important to look back at the design practices used over the last century. There are several approaches that are interesting and illustrate the progress made over the years. Beginning in the early 1900s there were no standards for designing lattice towers. During this time, the American Bridge Company was the leading provider of lattice towers and you might say they did it their way.

Several of their structures including tall river crossing towers are still in service. Their design approach is documented in their year 1925 publication “Transmission Towers” and is referenced for the study that will be discussed shortly. Materials science and engineering technologies continued to grow and advance.

By the 1960s, the practice of structural design had evolved and one approach was documented in the “Design Standards No. 10.” This publication came from the United States Department of the Interior, Bureau of Reclamation. It was one of the design methodologies used for design of lattice towers and hence is also referenced for this study, but structural design advancements did not stop there.

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