Koi Koi Grace
by on February 22, 2019
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Historically, the term “transmission structures’’ usually implied iron or steel latticed towers. The early “pylons’’ dating back to 1829 were iron structures; the basic shape of later pylons was mostly inspired by the famous Eiffel Tower. Figure 1.1 shows some of the early shapes and forms of transmission structures.

Single wood poles directly-embedded into the ground formed the bulk of the transmission structures family for a greater part of the 20th century. France in the late 1900s and later Belgium in 1924 began producing concrete poles. The first steel tubular transmission pole in USA was erected in 1958.

Wood H-frames and lattice steel towers became popular later on, dictated mostly by height, availability, strength and urban convenience. Prestressed concrete poles are also used in various places. The world record for the largest transmission structure is now held by China’s 500 kV Yangtze River Crossing double-circuit tower, 1152 ft (351 m) tall, supporting a maximum span of 7667 ft (2337 m) and weighing 8.4 million lbs (3.81 million kgs).

Little historical information is available on how foundations were designed for transmission structures in the early days. It is conceivable that some rule of thumb and field tests were used while determining how much a pole needs to be embedded into the ground. One of the earliest discussions on soil behavior in wood H-frames can be traced to 1943 (Hughes Brothers, 1943). Figure 1.2 illustrates the earth pressures below the ground on the legs of an H-frame.

 

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