The towers in the photos above and below are part of the system carrying electricity from the generating plant at Hoover Dam to Los Angeles. The photos are from the HAER survey “Hoover Dam, Los Angeles Bureau of Power & Light Lines 1-3, U.S. Highway 93, Boulder City, Clark County, Nevada” which is an accurate title even if it’s possible to briefly mistake it for a survey of the dam itself.
The electric lines were worthy of a HAER survey because “The transmission lines included pioneering technology in high-voltage transmission. The lines were crucial elements of the Hoover Dam electrical complex, particularly in the role played by Hoover Dam power in WWII industries and post-war development in southern California. At the time of construction, these lines transmitted electricity at a higher voltage than any other long-distance transmission line in the world.” I know relatively little about the development of electric technology, so I’ll happily take the word of the HAER historians.
By 1940, when these towers were built, bridge design had moved away from the spidery-thin trusses of the 1890s, towards trusses with fewer and heavier members, and, for short spans, towards girder bridges with large members. But the towers are very light trusses, over 100 feet high and 32 feet square at their bases. There are more than 2400 of these towers, spaced about 1000 feet apart. The loads on them are arguably more predictable than those on a bridge: the weight of the cables, and the wind load on the towers and cables. Given their design in the mid-1930s, I doubt that there was significant seismic analysis; empirical performance says that was not necessarily a problem. Predictable loads and the absence of cladding or other non-structural elements to confuse the load paths make the light trusswork possible.
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