American Scientist

The Washington Monument stands alone, tall and slender on a little hill, with its distance from any other structure making it seem even taller and more slender than it actually is. Last summer, when the world was celebrating the 50th anniversary of the Apollo mission’s “one small step for a man, one giant leap for mankind,” the image of a rocket projected on a side of the monument made it appear even taller and more slender still. Indeed, the masonry faux obelisk has a height 10 times the length of one side of its square base. Coincidentally, a height-to-width ratio of 10 to 1 is one of the more-or-less arbitrary definitions of a slender structure. Some engineers (and building codes) consider a ratio as small as 7 to 1 as the line of demarcation. Others take a ratio of 12 to 1 as the cutoff point. Whatever the number, the height-to-width ratio is sometimes referred to as the slenderness ratio, and sometimes as the aspect ratio. Whatever it is called, the definition of a slender structure remains somewhat subjective. Most people just know a slender tower when they see it, regardless of the numbers.