I receive a lot of letters, via both snail mail and email, from readers of my books and articles, and much of the correspondence comes with enclosures or attachments containing images of artistic, beautiful, clever, dramatic, elegant, fantastic, and sometimes just downright zany designs. What they all seem to have in common is that the sender of the image wants to share the joy of having come across some device, structure, or system that brings a smile to the face. There is just something about a good design that makes you feel proud to be part of the ever-creative and astounding human race.
Many of my correspondents are avid travelers, and they send me photos from around the world. Several of these peripatetic design enthusiasts are as enamored as I am of beautiful bridges, which I consider examples of pure engineering design, or, as the structural engineer, Princeton professor, and structural critic David Billington would term it, structural art. Unlike buildings, which hide their engineering behind architectural facades, bridges tend to lay bare their structural design features. The lines by which forces are directed and transferred are exposed and open to admiration, exploration, and fascination.
Among the most elegant and daring bridge types are the suspension and cable-stayed designs. Suspension bridges are said to have had their origins in hanging vines, but modern examples are made of steel. As high as skyscrapers their towers soar, supporting suspension cables that drape gracefully over, between, and beyond them, held in place by monumental anchorages.
The roadway of such a bridge can appear to be but a ribbon of steel hung from the cables, and in the Akashi Kaikyo Bridge -- Japanís outstanding example of the genre -- the main span alone stretches over a mile between the towers. The Brooklyn and Golden Gate bridges, the latter of which is celebrating its 75th anniversary this year, are among the most iconic of suspension bridges, and they are truly things of beauty. But my far-flung correspondents have sent me pictures of lesser-known spans that rival even these American designs, and I am always happy to have my vocabulary of bridges expanded.
A more recent type is often confused with the suspension, and that is the so-called cable-stayed bridge. This clever alternative arose in its modern form in the wake of World War II, when the superstructure of many a destroyed European bridge lay in the water beside its piers. It became a design challenge to rebuild on the same foundation a bridge with a superstructure that was lighter than the original but capable of carrying a heavier load of traffic without overtaxing the old foundations. The cable-stayed bridge form that arose has its roadway supported by multiple cables that stretch directly between it and the towers. The multiplicity of cables allows for a wide variety of patterns, thereby giving the bridge designer a greater freedom in establishing a distinctive appearance. This makes the bridge type popular with communities seeking a signature structure. Such aspirations typically include also the goal of attracting tourists with cameras, and indeed they will flock to see and photograph a distinctive new design.