How Bridges Stand Up

A bridge is a structure with a single stubborn purpose: to carry a load across a gap without falling into it. Everything about its design follows from the way it manages two opposing forces. When part of a structure is squeezed, it is said to be in compression; when it is stretched, it is in tension. Some materials, such as stone and brick, are excellent at resisting compression but weak in tension, while others, such as steel cable, are superb in tension. The art of the bridge builder lies in arranging materials so that each is used for the kind of force it handles best. A good design, in the end, is one in which nothing is asked to do a job it is unsuited for, and much of the history of bridges is the story of engineers learning this lesson, sometimes at great cost.

The simplest bridge of all is the beam, a flat deck laid across two supports, as when a plank is thrown over a stream. Under a load the top of the beam is compressed while its underside is stretched, and if the gap grows too wide the beam sags and eventually breaks under its own weight. For this reason beam bridges are cheap and easy but suited only to relatively short spans. To cross wider gaps, engineers have had to be more inventive. Even so, the humble beam remains by far the most common bridge in the world, carrying countless roads over rivers, valleys and other roads wherever the gap to be crossed is modest.

The arch offered an elegant answer, and it was used to spectacular effect by the Romans, whose stone arches still stand across Europe two thousand years later. In an arch the weight of the load is carried mainly in compression, travelling down the curve and pushing outwards on solid supports at either end. Because stone is so strong in compression, an arch can bear enormous loads and endure for centuries, which is why so many ancient examples survive. Its weakness is that it needs firm ground or heavy abutments to resist the outward push. The Romans grasped this instinctively, building their arches on solid foundations and repeating them in long rows of identical spans to carry roads and water across wide valleys, and the survival of so many of these structures is a lasting tribute to the strength of the form.

For the very longest spans, the suspension bridge is unrivalled. Here the deck hangs from cables that sweep between tall towers and are anchored firmly at each end. The cables are held under great tension, doing exactly the work that steel does best, while the towers carry the whole weight down in compression. Famous examples such as the Golden Gate Bridge, and later the record-breaking Akashi Kaikyo Bridge in Japan, cross distances that no beam or arch could ever hope to manage. The price of this reach is complexity and cost, for the towers must be sunk deep, the cables spun from many thousands of steel wires, and the anchorages built massive enough to hold the whole structure in tension for a century or more.

Yet strength against gravity is not the only danger a bridge must face. In 1940 the newly built Tacoma Narrows Bridge in the United States began to twist and heave in a moderate wind, its long, slender deck rippling like a ribbon until it tore itself apart and fell into the water below. No vehicle overloaded it; the wind alone had set it swaying. The disaster taught engineers that a bridge must be designed to resist the forces of moving air as carefully as those of any load, and the study of how wind flows around structures has been central to bridge design ever since. Today the decks of long bridges are shaped in wind tunnels and tested against gusts long before any steel is cut, and some carry special devices designed to damp out dangerous swaying before it can build.

Behind all these forms lies the long story of materials. The Romans had only stone and timber; the eighteenth and nineteenth centuries brought first cast iron and then wrought iron; and the modern age is built above all on steel and on reinforced concrete, in which steel bars lend tension-resisting strength to concrete that is otherwise strong only in compression. Each new material, lighter and stronger than the last, has allowed engineers to throw their bridges across ever greater gaps, for the ambition to span the longest distances always demands materials that are at once strong and light. Each advance in materials has therefore opened the way to a new generation of longer and bolder bridges, and engineers continue to experiment with fresh alloys and composites in the endless effort to span a little further still.