The Engineering of Suspension Bridges

Of all the structures that engineers build, few are as striking as the suspension bridge. With their slender roadways hung beneath sweeping curves of cable, these bridges can cross distances that no other design can match, leaping across wide rivers and deep bays in a single span. The largest of them stretch for well over a kilometre between their main supports, carrying streams of traffic high above the water. Their beauty is undeniable, but it is the engineering behind them, rather than their appearance, that makes such enormous spans possible. A suspension bridge is a careful solution to the problem of holding up a heavy road over a gap too wide for any simple beam to bridge.

The essential idea is to hang the roadway from above rather than to support it from below. In an ordinary beam bridge, a rigid deck rests on supports called piers, and the distance between piers is limited, because a long beam will sag and eventually break under its own weight. A suspension bridge escapes this limit by carrying the weight of the deck upward into strong cables. The roadway hangs from these cables, which in turn are held up at just a few high points. Because the load is gathered together and directed to a small number of supports, the towers can be spaced very far apart, and the great open span between them is left completely clear.

The main components of a suspension bridge are easy to identify once the idea is understood. Two tall towers rise from the ends of the central span and carry most of the structure's weight down to the ground. Over the top of each tower passes a main cable, which sweeps in a graceful curve from one side of the bridge to the other. From these main cables hang many thin vertical cables, called hangers or suspenders, and it is from the lower ends of these that the roadway, known as the deck, is actually suspended. The main cables do not stop at the towers; they continue down to the ground at each end, where they are fastened into massive blocks called anchorages.

Understanding the flow of forces is the key to understanding the bridge. The weight of the deck and the traffic upon it pulls down on the hangers, which pull down on the main cables. A cable is very strong when it is stretched, a kind of force engineers call tension, and it is this tension that the main cables carry. The cables transfer the load in two directions: downward through the towers, which are squeezed and so are said to be in compression, and outward along the curve of the cable towards the ends of the bridge. This outward pull is enormous, and it is the job of the anchorages to resist it. Buried deep and immensely heavy, the anchorages hold the ends of the cables firm and stop the whole structure from being dragged inward.

The main cables themselves are marvels of construction. Although each looks like a single solid rope, it is in fact made from many thousands of thin steel wires bundled tightly together. Steel wire is far stronger for its weight when many slender strands share the load than a single thick bar would be, and the bundle can be built up gradually, wire by wire, high above the water. Once all the wires are in place they are compacted and wrapped to protect them from rust, for the survival of the whole bridge depends on these cables enduring for a very long time.

One challenge that the early builders of these bridges did not fully anticipate is the effect of wind. A long, slender deck hanging in the air can be set swaying by a steady wind, and if the movement builds up rather than dying away, the results can be destructive. Engineers now study this behaviour carefully before a bridge is built, often testing scale models in wind tunnels to observe how the design responds to moving air. Modern decks are shaped so that the wind flows smoothly past them, and are stiffened so that they do not twist or oscillate dangerously. In this way lessons learned from earlier difficulties have been built into the safety of every large bridge that follows.

A suspension bridge is thus a conversation between a handful of simple ideas: weight pulling down, cables in tension carrying that weight to the towers, towers in compression passing it to the ground, and anchorages resisting the outward pull. Each part does one clear job, and the whole depends on the balance between them. When that balance is struck correctly, the reward is a structure that can span distances once thought impossible, standing for a century or more against the combined forces of gravity, traffic and wind.