The Engineering of the Suspension Bridge

Of all the structures built by engineers, few are as graceful, or as daring, as the great suspension bridge. Where an ordinary bridge rests its roadway on a row of solid columns, a suspension bridge does something that at first seems almost impossible: it hangs its roadway in the air from slender cables, allowing it to leap across distances that no other kind of bridge can match. The world's longest bridges are almost all of this type, striding in a single bound across wide rivers and deep straits where piers could never be built. Behind their apparent lightness lies a careful balance of forces that every part of the structure is designed to carry.

The roadway on which the traffic runs is called the deck. In most bridges the deck is supported from below, but in a suspension bridge it is supported from above, hanging from the cables that soar overhead. The deck itself does not touch the water or the valley beneath; it is held up entirely by its connection to the structure above it, and its whole task is to carry the weight of the vehicles that cross and to pass that weight upward to the cables that bear it. To keep it from swaying too much in the wind, the deck is usually made stiff and, in modern bridges, shaped so that the air flows smoothly over it.

High above the deck run the two main cables, which are the true muscles of the bridge. Each is not a single rope but a bundle of many thousands of thin steel wires, spun together into a cable that may be as thick as a tree trunk. These main cables are draped in a long, sweeping curve between the tops of the towers, dipping low over the centre of the span and rising to the towers at each side. Because the whole weight of the deck hangs from them, the main cables are stretched with tremendous force; they are held constantly under tension, which is the pulling force that a rope feels when a load hangs from it.

The weight does not pass directly from the deck to the main cables. Instead the deck is joined to the cables above it by a great many vertical steel ropes called hangers, which drop down from the sweeping main cables at regular intervals and hold the deck up like the strings of a puppet. The load of the traffic travels first into the deck, then up through the hangers, and finally into the main cables, gathering as it goes until the whole of it is carried by those two great curves of steel overhead.

From the main cables the weight is passed on to the towers. These rise high above the deck, and the cables pass over the very top of them before sweeping down again on the far side. The towers carry an enormous load pressing straight down upon them, for the entire weight of the bridge and its traffic is funnelled onto their tops and must be carried down to the solid ground or riverbed on which they stand. A tower is therefore held under compression, the squeezing force that a pillar feels when a heavy weight presses down upon it, and it must be built strong enough not to be crushed.

At each end of the bridge the main cables are fastened into the ground in huge blocks of concrete or solid rock known as anchorages. These are among the most important parts of the whole structure, for the cables, pulled tight by the hanging deck, are constantly trying to drag their ends inward towards the centre of the span. The anchorages hold the ends of the cables fast and resist this steady, powerful pull; without them the cables would simply be dragged loose and the bridge would collapse. Everything, in the end, depends on their firm grip on the earth.

The suspension design allows engineers to build across a very long span, but it brings its own dangers, the chief of which is the wind. Because the deck is light and hangs freely, a strong or gusting wind can set it swaying, and in a famous early disaster a bridge was shaken to pieces by the wind alone. Ever since, engineers have studied with great care how air moves around a bridge, shaping the deck and stiffening the structure so that the wind cannot build up a dangerous rhythm. The modern suspension bridge is thus a triumph not only of strength but of the delicate understanding of the forces that act upon it.