Reading passage
Of all bridge forms, the suspension bridge offers the greatest span for the least material. Its deck, the roadway on which traffic travels, does not support itself. Instead it hangs from vertical cables called hangers, which are attached to two main cables that drape over tall towers and are fastened into massive anchorages at each end of the crossing. Because the main cables work entirely in tension, pulling along their length rather than bending, they can be thin yet immensely strong, and spans of nearly two kilometres have become achievable.
The idea is ancient. Simple bridges of rope or bamboo have crossed gorges in the Himalayas and in China for many centuries, and Chinese engineers were building with wrought-iron chains by the fifteenth century. In Europe, the form matured during the Industrial Revolution. The Menai Suspension Bridge in Wales, designed by Thomas Telford and opened in 1826, carried the London to Holyhead road across a tidal strait on a main span of 176 metres, its deck suspended from sixteen iron chains, each link forged and tested before installation.
Iron chains, however, were heavy and limited in length, and the next leap came from spinning cables of steel wire high above the water. The master of this technique was John Roebling, whose Brooklyn Bridge in New York opened in 1883 after fourteen years of construction. Its main span of 486 metres dwarfed every earlier bridge. Four cables, each containing more than five thousand parallel wires bound together, carried a deck wide enough for trains, carriages, and pedestrians, and the bridge remains in daily use nearly a century and a half later.
Success bred confidence, and confidence nearly destroyed the form. Designers in the 1930s favoured ever slimmer, more graceful decks, forgetting that a bridge must resist not only weight but moving air. In November 1940 the Tacoma Narrows Bridge in Washington State, nicknamed Galloping Gertie for its habitual undulation, twisted itself apart in a wind of only 68 kilometres per hour. The film of its collapse taught engineers that aerodynamic forces could excite a structure the way a bow excites a violin string, and wind tunnel testing of sectional models became compulsory for every long-span design.
Modern giants apply those lessons on an enormous scale. The Akashi Kaikyo Bridge in Japan, opened in 1998, holds the record for the longest suspension span at 1991 metres, linking the city of Kobe with Awaji Island across a busy, storm-prone strait. During construction the great Kobe earthquake of 1995 shifted its two towers roughly a metre further apart, and the deck had to be lengthened to match. Its cables, each over a metre in diameter, contain enough wire to circle the globe several times, and its stiffening girder is shaped to slice through typhoon winds.
Keeping such structures alive is a discipline of its own. Water and road salt attack steel relentlessly, so modern practice pumps dry, dehumidified air through the inside of the main cables to halt corrosion of their wires. Hangers are replaced piecemeal without closing the bridge, paint systems are renewed in continuous cycles, and sensors embedded in towers and cables report movement to engineers in real time. A suspension bridge, its custodians like to say, is never finished; it is only maintained, and the cost of that care must be counted when the span is first proposed.
The suspension bridge is not always the right answer, and its dominance has been challenged. For medium spans, cable-stayed bridges, in which cables run straight from tower to deck without anchorages at the ends, are usually cheaper and stiffer. Some engineers also argue that suspension towers and anchorages impose heavy demands on foundations, ruling the form out where the ground is weak. Choosing between the two systems is therefore less a contest of elegance than a negotiation among geology, shipping lanes, wind, and cost, conducted long before any cable is spun.
From rope crossings in mountain gorges to a span measured in kilometres, the suspension bridge has repeatedly absorbed disaster and returned stronger. Each failure, from chains that snapped in frost to a deck that tore itself apart in a gale, entered the textbooks and lengthened the next span. The form's continuing appeal lies in that visible logic: two cables in tension, a deck held up by hangers, and a clear statement, legible even to a child, of exactly how the crossing is made.