Bridges—How Would We Manage Without Them?
“Praise the bridge that carried you over.”—George Colman, 19th-century English playwright.
WHEN was the last time you crossed a bridge? Did you even notice it? Millions of people cross bridges every day. We just take them for granted. We walk, ride, or drive over them or under them, perhaps without giving a thought to it. But what if they were not there?
For thousands of years, man and beast have been able to cross otherwise impassable gaps, whether a river, a chasm, or a ravine, thanks to all kinds of bridges. It is hard to imagine some cities without their bridges—Cairo, London, Moscow, New York, Sydney, and many others. Yes, bridges have an ancient past.
Bridges of the Past
Over 2,500 years ago, Queen Nitocris of Babylon built a bridge across the Euphrates River. Why? The Greek historian Herodotus answers: “[Babylon] was divided by the river into two distinct portions. Under the former kings, if a man wanted to pass from one of these divisions to the other, he had to cross in a boat; which must, it seems to me, have been very troublesome.” Using timber, baked brick, and stone blocks for building materials and iron and lead for mortar, Nitocris erected a bridge over one of the most famous rivers of ancient times.
Sometimes bridges have affected the course of history. When King Darius the Great of Persia set out on his military campaign against the Scythians, he wanted to take the fastest overland route possible from Asia to Europe. That meant leading his army of 600,000 men across the Bosporus Strait. It was dangerous to cross the strait by boat because of heavy fog and treacherous currents, so Darius lashed boats together until he had made a floating pontoon bridge 3,000 feet [900 m] long. Today, you don’t have to go to as much trouble as Darius did to cross the strait. You can make the trip in less than two minutes by automobile if you use the Bosporus bridges at Istanbul, Turkey.
If you are a Bible student, you may be able to think of an occasion when the lack of a bridge affected the course of history. Recall what happened when King Nebuchadnezzar of Babylon laid siege to the island city of Tyre. For 13 years he tried to conquer the city, but he was unable to do so, partly because there was no bridge between the island and the mainland. (Ezekiel 29:17-20) The island city was not to be conquered for another three hundred years, when Alexander the Great built a causeway from the mainland to the island.
By the first century, ‘all roads led to Rome,’ but the Romans needed bridges as well as roads to bind the empire together. Using stones weighing as much as eight tons each, Roman engineers built arched bridges that were so skillfully designed that some of them are still standing after more than two thousand years. Their aqueducts and viaducts were also bridges.
In the Middle Ages, bridges sometimes served as fortresses. In 944 C.E., the Saxons built a timber bridge across the Thames River in London to ward off an attack by the Danes. Almost three hundred years later, this timber bridge was replaced by Old London Bridge, celebrated in history and in rhyme.
By the time Queen Elizabeth I took the throne of England, Old London Bridge was no longer just a stone fortress. Buildings had been erected right on the bridge. There were shops on the main floor. And the upper floors? They served as living quarters for wealthy merchants and even for members of the royal court. London Bridge had become a center of London’s social life. The rents that were collected for the shops and residences helped pay for the maintenance of the bridge, and, yes, London Bridge was a toll bridge!
While Europeans were busy building bridges out of timber and stone, the Incas of South America were making them out of rope. One famous example is the bridge of San Luis Rey, which spanned the Apurímac River in Peru. The Incas took the fibers of a plant and twisted them together to make cables as thick as a man’s body. They laid the cables across stone pillars and then stretched them across the river. After securing the cables at both ends, they suspended a platform of wooden planks to make a roadway. Maintenance crews renewed the cables every two years. This bridge was so well constructed and maintained that it lasted for five hundred years!
Bridges and Our Changing Needs
Bridges must be able to resist earthquakes, strong winds, and temperature changes. As we have seen, until quite recently engineers used timber, brick, or stone in bridge construction. When the automobile came into use at the end of the 19th century, existing bridges needed to be improved and enlarged to accommodate heavier traffic.
The invention of the steam locomotive also gave impetus to bridge building and design. The most convenient rail routes often stretched across a wide channel or a deep chasm. Could a bridge be built to span the gap and support the weight as more and more freight cars were added? Cast iron bridges filled the need for a time. One of the most famous bridges of the early 19th century is the suspension bridge over the Menai Strait in North Wales, designed by the Scottish engineer Thomas Telford and completed in 1826. It spans 579 feet [176 m] and is still in use! But cast iron tended to be brittle, and bridge failures were common. Finally, in the late 1800’s, steel began to be manufactured. This material had properties that were suitable for use in building longer, safer bridges.
Types of Bridge Structures
There are seven main bridge designs. (See the box above.) Here, we will briefly discuss two of them.
Cantilever bridges have two massive towers, on opposite sides of the river. Beams are anchored to each tower, much as a diving board is fastened to the edge of a swimming pool. To complete the bridge, the beams are then joined in the middle by a rigid span.
Where there is a raging river or where the riverbed is extremely soft, cantilever construction is often preferred because it does not require that piers be sunk in the middle of the riverbed. Because of their rigidity, cantilever bridges are ideal for carrying such heavy traffic as railroad trains.
Perhaps you have seen an acrobat in a circus walk across a tightrope. Did you realize that he is actually walking across a bridge—a suspension bridge? Some suspension bridges in use today are not much more complicated than a tightrope. They may consist of a cable anchored at both ends with a basket hung on it. The passenger sits in the basket and propels himself at a slight downward angle until he reaches the other side. People around the world use simple rope bridges all the time.
Of course, you would hardly think of driving an automobile across a bridge made of rope. After iron-link chains and steel-wire cables were invented, it became possible to build suspension bridges that could support heavy loads. Modern suspension bridges may have a main span that extends 4,000 feet [1,200 m] or more. A suspension bridge usually consists of two piers made of steel, each supporting a tower. Steel cables, made up of thousands of wires, are secured to the towers and to the roadway below. The cables are the main supports of the weight of the traffic and the roadway. When properly constructed, a suspension bridge is one of the safest bridges in the world.
In the past, you may have taken bridges for granted. However, the next time you walk across a familiar bridge, ask yourself: ‘What do I know about this bridge? When was it built?’ Look at it closely. Is it a cantilever, suspension, or other type of bridge? Why was this particular design chosen?
Then, as you cross, look down and ask yourself, ‘How would I manage without it?’
[Box/Picture on page 12]
BRIDGE DESIGNS
1. GIRDER BRIDGES are often used on highways. The girders rest on piers or abutments. These bridges can span up to 1,000 feet [300 meters].
2. TRUSS BRIDGES are supported by trusses shaped like triangles. These bridges, often used for railways, are built to span canyons, rivers, and other obstacles.
3. In ARCH BRIDGES each span forms an arch. This is one of the oldest types of bridge. The Romans built this kind of arch in their aqueducts and viaducts and used a keystone to lock the arch. Many are still standing today.
4. CABLE-STAYED BRIDGES resemble suspension bridges except that the cables are connected directly to the towers.
5. MOVABLE BRIDGES can be raised or swung around to allow ships to go through. London’s Tower Bridge is a good example.
6. CANTILEVER BRIDGES are explained in the main text.
7. SUSPENSION BRIDGES are explained in the main text.—World Book Encyclopedia, 1994.
[Chart on page 13]
SOME FAMOUS BRIDGES
SUSPENSION
Storebaelt Denmark 5,320 ft [1,624 m]
Brooklyn U.S.A. 1,595 ft [486 m]
Golden Gate U.S.A. 4,200 ft [1,280 m]
Jiangyin Yangtze China 4,544 ft [1,385 m]
CANTILEVER
Forth (two spans) Scotland 1,710 ft [521 m] each
Quebec Canada 1,800 ft [549 m]
Mississippi River U.S.A. 1,575 ft [480 m]
STEEL ARCH
Sydney Harbour Australia 1,650 ft [500 m]
Birchenough Zimbabwe 1,080 ft [329 m]
CABLE-STAYED
Pont de Normandie France 2,808 ft [856 m]
Skarnsundet Norway 1,739 ft [530 m]
[Picture on page 10]
Modern girder bridge above ancient arch bridge in Almería, Spain
[Picture on page 13]
Brooklyn Bridge, New York, U.S.A. (suspension)
[Picture on page 13]
Tower Bridge, London, England (movable)
[Picture on page 13]
Sydney Harbour Bridge, Australia (arch)
[Picture on page 13]
Seto Ohashi, Japan (cable-stayed)