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A Species Of Bridge Builders

We've tamed steel, stone, lumber and even living vegetation, all in an effort to reach the places, people and things we desire. We are a species of bridge builders. Since time out of mind, humans have engineered structures to surmount obstacles. Although the concept itself is as simple as felling a tree across a creek, bridge design and construction entails serious ingenuity. Artists, architects and engineers pour vast resources into bridge construction and, in doing so, reshape the very environment in which we live.

As a result, we inhabit a planet of bridges, some as ancient as Greece's 3,000-year-old Arkadiko bridge or as unchanged as India's 500-year-old Meghalaya living bridges, which are coaxed into existence from growing tree roots. Countless others have fallen into the ravines and rivers they span, as humans continue to tackle ever more ambitious bridges and construction

A London bridge has existed since the Roman occupation nearly 2000 years ago. The first bridge across the Thames in the London area, probably a military pontoon bridge, was built of wood by the Romans around AD 50. Around AD 55, a piled bridge was constructed, and the Romans built a small trading settlement next to itâ-¬"the town of Londinium. The settlement and the bridge were destroyed in a revolt led by Queen Boudicca in 60 AD.

The bridge fell into disrepair after the Romans left. As Londinium was also abandoned, there was little need for a bridge at this point, and in the Saxon period the river was a boundary between the hostile kingdoms of Mercia and Wessex. With the impact of the Viking invasions, the reconquest of the Roman city by the kings of Wessex and its reoccupation by Alfred the Great, the political conditions arose for a Saxon bridge to be built. However there is no archaeological evidence for a bridge before Aethelred's reign and his attempts to stem the Sweinian invasions of the 990s. A much later skaldic tradition states that the bridge was pulled down by the Norwegian prince Olaf in 1014, to assist the Anglo-Saxon King Aethelred to divide the forces of the Danes who held the walled City of London and Southwark, on either side of the river; thus regaining London. This episode has been thought to have inspired the well-known nursery rhyme "London Bridge Is Falling Down".

In 1173, Peter de Colechurch, "Warden of the Brethren of the Bridge," soon proposed to replace the timber bridge with a stone one, almost certainly required by the popularity of the Thomas Becket cult and the associated pilgrimage from the bridge to Canterbury. Construction began under de Colechurch's direction in 1176. A chapel was built near the centre of the bridge (dedicated to the recently martyred and canonised Becket who, appropriately, had been born in the parish of St Mary Colechurch). St. Thomas Chapel was grander than hi-town parish churches; it even had a river-level entrance for fishermen and those who taxied passengers across the river. The new bridge took 33 years to complete and was finished in 1209, during the reign of King John. John licensed the building of houses on the bridge, as a direct means of deriving revenue for its maintenance, and it was soon colonised by shops.

By the end of the 18th century, it was apparent that the old London Bridge â-¬" by then over 600 years old â-¬" needed to be replaced. It was narrow and decrepit, and blocked river traffic. The bridge was eventually replaced by a structure of five stone arches, designed by engineer John Rennie. The version of London Bridge that was rebuilt at Lake Havasu consists of a concrete frame with stones from Rennie's

There are more than half a million bridges in the United States, and you rely on bridges every day to cross obstacles like streams, valleys, and railroad tracks. Since ancient times, engineers have designed bridges to withstand all forces of nature. Bridges are too often an unadmired aspect of the architectural world.

In eighteen hundred, the northeastern United States was a country in need of bridges. It is a fairly narrow coastal plain cut by many short rivers and creeks. In the "tidewater" region, these little streams and the great estuaries such as the Chesapeake and Delaware Bays had been highways and lifelines. But now the population was surging beyond the tidewater region, drawn both by the growth of agriculture and the call of water-powered industrialization. Inland farmers needed overland transport, and that meant fords or bridges. But the water-powered mills sought out the very places where the streams could not be forded -- the falls and rapids -- and they too needed transportation.

So bridges were needed. The American northeast was a forest country: wood was a plentiful building material, especially in the remote areas where the smaller bridges were needed. And the climate favored wooden construction. The climate of the region is harsh, by European standards -- hot in the summer and icey in the winter, with a freeze-thaw cycle that would overturn stone pavings. But this sort of climate is less destructive of wood than the mild, moist climate of Britain. So wooden bridges there would be.

A bridge is designed for trains, pedestrian or road traffic, a pipeline or waterway for water transport or barge traffic. An aqueduct is a bridge that carries water, resembling a viaduct, which is a bridge that connects points of equal height. A road-rail bridge carries both road and rail traffic. Bridges are subject to unplanned uses as well. The areas underneath some bridges have become makeshift shelters and homes to homeless people, and the undersides of bridges all around the world are spots of prevalent graffiti. Some bridges attract people attempting suicide, and become known as suicide bridges.

Rope bridges, a simple type of suspension bridge, were used by the Inca civilization in the Andes mountains of South America, just prior to European colonization in the 16th century. With the Industrial Revolution in the 19th century, truss systems of wrought iron were developed for larger bridges, but iron did not have the tensile strength to support large loads. With the advent of steel, which has a high tensile strength, much larger bridges were built, many using the ideas of Gustave Eiffel.

The beam bridge consists of a horizontal beam supported at each end by piers. The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 feet.

The continuous span gives beam bridges the ability to span great distances. A single beam bridge rarely spans more than 250 feet. But, as in the Chesapeake Bay Bridge-Tunnel, several beam bridges can be linked together, creating what is called a continuous span.

The movable bridge like the George P. Coleman Bridge, has a deck that moves. A swing bridge has a deck that rotates around a center point; a drawbridge has a deck that can be raised and lowered; a bascule bridge deck is raised with counterweights like a drawbridge; and the deck of a lift bridge is raised vertically like a massive elevator.

The truss bridge consists of an assembly of triangles. Truss bridges are commonly made from a series of straight, steel bars. The Firth of Forth Bridge in Scotland is a cantilever bridge, a complex version of the truss bridge. Rigid arms extend from both sides of two piers. Diagonal steel tubes, projecting from the top and bottom of each pier, hold the arms in place. The arms that project toward the middle are only supported on one side, like really strong diving boards. These "diving boards," called cantilever arms, support a third, central span.

In the late 1800s, a railway bridge across Scotland's Firth of Tay swayed and collapsed in the wind. Seventy-five passengers and crew on a passing night train died in the crash. It was the worst bridge disaster in history. So when engineers proposed bridging the even wider Firth of Forth, the Scottish public demanded a structure that looked like it could never fall down. They got it.

Chief engineers Sir John Fowler and Benjamin Baker came up with the perfect structural solution: a cantilever bridge. The Firth of Forth Bridge is made of a pair of cantilever arms, or beams "sticking out" from two main towers. The beams are supported by diagonal steel tubes projecting from the top and bottom of the towers. These well-secured spans actually support the central span. This design makes the Firth of Forth Bridge one of the strongest -- and most expensive -- ever built.

But not everyone liked the design. The poet and artist William Morris declared it "the supremest specimen of all ugliness." Ugly or not, the Firth of Forth is a safe bridge. Even today, the highest winds barely shake this enormous structure. This is exactly what the people of Scotland needed after the Tay Bridge disaster. Unfortunately, a cantilever of this size comes with a hefty price tag. This is why very few like it have ever been built again.

The arch bridge has great natural strength. Thousands of years ago, Romans built arches out of stone. Today, most arch bridges are made of steel or concrete, and they can span up to 800 feet.

Building an arch bridge isn't easy, since the structure is completely unstable until the two spans meet in the middle. For years, engineers used a technique called centering, in which a wooden form supported both spans until they locked together at the top. A newer method supports the spans using cables anchored to the ground on either side of the bridge. This is how the New River Gorge Bridge in West Virginia was built.

The New River carves a deep gorge through southern West Virginia. For years, in order to cross the New River Gorge, drivers were forced to take a 40-mile detour or carefully wind down narrow mountain roads. Despite its scenic beauty, the New River Gorge was a major obstacle. It wasn't until the completion of the New River Gorge Bridge in 1977 that this problem was solved.

The New River Gorge was an ideal location for a steel arch bridge. The solid rock on both sides of the gorge would resist the outward thrust of the arch, making tall towers and deep piers unnecessary. In June 1974, cables were strung between temporary towers located on each side of the gorge. The steel sections of the arch bridge were pieced together over the gorge by trolleys running on these cables. After three years of construction and $37 million, the new bridge reduced a 40-minute drive around one of America's oldest rivers to less than one minute.

Today, the New River Gorge Bridge is the world's longest spanning, steel single-arch bridge. Soaring 876 feet above the rugged whitewaters of West Virginia's New River, it is also the second tallest bridge in the United States.

The suspension bridge can span 2,000 to 7,000 feet -- way farther than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.

The cable-stayed bridge, like the suspension bridge, supports the roadway with massive steel cables, but in a different way. The cables run directly from the roadway up to a tower, forming a unique "A" shape.

Cable-stayed bridges, like the Sunshine Skyway in Florida, require less cable and can be built much faster than suspension bridges. Cable-stayed bridges are becoming the most popular bridges for medium-length spans (between 500 and 3,000 feet).

Completed in 1987, the Sunshine Skyway is the world's longest cable-stayed concrete bridge. It is probably the best known of the several dozen cable-stayed bridges that have been built in the United States since the late 1970s. Its popularity may be due to its unique color -- its cables are painted a bright taxicab yellow -- but the bridge also boasts an interesting history.

The Sunshine Skyway isn't the first bridge to span the broad mouth of the Tampa Bay. In fact, a four-mile steel cantilever bridge used to live where the new Sunshine Skyway now stands. But during a violent thunderstorm on the morning of May 9, 1980, the freighter Summit Venture plowed into the cantilever bridge. More than 1,000 feet of the bridge fell into the bay, killing 35 motorists and bus passengers instantly.

The Florida Department of Transportation began construction on a safer Sunshine Skyway Bridge only days later. more than 300 precast concrete segments were linked together with high-strength steel cables to form the roadway. Protecting the new bridge from ships was a big priority, so they installed large concrete islands, called dolphins, around each of the bridge's six piers to absorb unwanted impact. Since it opened to traffic in 1987, the sleek, new Sunshine Skyway has won dozens of engineering and design awards.

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