BRIDGES

There are more than half a million bridges in the United States, and you rely on them every day to cross obstacles like streams, valleys, and railroad tracks. But do you know how they work? Or why some bridges are curved while others are straight? Engineers must consider many things -- like the distance to be spanned and the types of materials available -- before determining the size, shape, and overall look of a bridge. Since ancient times, engineers have designed three major types of bridges to withstand all forces of nature.

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 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.

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.

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.

Beam Bridge: ForcesWhen something pushes down on the beam, the beam bends. Its top edge is pushed together, and its bottom edge is pulled apart.

Truss Bridge: ForcesEvery bar in this cantilever bridge experiences either a pushing or pulling force. The bars rarely bend. This is why cantilever bridges can span farther than beam bridges.

Arch Bridge: ForcesThe arch is squeezed together, and this squeezing force is carried outward along the curve to the supports at each end. The supports, called abutments, push back on the arch and prevent the ends of the arch from spreading apart.How are arch bridges built?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

Cantilever bridge: Firth of Forth

Ancient Roman aqueduct

Suspension bridge: Golden Gate Bridge

Arch bridge being constructed with cables1

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.

Suspension Bridge: ForcesIn all suspension bridges, the roadway hangs from massive steel cables, which are draped over two towers and secured into solid concrete blocks, called anchorages, on both ends of the bridge. The cars push down on the roadway, but because the roadway is suspended, the cables transfer the load into compression in the two towers. The two towers support most of the bridge's weight.

Cable-Stayed BridgeThe 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). 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.

Firth of Forth BridgeVital Statistics:Location: South Queensferry and North Queensferry, ScotlandCompletion Date: 1890Cost: $15 millionLength: 8,276 feetType: CantileverPurpose: RailwayMaterials: SteelLongest Single Span: 350 feet (center span)Engineer(s): Benjamin Baker, John Fowler

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.

Fast Facts: The Firth of Forth Bridge may not have been a bridge at all -- engineers considered building a tunnel, but abandoned

the idea because it seemed too risky. The Firth of Forth was the first bridge built primarily of steel.

It took 54,000 tons of steel; 194,000 cubic yards of granite, stone, and concrete; 21,000 tons of cement; and almost seven million rivets to build the Firth of Forth Bridge.

Vital Statistics:Location: London, EnglandCompletion Date: 1894

Cable-stayed bridge: Sunshine Skyway Bridge

Continuous span beam bridge

Swing bridge: George P. Coleman Bridge

Tower Bridge

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Length: 880 feetType: Movable (bascule), suspensionPurpose: Roadway, pedestrianMaterials: Steel, masonryLongest Single Span: 200 feetEngineer(s): Sir Horace Jones, Sir John Wolfe-Barry

By the end of the 19th century, the city of London had outgrown itself. Thousands of cars and pedestrians relied on a single bridge -- the London Bridge -- to travel in and out of the capital city each day. The traffic jams were unbearable. So it was with great anticipation that Londoners awaited the completion of a new bridge across the Thames, the Tower Bridge, designed by Sir Horace Jones and engineered by Sir John Wolfe-Barry.

The new bridge would have two towers that would rise 200 feet above the Thames. A pair of glass-covered walkways would stretch between the two towers for pedestrians. Steam engines would raise and lower the bascules, or movable roadways, in less than two minutes to allow boats to pass. Londoners were thrilled. Once the bridge was completed in 1894, however, the public was appalled with the results. Jones' original design was simple and had a medieval style. But Jones died in 1887, and Barry added his own artistic touch. When the Tower Bridge opened to traffic in 1894, the journal The Builder cursed the bridge, calling it "the most monstrous and preposterous architectural sham that we have ever known." But public opinion mellowed over time, and today, the Tower Bridge is one of London's best loved landmarks.

Fast Facts: The Tower Bridge is the only movable bridge of the 29 bridges on the Thames River. When the bridge opened to river traffic in 1894, it was raised and lowered 1,000 times per year. Today, few ships

actually travel on the Thames, so the bridge opens less than 100 times per year. In 1952, a London bus had to leap from one bascule to the other when the bridge began to rise with the bus still on it.

Luckily, no one was injured.

Akashi Kaikyo Bridge

Vital Statistics:Location: Kobe and Awaji-shima, JapanCompletion Date: 1998Type: Suspension

Length: 12,828 feetPurpose: Roadway Cost: $4.3 billionMaterials: SteelLongest Single Span: 6,527 feetEngineer(s): Honshu-Shikoku Bridge Authority

In 1998, Japanese engineers stretched the limits of bridge engineering with the completion of the Akashi Kaikyo Bridge. Currently the longest spanning suspension bridge in the world, the Akashi Kaiko Bridge stretches 12,828 feet across the Akashi Strait to link the city of Kobe with Awaji-shima Island. It would take four Brooklyn Bridges to span the same distance! The Akashi Kaikyo Bridge isn't just long -- it's also extremely tall. Its two towers, at 928 feet, soar higher than any other bridge towers in the world. The Akashi Strait is a busy shipping port, so engineers had to design a bridge that would not block shipping traffic. They also had to consider the weather. Japan experiences some of the worst weather on the planet. Gale winds whip through the Strait. Rain pours down at a rate of 57 inches per year. Hurricanes, tsunamis, and earthquakes rattle and thrash the island almost annually. How did the Japanese engineers get around these problems? They supported their bridge with a truss, or complex network of triangular braces, beneath the roadway. The open network of triangles makes the bridge very rigid, but it also allows the wind to blow right through the structure. In addition, engineers placed 20 tuned mass dampers (TMDs) in each tower. The TMDs swing in the opposite direction of the wind sway. So when the wind blows the bridge in one direction, the TMDs sway in the opposite direction, effectively "balancing" the bridge and canceling out the sway. With this design, the Akashi Kaikyo can handle 180-mile-per-hour winds, and it can withstand an earthquake with a magnitude of up to 8.5 on the Richter scale!

Fast Facts: The bridge is so long, it would take eight Sears Towers laid end to end to span the same distance. The length of the cables used in the bridge totals 300,000 kilometers. That's enough to circle the earth 7.5 times! The bridge was originally designed to be 12,825 feet. But on January 17, 1995, the Great Hanshin Earthquake

stretched the bridge an additional three feet. The bridge holds three records: it is the longest, tallest, and most expensive suspension bridge ever built.

Garabit Viaduct

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Vital Statistics:Location: Massif Central, FranceCompletion Date: 1884Length: 1,853 feetType: ArchPurpose: RailwayMaterials: Wrought ironLongest Single Span: 541 feetEngineer(s): Gustave Eiffel

In the late 1800s, a mountainous barrier blocked the railways from reaching Southern France. For years, engineers tried to figure out a way to bridge the windy Garabit Valley in France's Massif Central. Finally, one of the era's best engineers, Gustave Eiffel, came up with a brilliant solution. He built a huge wrought-iron arch in record time with just a minimal amount of material. How did he do it? Rather than building his bridge with thick, solid beams, Eiffel used beams with lots of holes -- holes in the shapes of triangles. Eiffel knew that if his bridge was made of thick, solid beams, it would be very heavy and the beams would rattle in the wind. But if he used a series of open triangles, called a truss, the gusty wind in the valley would blow right through them. Not only is the truss pattern lightweight; it’s very stable as well. Depending upon the position of a train on the bridge, the connecting vertical and diagonal segments are pulled into tension and pushed into compression -- forces that resist one another. A push on one segment is resisted by an opposite pull from another, all along its length. So the bridge remains strong and rigid, despite its lightness.

Fast Facts: It took 38 tons of red paint to coat the entire bridge. For many years, the Garabit Viaduct remained the tallest bridge in the world. The single railroad track crosses the

Garabit Valley, 400 feet above the Truyere River. That’s half as high as the Eiffel Tower! Gustave Eiffel's tremendous success with the Garabit Viaduct, and later with the framework of the Statue of Liberty

and Paris' Eiffel Tower, earned him the nickname "magician of iron."

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