BRIDGE CONSTRUCTION
BACKGROUND INFORMATION:
Bridges are structures which carry people and vehicles across natural or man-made obstacles. As early roads connected villages and towns, people traveled by foot or with carts and wagons. Although a person carrying a large bundle might be able to cross a stream by swimming or stepping on stones, as horse drawn vehicles with heavy loads needed to cross more dangerous terrain, permanent sturdy bridges became an important part of transportation systems.
Early bridges were made from local materials such as wood, stone and fibers. Today, most bridges have a concrete, steel, or wood framework with an asphalt or concrete roadway. Based on the length of the barrier to be crossed, the amount and type of traffic as well as forces of nature (wind, tide, flood) different materials and shapes of bridges are used.
There are many types of bridges such as arch bridges, girder bridges, truss bridges, cantilever bridges, cable-stayed bridges, suspension bridges and moveable bridges. Many bridges are actually combinations of different types of bridges -- and no two bridges are identical! Most bridges are held up by at least two supports set in the ground called abutments. Some bridges have additional supports along the middle of the bridge called piers. A span is the distance between two supports, either two piers, a pier and an abutment or two abutments. Many short bridges are supported only by the abutments and are called single-span bridges. Longer bridges usually have one or more piers to support them and are known as multi-span bridges.
How Bridges Work
Most machines do some type of work. You know an engine, windmill or other machine is generating a force or work because you can see the motion. Although a bridge does not have spinning moving parts like other machines, it is still doing work.
What is a bridge's work? A bridge needs to be strong enough to support its own weight plus the load of passengers and vehicles travelling on it against the pull of gravity. A bridge works against the pull of gravity. If you hold a facial tissue between your hands and place a heavy book on top of a facial tissue, you know the book will break through the tissue. The tissue is not strong enough to support the weight of the book.
How do bridge's work? Although there are many types of bridges most bridges work by balancing compression and tension. Place a flexible object like an eraser, sponge, or small piece of bread between your thumb and index finger. Press your fingers together. One side of the object will bend inwards and shorten while the other will bend outwards and lengthen. The shorter side has been compressed, while the other side is under tension. Bridge components experience these tension and compression stresses.
The Upper Side Has Been Compressed, The Lower Side Is Under Tension
Bridge materials, like stone, wood, steel and concrete, all have different strengths. For example, steel can be much stronger than wood. Some materials are easier to form a particular shape. Different materials can withstand different amounts of compression and tension. For example, stone can withstand a lot of compression, but under a lot of tension will break. Steel is very flexible and can endure a lot more compression and tension then materials like iron, wood or stone. Engineers will select materials and a bridge design, based on the strength of the material, amount and type of stress a material can withstand and other bridge requirements (length, terrain, etc.).
Different Types Of Bridges
Arch bridges are structures in which each span forms an arch. The arch bridge is one of the oldest types of bridges. Early arch bridges were made from stone. The spans range up to about 1700 ft.
Arch Bridge
Bixby Creek Bridge, Monterey, California
Photo Courtesy G. Donald Bain, The Geo-Images Project at the University of California, Berkeley
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Girder bridges are made of beams called girders. The ends of the beams or girders rest on piers or abutments. The span length of girder bridges ranges up to about 1000 ft.
Girder Overpass Oakland, California
Truss bridges are supported by frameworks called trusses. Trusses are beams arranged to form triangles.
Truss Bridge Worcester, UK
Photo Courtesy D. Locke, Bridge Building
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Cantilever bridges consist of two independent beams, cantilevers, that extend from opposite banks of a waterway. Cantilever bridges have spans as long at 1800 ft.
Cantilever Section of the Bay Bridge, California
Cable-stayed bridges have roadways that hang from cables. The cables are connected directly to towers.
Cable-Stayed Bridge
Sabrina Foot-Bridge Worcester, UK
Photo Courtesy D. Locke, Bridge Building
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Moveable bridges have roadway that is moved to provide enough clearance for boats or large ships to pass. An example of a moveable bridge is a drawbridge that tilts the roadway upward.
Drawbridge and Closeup of Drawbridge Mechanism Larkspur, California
Suspension bridges may be the most impressive type of bridge with their long main span and beauty. These bridges have a roadway that hangs from steel cables supported by two high towers. The difference between suspension bridges and cable-stayed bridges is that suspension bridge cables are not directly connected to the towers. The cables of a suspension bridge are not connected to the bridge - the cables pass through a hole in the top of the towers.
A suspension bridge has at least two main cables. These cables extend from one end of the bridge to the other. Suspender cables hang from these main cables. The other end of the suspender attaches to the roadway.
SUSPENSION BRIDGES
NAME / LOCATION / MAIN SPANLENGTH IN FEET / TOTAL
LENGTH IN FEET
Akashi-Kaikyo / Kobe City - Awajishima Island, Japan / 6,532 / 12,831
Humber / River Humber, England / 4,626 / 7,283
Verranzano-Narrows / Brooklyn-Staten Island, NY / 4,260 / 6,690
Golden Gate / San Francisco, CA / 4,200 / 8,981
Mackinac / Mackinaw City, MI / 3,800 / 8,614
Brooklyn / Brooklyn, NY / 1,595 / 5,989
Suspension bridges have the longest spans in the world and are used to cross great distances. These types of bridges are used to cross deep water channels, cannons or gorges, where construction of supporting piers can be difficult. The towers can be placed far apart eliminating the need for multiple towers and piers. Some suspension bridges have a main span longer then 4000 ft. The longest suspension bridge in the world is in Japan (the Akashi-Kaikyo Bridge) and has a span over a mile long. The largest bridges have cables several feet wide which weigh thousands of pounds per foot. For that reason the cables are spun in place.
Suspension Bridge - Golden Gate Bridge, San Francisco, California
The cables must be secured and anchored past the ends of the bridge. Why? Look at Figures 1a and 1b. In Figure 1a the bridge is not anchored to the land. What happens to the bridge?
Figure 1a ------Figure 1b
To feel the difference between a cable that ends at the tower top and one that goes over it and continues to the ground, grab your head with your right hand and gently pull (Figure 2a). If you try to keep your head straight, your neck will feel compressed, but also pulled to the right.
Figure 2a ------Figure 2b
Now interlace the fingers of your hands, put them over your head and pull with both arms (Figure 2b). Your head and neck will feel compressed, but your neck will not feel a pull to the right or left, since the force of your right arm is balanced by that of your left arm.
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