Suspension bridge
A suspension bridge is a type of bridge that is supported by cables, typically designed to span long distances, making them particularly useful for crossing waterways. These bridges consist of two main towers connected by horizontal cables, known as suspension cables, which are secured to large concrete anchorages at each end. Vertical cables, called suspender cables, extend down from the suspension cables to support the bridge deck, which is the roadway for vehicles and pedestrians.
Historically, suspension bridges are among the oldest bridge designs, initially constructed using natural materials like vines and later evolved to use iron chains and, eventually, steel cables. Notable examples include the Akashi Kaikyo Bridge in Japan, recognized as the longest suspension bridge in the world, and the iconic Golden Gate Bridge in San Francisco.
Engineers design these structures with careful consideration of the forces at play, primarily compression and tension, to ensure stability and safety. Modern advancements have addressed earlier design flaws, enhancing their resilience against high winds and potential cable failures. Overall, suspension bridges exemplify significant engineering achievements, combining functionality with aesthetic appeal while serving as vital transportation links in many regions.
Suspension bridge
As its name suggests, a suspension bridge is suspended in the air by cables. One of the earliest bridges, a suspension bridge has two towers connected by horizontal cables called suspension cables, which are attached to anchorages, or large concrete blocks at each end of the bridge. Other cables hang vertically from the suspension cables. These vertical cables, called suspender cables, hold the deck, or roadway, of the bridge.
Suspension bridges are ideal for spanning long distances, especially waterways. The Akashi Kaikyo Bridge in Japan is the world's longest suspension bridge. It spans 1,991 miles (1,917 kilometers) across the Akashi Strait to connect Awaji Island to the city of Kobe on the Japanese mainland. Two million workers spent ten years building the Akashi Kaikyo Bridge, which may be Japan's greatest engineering feat. Other famous suspension bridges include the Golden Gate Bridge in San Francisco, California; the Brooklyn Bridge in New York City; and the George Washington Bridge in Fort Lee, New Jersey.
Background
Suspension bridges are among the earliest types of bridges. Primitive peoples used vines to create suspension bridges so they could cross gorges and canyons. To cross these early suspension bridges, they hung on the vines and moved hand over hand.
Other early suspension bridges used iron chains as cables. Chinese bridge builders began constructing this type of bridge in the eighth century; they placed wooden planks between sets of chains. A similar chain suspension bridge was constructed in the United States in 1801 by James Finley to enable travelers to cross a creek in Westmoreland County, Pennsylvania. Finley patented the design of the bridge in 1808 and built more than a dozen small chain suspension bridges.
During the mid-1800s, John Roebling, an American, improved on the original design of the chain suspension bridge by making the deck more rigid to keep it from vibrating. Roebling also came up with a method to create cables in the place where they would be installed by spinning many strands of wire. His method proved much more effective than transporting preconstructed cables to the building site and then struggling to make them fit. Roebling oversaw the construction of a railway bridge that spanned the Niagara River gorge. Construction of this bridge was completed in 1855.
Roebling also designed New York's Brooklyn Bridge but died in 1869 while surveying the site. His thirty-two-year-old son, Washington Roebling, took his place and oversaw the construction of the enormous bridge that spans the East River to connect the boroughs of Brooklyn and Manhattan. More than six hundred workers constructed the bridge over fourteen years. At least twenty-four workers died from falls and other disasters.
Perhaps the most photographed bridge in the world, the Golden Gate Bridge in San Francisco, was built by Joseph Strauss between 1933 and 1937, despite concerns about the area's powerful winds and blinding fog. The orange bridge's twin towers stretch 746 feet (227 meters) into the sky. Eleven workers perished during the four-year construction of the bridge, but nineteen others were saved by a safety net that had been installed under the bridge. (Such a net became mandatory during bridge construction.)
Construction of the exceptionally sleek Tacoma Narrows Bridge in Washington State was completed in 1940. At the time, it was the third-longest suspension bridge in the world. The bridge spanned 853 miles (1,373 kilometers) across the Tacoma Narrows, linking Tacoma to the Gig Harbor Peninsula. Workers became concerned about the safety of the bridge when it swayed from side to side in heavy wind and the deck seemed to ripple. During a storm four months after its construction, a 600-foot (183-meter) section of the bridge broke free, causing the main span to collapse. Fortunately, a toll operator had closed the bridge because of the storm, and no one was injured.
Overview
The deck of a suspension bridge is hung on cables. Designed to span long distances, this type of bridge most often has two towers, which are usually made of strong steel and extend into the hard rock below the bridge. Suspension cables are draped horizontally from tower to tower. Vertical cables called suspenders connect to the suspension cables and hold the deck. Two anchorages—enormous concrete blocks—are at each end of the bridge and are attached to rock formations.
Engineers carefully design a suspension bridge so that the forces pushing and pulling on the bridge work together and achieve balance. Compression and tension are two of these forces. Compression is a pressing force on the bridge, and tension is a pulling force. The cars, buses, and people traveling on the bridge are called the load.
When a load is on the deck, compression forces push downward. The suspenders transfer the force to the suspension cables, which transfer the force to the towers. The towers dissipate this force into the ground underneath them. The weight on the deck also causes tension. This tension stretches the suspension cables, is transferred to the anchorages, and is then dissipated into the rock or ground. A suspension bridge achieves a balance between compression and tension.
Most suspension bridges are similarly designed, and their components are made of steel. For example, a bridge's towers and cables are constructed from steel. The girders, or beams, used to make the deck rigid are also made of steel. However, engineers may alter the design of a bridge and select different materials to better suit the area in which a bridge is being built. They will consider such factors as whether an area is prone to earthquakes, whether the water underneath the bridge is freshwater or saltwater, and whether the towers need to be protected from collisions with ships.
Engineers have remedied some design flaws in suspension bridges over the years. For example, the decks on some early suspension bridges were thin, which made the bridges unstable in high winds. Modern suspension bridges have a thick bridge deck with steel girders underneath to make it rigid so it does not sway.
Early suspension bridges that used chains instead of cables were also problematic—they would collapse if a chain broke. Using steel cables solved this problem. A suspension bridge with a broken cable is still safe and functional.
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