Bridge
For ages man has built bridges to allow transport over streams, rivers, gorges, and other transportation routes. The primitive clapper bridge--a stone slab-and-pier construction--and the basic beam bridge in which a stone or tree breached a gap, are among the earliest bridges.
The Romans used the semi-circular masonry arch in their bridge-building and also used timber, while in China segmental arch bridges were constructed to span greater distances. As commerce escalated, the load requirements of roads and bridges became greater, and complex bridge engineering was crucial to ensure against collapses. The late eighteenth century saw the introduction of iron and steel into bridge-building, with the stronger cast-iron eventually replacing wrought iron. The bridge-builders of Great Britain were particularly innovative and showed a keenness for competition. Such names as Thomas Telford and Isambard K. Brunel are associated with magnificent early suspension bridges.
Bridges are of three basic types that encompass a range of variations: the beam, the arch, and the suspension. Beam or girder bridges press downward on supports, while arch bridges are compressed and push outward against the supports, placing the burden on the endpoints or abutments. The suspension bridge relies on tension between cables and exerts a pull on its end anchorages; its earliest form was constructed of vines, bamboo, or cane tied to tree trunks. The need to span greater distances and deeper channels has led to the reliance on suspension bridges rather than on piers and arches.
Truss bridges—bridges with rigid frameworks for support—were simple to build and were typically American. Cantilever bridges, a variation on the beam or girder, were also developed in the later part of the 1800s and could extend three to four times farther than the diagonal truss. The first cantilever bridge was built across the Main River in Germany in 1867. The renowned Firth of Forth Bridge was built in Scotland in 1889 and was the first long-span railway bridge made of steel.
In 1874, American James Eads (1820-1887) designed a magnificent steel arch bridge with two decks across the Mississippi River at St. Louis, marking the beginning of the replacement of iron bridges with steel. Iron bridges tended to collapse after ten years or so of continued stress, and steel was far more durable. John Roebling pioneered the use of steel cable in the construction of the Brooklyn Bridge. He died in 1869, but his son, Washington Augustus Roebling (1837-1926), engineered the project and saw its completion in 1883.
Variations on these types of bridges include the covered bridge, first built by Timothy Palmer 1806 across Philadelphia's Schuylkill River. The covering over the framework protected the beams from rot and the deck from snow and ice. Palmer's innovation was popular, and covered bridges were common sights in the southern and midwestern United States, and especially in New England.
Pontoon bridges are generally used during wartime, replacing destroyed or blocked bridges or facilitating crossings where no bridges exist. The earliest pontoon bridges were used by Persian armies in the fifth century B.C. Primitive pontoons consisted of boats placed side by side and overlaid with planks. Modern pontoon bridges consist of a deck laid across airtight floats tethered or anchored to the bottom and were used extensively during World War II.
Another type of bridge used in military campaigns is the Bailey bridge. Invented in 1941 by England's Sir Donald Bailey (1901-1985), it consists of interchangeable steel panels held together with steel pins that can be quickly assembled on shore with hand tools, then unrolled across a river on temporary piers. An entire class of bridge is the movable deck bridge, used most commonly to span narrow shipping channels where the height of passing boats is greater than that of the deck.
The most common type of movable bridge is the drawbridge, or bascule bridge, which is either hinged at one end, permitting the entire deck to swing upward like a gate, or split at the middle, allowing both halves to swing upward. The vertical lift bridge uses weights to raise the entire center deck and its truss above the shipping channel. One of the most well known of these is the Aerial Bridge at Duluth harbor. Other verticals are used in the St. Lawrence Seaway and in many harbors of the world. Swinging bridges pivot the bridge structure on its pier and are useful in lock and dam situations. Only the bridge span over the lock is moved to allow towboats to pass.
Early suspension bridges used iron rods or chains. In the later nineteenth century, steel cables came into use. Some bridge decks have been suspended from arches, like the Sydney Harbour Bridge (1932) in Australia. Several classic bridges, like the George Washington (1931) in New York and the Golden Gate (1937) in San Francisco, were built during the early half of the twentieth century. Suspension bridge construction reached its peak between the late 1950s and early 1970s.
Modern construction materials are making it possible to build bridges of simpler design and across greater expanses. Reinforced and prestressed concrete and high-strength steel are allowing for the return to simple self-supporting bridge decks reminiscent of the early beam bridge. The Interstate Highway system of the United States has made extensive use of the continuous box-and-plate girder bridge as a result. Suspension bridges are now being built at record lengths, crossing channels previously considered unbridgeable. The longest spanning suspension bride in the United States (and sixth in the world) is the Verrazano-Narrows bridge in New York City, completed in 1964 and spanning 4,259 ft (1,298 m). The longest suspension bridge in the world is the Akashi Kaikyo Bridge, the final link in a chain of spans that connects Japan's four main islands. Completed in 1998 after ten years of work, it measures 12,828 ft (3,910 m), or 2.43 mi (3.91 km). Its center span alone measures 6,529 ft (1,990 m). With the nearest seismic fault 90 miles (145 km) away, the bridge was designed to withstand an earthquake that measures 8.5 on the Richter scale.
The longest bridge in the world is the over-water highway bridge over the Lake Pontchartrain Causeway in Louisiana, spanning 34.3 mi (55.2 km). The twin spans of the Causeway are made of pre-stressed panels supported by over 9,000 concrete pilings. The first span opened to the public in 1956, and the second in 1969. The result was one-way driving safety on straight, two-lane railed surfaces 80-feet apart connected by seven crossovers that function as pull-over areas for auto emergencies.
The future for bridge design includes "smart bridges," which have fiber optic sensors embedded into their structure to monitor their health. The sensors are linked to a computer that monitors the bridge for cracks, strains, and road-salt corrosion, minimizing the need for on-site maintenance. (An estimated 230,000 bridges in the United States alone are in need of serious repair.) Efforts are also underway to construct bridges of composite materials that require virtually no maintenance, such as a 36-ton bridge in Butler County, Ohio. It is made of lightweight, corrosive-resistant polymers, and is expected to last 150 years, about three times longer than conventional bridges. Such bridges cost about twice that of a normal bridge, which has hindered their widespread use. In other prototypes, high-performance concrete (HPC) allows longer beams to be used, and is more resistive to the salt erosion which corrodes the reinforcing steel bars inside the concrete beam.
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