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Bridges

BRIDGES

BRIDGES. Bridges have been essential to America's growth, and countless types were devised to carry highways, railroads, and even canals. Location, materials, cost, traffic, and the ingenuity and creativity of bridge engineers all have influenced the evolution of American bridge technology.

The Colonies and the Early Republic

Large-span bridge building in North America began with the Charles River Bridge at Cambridge, Massachusetts, in 1662. Its pile and beam construction was not unlike that used for centuries in Europe. The design placed heavy timber beams across piles that were hand driven into the riverbed. Side members were then tied together by cross beams, and wood decking was attached to stringers running parallel to the sides. Spans built this way were limited by the length of available timber and the depth of the water.

A more versatile bridge form, the truss, first came into use in the United States during the late eighteenth century. Trusses composed of a series of triangles were assembled from short lengths of timber that, depending upon their location, resisted the forces of either compression or tension. Because of the way in which it was assembled, the truss bridge could be lengthened to span distances far greater than the simple pile and beam bridge. Structural integrity in these bridges came from a balance of the opposing forces inherent in their construction. Because these spans were not self-supporting during construction,


however, they were built on false work or framing that was later removed. As with all bridges regardless of type, they had to be designed to support their own weight or dead load, as well as the moving weight or live load that passed over them.

During the first two decades of the nineteenth century, Theodore Burr of Connecticut was one of the best-known American bridge builders. His 1817 patent for a combination arch and truss design was widely used in covered bridges. He had erected some forty-five highway spans in New York, New Jersey, and Pennsylvania by the time of his death in 1822. The wooden walls and roofs that were typical of covered bridges were necessary to protect the truss's countless wooden joints from the ravages of the weather.

By far the most lasting bridges built in the eighteenth and early nineteenth centuries were those made of masonry, but masonry construction was expensive and a shortage of qualified masons in the early Republic limited the number that were constructed. Those bridges were most often built in the form of an arch and assembly proceeded on timber falsework, for these bridges were self-supporting only after the last stone was put in place. When America's first railroads began laying out their routes in the 1820s and 1830s, their bridges were apt to be of stone, since once erected such bridges required little attention and were highly durable. As the pace of railroad expansion quickened toward the mid-nineteenth century, however, the need for bridges burgeoned and permanency was abandoned for expediency. The prevailing attitude was that quickly assembled and even temporary timber bridges and trestles could be replaced at some later date with more lasting structures once a rail line was producing revenue. The railroads' need for quickly constructed spans spurred the development of the truss bridge, constructed primarily of wood, throughout the first half of the nineteenth century.

The Expanding Nation

Individual types of truss bridges can be identified by the way their members were assembled. A large number of designs were patented during the nineteenth century. Some trusses were of no practical value, while others were over-engineered or too expensive to build. Those popular during the nineteenth century did not necessarily find similar acceptance in the twentieth century. In time, the broad range of trusses was gradually reduced to a few basic types that proved to be the strongest and most economical to build. By the early twentieth century, the Pratt and Warren were the most commonly used trusses and into the 1920s the truss was the most common bridge type in America.

As the railroads used ever heavier and faster rolling stock, it was necessary to replace wooden bridges with heavier and sturdier construction. Beginning in the 1840s, cast and wrought iron were being substituted for wood members in some bridges and during the 1850s railroads began turning to bridges made entirely of iron. During the 1870s, steel production increased greatly and the price fell to levels that made it reasonable for use in bridges. By 1930, the expansion of American railroads was over and their influence on the structural development of bridges was at an end.

Two significant advances in bridge technology took place in the late 1860s and early 1870s with the construction of the long-span metal arch bridge across the Mississippi River at St. Louis. First, not only was this bridge the first major spanning of North America's largest river, but engineer James B. Eads specified the use of steel in the bridge's arch members. The three tubular arches rested on masonry piers built on wooden caissons sunk in the riverbed. Second, this was the initial use in the United States of the technique in which excavation work inside a caisson took place in an atmosphere of compressed air. Prior to that workers had labored under water or in areas where water was diverted in some way. Air pressure within the caisson equaled the force exerted by the river water outside and the shell did not flood as excavation work inside progressed down toward bedrock. This technology was crucial in the successful execution of all subsequent subaqueous foundation work.

Limitations imposed by location have forced bridge builders to be innovative. It would be impractical, if not impossible, to erect a bridge across wide, deep ravines if the bridge required the support of extensive false work during construction. As a result, a method evolved that avoided the use of staging. In October 1876, engineer Charles Shaler Smith embarked on the construction of the first modern cantilever railroad span to bridge the 1,200-foot-wide and 275-foot-deep valley of the Kentucky River. Smith refined a bridge-building technique little used outside of ancient China. Cantilever construction employed counter balancing forces so that completed segments supported ongoing work as it progressed inward toward the span's midpoint.

Although the suspension bridge was not new in 1842, its future form was forecast when Charles Ellet's Philadelphia wire suspension bridge was opened to highway traffic that year. Suspension bridges in which the roadway or deck was suspended from heavy wrought iron chains had been built for years. For the first time in a major American span, the deck was suspended from relatively lightweight wire cables. Ellet used a European cable-making technique in which cables were composed of a number of small-diameter parallel wires. The shape of each cable was maintained and its interior protected when the bundle's exterior was wrapped with additional wire. The scale of the bridges that followed increased tremendously, but the basic technology for cable making remained the same.

Civil engineer John A. Roebling was the preeminent suspension bridge designer of the nineteenth century. His career began with a suspension canal aqueduct at Pittsburgh in the 1840s, and each of his following projects reflected a growing skill and daring. His combined railroad and highway-carrying suspension bridge across the Niagara River gorge was completed in 1855. In it a suspended double-deck wooden truss carried the two roadbeds. Although other types of bridges would be built to carry highway and urban rail systems, this was the lone example of a suspension bridge constructed to carry both. The overall design and appearance of his Ohio River suspension bridge at Cincinnati in 1867 foretold of his plans for New York City's even larger and monumental Brooklyn Bridge of 1883. The extensive use of steel throughout the bridge, and especially for its cables, was a watershed in bridge technology.

The Early Twentieth Century

A number of large suspension bridges were built during the first half of the twentieth century. They were ideal for spanning the broad waterways that seemed to stand in the way of the growth of modern America. Neither their construction nor their final form posed an impediment to the nation's busy waterways. During the first four decades of the century, New York City was the focus of much of that construction. The city's boroughs were joined by the Williamsburg (1903), Manhattan (1909), Triborough (1936), and Bronx-Whitestone (1939) suspension bridges. However, none compared in size to the magnificent George Washington Bridge, completed in 1931. It was the first bridge linking Manhattan and New Jersey, and represented a remarkable leap forward in scale. Its 3,500-foot-longsuspended span was double the length of the next largest. The bridge's four massive suspension cables passed over towers soaring more than 600 feet high and its roadway was suspended 250 feet above the Hudson River. It was never truly completed, as the masonry facing called for on each of its steel towers was omitted because of the Great Depression.

The federal government responded to the depression by funding many massive public works projects. Partially as a form of unemployment relief, San Francisco undertook the construction of two great suspension bridges. They spanned greater distances than any previously built bridges. Strong Pacific Ocean currents and the depth of the water in San Francisco Bay made the construction of both the San FranciscoOakland Bridge (1936) and the Golden Gate Bridge (1937) particularly challenging, and no part of the project required more technical expertise than building the subaqueous tower piers.

Triumphs in bridge building have been tempered by failures and perhaps no span has received more notoriety because of its collapse than did the Tacoma Narrows Suspension Bridge across Puget Sound in Washington State. Beginning in the late 1930s, its construction progressed uneventfully until the bridge was completed in July 1940. The valley in which the bridge was built was subject to strong winds and gusts that set the bridge in motion even while it was under construction. These wind-induced undulations increased in frequency as the bridge neared completion. So noticeable were the span's movements that they earned the bridge the sobriquet Galloping Gertie. Several months after its opening, the bridge was subjected to a period of intense high winds, during which it literally tore itself apart. The rising, falling, and twisting of the deck was so violent that it broke loose from its suspender cables and crashed into the sound. It was later determined that the failure resulted from the bridge being too flexible. The narrow deck and the shallow profile of the steel girders supporting the deck provided little resistance to aerodynamic action. The bridge's collapse prompted a reevaluation of suspension bridge design and resulted in a move away from flexible designs toward much stiffer and wind-resistant construction. A redesigned span across Puget Sound was completed in 1950.

Of the few suspension spans built in the United States after the middle of the twentieth century, one of the more remarkable was the Verrazano-Narrows Bridge, which opened in November 1964. The bridge, situated across the entrance to New York Harbor, connected Staten Island to Brooklyn. It was designed by engineer Othmar H. Ammann, who during his career designed a number of New York City's bridges, including the George Washington Bridge. While no new techniques were introduced in its construction, the bridge is remembered for it huge overall dimensions and the unprecedented size of its individual parts as well as the speedfive yearswith which it was erected.

The Late Twentieth Century

Although the first cable stay bridges appeared in seventeenth-century Europe, this type of bridge emerged in a rationalized form only during the 1950s. Their con-figuration may vary in appearance and in the complexity of the tower or towers as well as in the symmetry and placement of the cables. The most recognizable spans are characterized by a single tower or mast and multiple diagonal cables that, if arranged in a single vertical plane, pass over the tower and are affixed at opposite points along the center line of the deck. Decks can be assembled in cantilever fashion from sections of pre-cast, pre-stressed concrete. They offer many of the advantages of suspension bridges, yet require neither the lengthy and costly process of cable spinning nor large cable anchorages. Their overall load-bearing capacity is less than the more complex suspension bridge. One of the most notable American examples is the Sunshine Skyway Bridge completed across Tampa Bay, Florida, in 1987.

With the expansion of American railroads nearing its end, highways became a major factor in bridge design and construction during the 1920s. As a result, the majority of spans constructed during the remainder of the twentieth century were relatively light, reinforced, and prestressed concrete highway bridges. Reinforced concrete bridge construction, in which steel imbedded in the concrete controls the forces of tension, was introduced in the United States in the late nineteenth century. Eventually, a variety of reinforcing systems were patented. The interstate highway system's rapid growth during the 1950s and 1960s fostered the widespread use of pre-stressed concrete beam bridges. Beams fabricated in this way were strengthened by built-in compressive forces. These bridges became the most common type of span in late-twentieth-century America.

BIBLIOGRAPHY

Condit, Carl W. American Building Art: The Nineteenth Century. New York: Oxford University Press, 1960.

. American Building Art: The Twentieth Century. New York: Oxford University Press, 1961.

Jackson, Donald C. Great American Bridges and Dams. Washington, D.C.: Preservation Press, 1988.

McCullough, David. The Great Bridge. New York: Simon and Schuster, 1972.

Petroski, Henry. Engineers of Dreams: Great Bridge Builders and the Spanning of America. New York: Knopf, 1995.

Scott, Quinta, and Howard S. Miller. The Eads Bridge. Columbia: University of Missouri Press, 1979.

Van der Zee, John. The Gate: The True Story of the Design and Construction of the Golden Gate Bridge. New York: Simon and Schuster, 1986.

Wittfoht, Hans. Building Bridges: History, Technology, Construction. Dusseldorf, Germany: Beton-Verlag, 1984.

William E. Worthington Jr.

See also Brooklyn Bridge ; Eads Bridge ; George Washington Bridge ; Golden Gate Bridge ; Toll Bridges and Roads ; Verrazano-Narrows Bridge .

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Bridges

Bridges

Bridges are structures that provide a means of crossing natural barriers, such as rivers, lakes, or gorges. Bridges are designed to carry railroad cars, motor vehicles, or pedestrians. Bridges also support pipes, troughs, or other conduits that transport materials, such as an oil pipeline or a water aqueduct.

Humans have been constructing bridges since ancient times. The earliest bridges were probably nothing more than felled trees used to cross rivers or ditches. As civilization advanced, artisans discovered ways to use stone, rock, mortar, and other natural materials to construct longer and stronger bridges. Finally, as physicists and engineers began to develop the principles underlying bridge construction, they incorporated other materials such as iron, steel, and aluminum into the bridges they built. There are four major types of bridges: beam, cantilever, arch, and suspension.

Forces acting on a bridge

Three kinds of forces operate on any bridge: the dead load, the live load, and the dynamic load. Dead load refers to the weight of the bridge itself. Like any other structure, a bridge has a tendency to collapse simply because of the gravitational forces acting on the materials of which the bridge is made. Live load refers to traffic that moves across the bridge as well as normal environmental factors such as changes in temperature, precipitation, and winds. Dynamic load refers to environmental factors that go beyond normal weather conditions, factors such as sudden gusts of wind and earthquakes. All three factors must be taken into consideration in the design of a bridge.

Words to Know

Abutment: Heavy supporting structures usually attached to bedrock and supporting bridge piers.

Bedrock: Portion of Earth's mantle made of solid rock on which permanent structures can be built.

Dead load: The force exerted by a bridge as a result of its own weight.

Dynamic load: The force exerted on a bridge as a result of unusual environmental factors, such as earthquakes or strong gusts of wind.

Live load: The force exerted on a bridge as a result of the traffic moving across the bridge.

Piers: Vertical columns, usually made of reinforced concrete or some other strong material, on which bridges rest.

Suspenders: Ropes or steel wires from which the roadway of a bridge is suspended.

Truss: A structure that consists of a number of triangles joined to each other.

Beam bridges

The simplest type of bridge consists of a single piece of material that stretches from one side of a barrier to the other side. That piece of materialcalled a beam or girderrests directly on the ground on each side or is supported on heavy foundations known as piers. The length of a beam bridge is limited by the weight of the beam itself plus the weight of the traffic it carries. Longer beam bridges can be constructed by joining a number of beams to each other in parallel sections.

Cantilever bridges

A cantilever bridge is a variation of the simple beam bridge. A cantilever is a long arm that is anchored at one end and is free to move at the opposite end. A diving board is an example of a cantilever. When anchored firmly, a cantilever is a very strong structure. It consists of three parts: the outer beams, the cantilevers, and the central beam. The on-shore edge of the outer beam is attached to the ground itself or to a pier (usually a vertical column of reinforced concrete) that is sunk into the ground. The opposite edge of the outer beam is attached to a second pier, sunk into the ground at some distance from the shore. Also attached to the off-shore pier is one end of a cantilever. The free end of the cantilever extends outward into the middle of the gap between the shores. The cantilevers on either side of the gap are then joined by the central beam.

Caisson

To build bridge piers, workers need a water-free environment to excavate or dig the foundations. This is achieved by using a caisson, a hollow, water-tight structure made of concrete, steel, or other material that can be sunk into the ground. When building a bridge over a river, workers sink a caisson filled with compressed air into the river until it reaches the river bottom. The workers then go into the caisson and dig out soil from the riverbed until they come to bedrock. The caisson, which has sharp bottom edges, continually moves downward during the digging until it comes to rest on bedrock. Concrete is then poured into the caisson to form the lowest section of the new bridge pier.

Trusses. The strength of a cantilever bridge (or any bridge) can be increased by the use of trusses. A truss is structure that consists of a number of triangles joined to each other. The triangle is an important component of many kinds of structures because it is the only geometric figure that cannot be pulled or pushed out of shape without changing the length of one of its sides. The cantilever beam, end beams, and joining beams in a cantilever bridge are often strengthened by adding trusses to them. The trusses act somewhat like an extra panel of iron or steel, adding strength to the bridge with relatively little additional weight. The open structure of a truss also allows the wind to blow through them, preventing additional stress on the bridge from this force.

Arch bridges

The main supporting structure in an arch bridge is one or more curved elements. The dead and live forces that act on the arch bridge are transmitted along the curved line of the arch into abutments or supporting structures at either end. These abutments are sunk deep into the ground, into bedrock if at all possible. They are, therefore, essentially immovable and able to withstand very large forces exerted on the bridge itself. This structure is so stable that piers are generally unnecessary in an arch bridge.

The roadway of an arch bridge can be placed anywhere with relationship to the arch: on top of it, beneath it, or somewhere within the arch. The roadway is attached to the arch by vertical posts (ribs and columns)

if the roadway is above the arch, by ropes or cables (suspenders) if the roadway is below the arch, and by some combination of the two if the roadway is somewhere within the arch.

Suspension bridges

In a suspension bridge, thick wire cables run across the top of at least two towers and are anchored to the shorelines within heavy abutments. In some cases, the roadway is supported directly by suspenders from the cables. In other cases, the suspenders are attached to a truss, on top of which the roadway is laid. In either case, the dead and light loads of the bridge are transmitted to the cables which, in turn, exert stress on the abutments. That stress is counteracted by attaching the abutments to bedrock.

The towers in a suspension bridge typically rest on massive foundations sunk deep into the riverbed or seabed beneath the bridge itself. The wire cables that carry the weight of the bridge and its traffic are made of parallel strands of steel wire woven together to make a single cable. Such cables typically range in diameter from about 15 inches (38 centimeters) to as much as 36 inches (91 centimeters).

Movable bridges

Traditionally, three kinds of movable bridges have been constructed over waterways to allow the passage of boat traffic. In a swing bridge, the roadway rotates around a central span, a large, heavy pier sunk into the river bottom. In a bascule bridge, the roadway is raised like an ancient drawbridge. It can be lifted either at one end or split in two halves in the middle, each half rising in the opposite direction. In a vertical-lift bridge, the whole central portion of the bridge is raised straight up by means of steel ropes.

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Bridges

Bridges

Sources

Bottlenecks. In the late summer of 1787 George Washington made the mistake of trying to cross the stream at Elkton, Maryland, on what he described as an old rotten, and long disused bridge. One of his carriage horses fell through, and only the prompt assistance of some bystanders saved the first citizen of the republic from plummeting into the water. Experiences such as Washingtons were not uncommon in the early national era. Roads were often no more than mud tracks, and few streams could boast permanent all-weather bridge crossings. Most travelers relied on ferries and fords, both subject to the vagaries of the weather, even to cross major rivers. The lack of reliable bridges and roads represented more than just an occasional danger or inconvenience to travelers. During the War of 1812 the nations armies found the unreliable road-and-bridge system a serious impediment to the timely movement of troops and supplies, especially in the West. In an expanding postwar economy, transportation bottlenecks at river crossings obstructed the free flow of trade goods between the countryside and the ever-growing port and manufacturing cities.

Burr. In the decades after the War of 1812 the threat to national defense and economic growth posed by bad roads and bridges prompted both private investors and the state and federal governments to expend vast sums on infrastructure, including turnpikes, canals, railroads, and the bridges that carried all of these arteries across the nations waterways. America could claim few trained civil engineers qualified to design these bridges, but a generation of bridge-building craftsmen arose to meet the unique design and construction challenges presented by Americas great rivers. Theodore Burr was one who learned his bridge-building skills as a carpenter rather than in the engineering classroom. Burr used his innovative wooden arch-truss designs to bridge the Hudson, the Delaware, and the Mohawk, all broad rivers with heavy currents and teeming with boat traffic, before moving on to his masterpiece, the McCalls Ferry Bridge over the Susquehanna. When completed in 1815 the McCalls Ferry Bridge was the longest wooden-arch span in the country.

The Truss Design. Burrs arch-truss bridges impressed the public and professionals alike, but the wooden truss bridges of Ithiel Town were easier to construct, and the cantilevered arch-truss bridges of Louis Wernwag were more successful in the long run. Wernwag finished the Colossus of Philadelphia in 1812, a 340-foot clear span cantilevered wooden bridge that became the admiration of artists and engineers from the United States and overseas. The Colossus was the first modern cantilever, and Wernwag built some thirty such bridges over the next three decades throughout the Northeast and Midwest. Towns bridges, on the other hand, did away with the arch entirely, relying instead on a bridge framework of wooden planks crossed in a diamond pattern and secured with wooden pins (much like a garden lattice) resting on stone piers. Simple in design and made with easily obtained materials, Towns truss bridge became popular for the highways and earliest railroads of the nineteenth century.

Railroad Bridges. All-wood spans were more than adequate for most of the traffic in prerailroad America. But with the growth of extensive rail networks in the 1840s and 1850s engineers needed bridges that could span wide river valleys, withstand the weight and vibration of massive trains, and still avoid the expense and stream obstruction of multiple-pier wooden spans. These requirements were especially important in bridging rivers such as the Mississippi or the Ohio, where heavy barge and steamboat traffic necessitated wide channels and tall vertical clearances (the smokestacks on some steamboats were over 75 feet tall). A new generation of engineers, including William Howe, Caleb and Thomas Pratt, and Squire Whipple, managed to solve these problems by fine-tuning Towns basic truss design and by utilizing more cast and wrought iron in the planning and construction of railroad bridges. As Pittsburgh foundries increased the volume and quality of their iron (and eventually steel) production, all-metal Pratt, Howe, and Whipple truss bridges became the standard railroad spans nationwide.

Suspension Bridges. In the winter of 1816 the first wire suspension bridge in America, a passenger toll path only 2 feet wide and 408 feet long spanning the Schuylkill River at Philadelphia, collapsed under snow and ice. Almost two decades earlier James Finley, a western Pennsylvania judge, had come up with the idea of using iron chains stretched over stone piers to hold up a level floor over a span of water. While Finleys first suspension bridges never exceeded 70 feet, he claimed that they would someday safely cross open spaces and waterways 1, 000 feet wide. By the last half of the nineteenth century bridge designers in the United States and Europe were proving Finleys predictions practicable. Suspension bridges offered strength and safety (when braced for wind) over long stretches of water, with the added advantage of fewer piers and thus less obstruction to navigation. The latter benefit became especially important when bridging active shipping channels at busy ports such as New York.

THE MCCALLS FERRY BRIDGE

Theodore Burr bragged that the wooden arch-truss bridge over the Susquehanna River that he designed in 1815 contained the greatest [arch] in the world at three hundred and sixty feet four inches It was certainly one of the most difficult bridges in America to build. At the spot Burr chose, the Susquehanna was swift, almost one hundred feet deep, and subject to ice floes during the spring thaw. Instead of working in the dangerous main channel Burrs laborers constructed the bridges central arch in upright sections at a spot a quarter-mile downstream, then used the frozen river of January 1815 to swing it upstream and raise it into place. Unfortunately, the river was not entirely frozen, and blocks of ice mixed with slush had stacked up at the narrows to a depth of sixty to eighty feet where the bridge piers stood ready for the deck. Somehow his workmen manhandled the huge arch into place with only one accident: a worker who plunged fifty-four feet to the river yet survived. Bonfires and ample amounts of liquor heralded the completion of the span, but the bridge fell victim within two years to another ice jam and was never replaced.

Sources: Llewellyn N. Edwards, A Record of History and Evolution of Earty American Bridges (Orono: Maine University Press, 1959);

Richard Shelton Kirby and others, Engineering in History (New York: McGraw Hill, 1956).

Sources

Llewellyn N. Edwards, A Record of History and Evolution of Early American Bridges (Orono: Maine University Press, 1959);

Donald C. Jackson, Great American Bridges and Dams (Washington, D.C.: Preservation Press, 1988);

Lee H. Nelson, The Colossus of 1812: An American Engineering Superlative (New York: American Society of Civil Engineers, 1990);

Ted Ruddock, Arch Bridges and Their Builders, 17351835 (Cambridge, U.K.: Cambridge University Press, 1979).

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Bridges

BRIDGES

Structures constructed over obstructions to highways or waterways, such as canals or rivers, in order to provide continuous and convenient passages for purposes of transportation. A bridge includes the necessary abutments and approaches that make it accessible. A public bridge that spans obstructions to a public highway is built on land owned by the state government for public use, while a private bridge is built on private property for the use of particular individuals who own it.

The construction of public bridges is a function of the state government by virtue of statute and is limited only by contractual or constitutional provisions. A state may exercise its power directly or delegate it to governmental agencies, such as a state highway commission. Cities and municipalities may erect bridges within their borders if authorized to do so by the state legislature. If a bridge is to be built within the borders of a state, the state has control of the project; but if the bridge connects two states, both states share involvement in the venture but must yield to the power of the federal government to supervise matters that have an effect on interstate commerce.

The state determines the location of a bridge subject to public safety and convenience considerations. It may grant a franchise (special privilege) to erect the bridge to a private bridge company that is chartered to build and maintain bridges. Such a corporation is considered a business affected with a public interest. A state agency may be organized to receive a franchise to construct a bridge.

The money needed to finance the construction of a bridge is usually raised by appropriations designed for the project—the sale of bonds pursuant to statute, special assessments, or taxation. The legislature decides whether construction expenses will be borne by the entire state or apportioned among its various subdivisions. It may create special taxing districts to finance the project as long as the district receives a proportional benefit from the bridge. State taxes cannot be used to defray the expense of purely local bridge obligations.

A reasonable toll may be charged for using the bridge when authorized by statute. The revenue collected can be used for governmental purposes as well as for the operating and maintenance expenses of the bridge.

The duty to maintain and repair bridges rests with the government agency or private company charged with their operation and maintenance. Statutes frequently require warning signs on guardrails and bridge approaches to caution drivers against known dangers. Civil or criminal liability may be imposed for damages resulting from the failure to maintain a bridge properly. No liability generally, exists, however, for any damages incurred by an adjoining landowner from negligence or other wrongful conduct in the construction or maintenance of a bridge by a municipality or government agency unless provided by statute.

A government entity is often shielded from liability for general harm to persons or property caused by negligent construction, repair, or maintenance of bridges under the theory of sovereign immunity pursuant to statute. For example, in the case of Hansen v. State Dept. of Transportation, 1998 S.D. 109 (1998), plaintiff Hansen was seriously injured after driving her vehicle into an unmarked construction hole on an interstate highway bridge. The South Dakota Supreme Court affirmed a lower court's decision to dismiss the case on the basis that sovereign immunity barred Hansen from suing the state's department of transportation. Many states have modified their immunity statutes to permit claims premised on gross negligence; others draw a distinction between ministerial (bound by judicial command) and discretionary duties, allowing claims only for negligence in the performance of ministerial duties or functions.

Private companies may be liable for harm if the law in the jurisdiction so provides. When a pedestrian bridge over Interstate Highway 29 in North Carolina collapsed in May 2000 at Lowe's Motor Speedway, 107 persons were injured. More than half filed suits, many naming as defendants the speedway, the bridge builder (a private corporation), and the maker of a grout substance that corroded the steel supporting the bridge. As of early 2003, none of the cases had yet completed trial.

further readings

Levy, James. 2003. "Corporate Defendants in Lowe's Motor Speedway Collapse Blame One Another." Charlotte Observer (February 1).

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bridges

bridges. Britain's pre-1100 wooden bridges do not survive, except as place-names, but extensive activity continued throughout the Middle Ages, and few new bridge sites were added before 1750 to those identifiable in 1500. Bridge-building was one of the three communal obligations, an object for pious work, and for seigneurial enterprise. There had been a Roman London bridge, but its continuous history dates from that constructed of wood in 993, and rebuilt of stone from 1176. Some early bridging works were extensive: Aldreth Causeway (c.1100) was one of two constructed across the fenland to the Isle of Ely. Many towns levied pontage tolls to erect and maintain bridges, and there were also religious links: Queen Matilda endowed Barking abbey to maintain Bow bridge, c.1150, and chapels were recurrent features of medieval bridges, as at Wakefield and Derby. Bridging points formed the core of medieval new towns, such as Chelmsford (c.1200) and Leeds (1207). Bridges replaced many fords and ferries in the later Middle Ages, and indicated widespread growth of vehicular traffic. Medieval stone bridge construction was robust and coped with much traffic growth, and widening and strengthening engineering works did not become generalized until after 1750.

Responsibility for bridges lay with parishes or landowners, but uncertainty led to the statute of Bridges (1531) providing for fall-back maintenance by the county, and by 1602 48 bridges had become the responsibility of the West Riding. Heavily used bridges also required special procedures: private Acts were employed increasingly in the 18th cent. to create commissions to finance major bridges and new crossings by tolls, beginning with Westminster (1736) initiated with a dedicated lottery. General county responsibility did not come until 1888. Private Acts were thus used for many of the eight bridges added to London before 1820 (including Kew in 1759), and three of these—Vauxhall, Waterloo, and Southwark (1816, 1817, 1819)—effectively created the new suburbs of Brixton, Kennington, and Camberwell. Trunk turnpike development built major bridges, notably on Telford's Holyhead road at Conwy and the Menai Straits (1815–19).

Significant development functions came also with the great aqueducts at Barton on the Bridgwater canal (1761), which obviated the need for locks, and Telford's cast-iron construction at Pontcysyllte (1805) carrying the Ellesmere canal over the Dee at 127 feet. Ironbridge, Coalport (1779), and Sunderland (1796) pioneered the application and diffusion of cast iron to bridge construction, and laid the basis for the railways, although Brunel and Cubitt still used timber extensively in the 1840s. Railways employed brick and cast and wrought iron to bridge Britain's great estuaries and rivers between the 1840s and the 1870s: the Dee, the Severn, the Tamar, the Solway Firth, the Menai Straits, and the Tay. Even before the failure of materials and design on the last (1879), steel had established itself as the critical construction material, and demonstrated its strength in the Forth bridge (completed 1890).

Twentieth-cent. bridge-building was correspondingly dominated by steel and concrete for roads. The great railway crossings were replicated from the 1960s by road bridges, all bar the Tay on the suspension principle, and new crossings of the Thames and Humber were added. Lightweight box-girder construction speeded the building of over-bridges from the 1960s, despite initial collapses, and motorway and trunk road building provided an immense stimulus to their use: the first stretch of the M1, opened in 1959, had 183 bridges in its 75 miles, and was constructed in 586 days.

J. A. Chartres

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