BRIDGE

structure that spans horizontally between supports, whose function is to carry vertical loads. The prototypical bridge is quite simpletwo supports holding up a beamyet the engineering problems that must be overcome even in this simple form are inherent in every bridge: the supports must be strong enough to hold the structure up, and the span between supports must be strong enough to carry the loads. Spans are generally made as short as possible; long spans are justified where good foundations are limitedfor example, over estuaries with deep water. All major bridges are built with the public's money. Therefore, bridge design that best serves the public interest has a threefold goal: to be as efficient, as economical, and as elegant as is safely possible. Efficiency is a scientific principle that puts a value on reducing materials while increasing performance. Economy is a social principle that puts value on reducing the costs of construction and maintenance while retaining efficiency. Finally, elegance is a symbolic or visual principle that puts value on the personal expression of the designer without compromising performance or economy. There is little disagreement over what constitutes efficiency and economy, but the definition of elegance has always been controversial. Modern designers have written about elegance or aesthetics since the early 19th century, beginning with the Scottish engineer Thomas Telford. Bridges ultimately belong to the general public, which is the final arbiter of this issue, but in general there are three positions taken by professionals. The first principle holds that the structure of a bridge is the province of the engineer and that beauty is fully achieved only by the addition of architecture. The second idea, arguing from the standpoint of pure engineering, insists that bridges making the most efficient possible use of materials are by definition beautiful. The third case holds that architecture is not needed but that engineers must think about how to make the structure beautiful. This last principle recognizes the fact that engineers have many possible choices of roughly equal efficiency and economy and can therefore express their own aesthetic ideas without adding significantly to materials or cost. Generally speaking, bridges can be divided into two categories: standard overpass bridges or unique-design bridges over rivers, chasms, or estuaries. This article describes features common to both types, but it concentrates on the unique bridges because of their greater technical, economic, and aesthetic interest. in electrical measurement, instrument for measuring electrical quantities. The first such instrument, invented by British mathematician Samuel Christie and popularized in 1843 by Sir Charles Wheatstone, measures resistance by comparing the current flowing through one part of the bridge with a known current flowing through another part. The Wheatstone bridge has four arms, all predominantly resistive. A bridge can measure other quantities in addition to resistance, depending upon the type of circuit elements used in the arms. It can measure inductance, capacitance, and frequency with the proper combination and arrangement of inductances and capacitances in its arms. either of two card games, auction bridge and contract bridge, that are derived from whist and retain its essential features. These features are the following: four players participate, two against two in partnership. They play with a 52-card pack, all of whose cards are dealt face downward one at a time, clockwise. When play begins, the object is to win tricks, consisting of one card from each player in rotation. The players must, if able, contribute a card of the suit led, and the trick is won by the highest card. All tricks taken in excess of the first six tricks are known as odd tricks. Before play begins, a suit may be designated the trump suit, in which case any card in it beats any card of the other suits. For further details of basic play common to both bridge and whist, see whist. The whist and bridge family of games is of English origin, having evolved gradually from several other games, principally one called triumph, a name that became corrupted to trump. The name whist probably originated in the early 17th century, and by the mid-18th century whist had become the preeminent card game among the upper classes in both western Europe and North America. Whist was supplanted by bridge whist in the 1890s. The latter in turn evolved into auction bridge in the first decade of the 20th century, and contract bridge was developed in the 1920s and '30s. From the early 18th century, whist, bridge whist, auction bridge, and contract bridge have each reigned in turn as the most intellectually stimulating of all card games. Successive improvements in various features of the games have greatly enlarged the scope for inferential reasoning, psychological stratagems, and partnership cooperation. structure that surmounts an obstacle (e.g., a river or declivity) and is used as a passageway for pedestrian, motor, or rail traffic. The simplest bridge is the beam (or girder) bridge, consisting of straight, rigid beams placed across a span. The most basic example of this type, and probably the oldest form of bridge, is a log set over a stream. Support for the girder bridge comes from the foundations at either end, which bear the entire weight of the bridge and its traffic. A major improvement over the simple beam was the circular arch, the basic design of which was perfected by the ancient Romans. Because the forces on an arch bridge extend outward as well as downward (a condition called compression), they could be built with longer spans and with masonry blocks rather than with single timbers or slabs of stone. A refinement of the beam, called a cantilever, is used to construct bridges with even longer spans. In simple cantilever bridges, such as those that are found in many parts of Asia, the structure rests on crisscrossed log foundations with narrow bases that flare outward as they rise to support the roadway. A modern cantilever bridge may use only two intermediate piers set a short distance from either bank. These act something like fulcrums. The first span of the bridge extending from bank to pier is called the anchor arm, and the next span projecting toward the middle is called the cantilever arm. The cantilever arms from either end are joined at the middle by a suspended span. Top structures connect the anchor arms to towers above the piers, and the cantilever arms support the midsection by the force of tension. Downward forces are absorbed by the piers. The weight of a suspension bridge is also partially supported by intermediate piers crowned with towers; but instead of the rigid superstructure of a typical cantilever, the main support members of a suspension bridge are parallel cables composed of thousands of individual strands of wire. These run, supported by each tower and curving down at the middle, the entire length of the bridge and are anchored on either end. The deck (roadway) is supported by hanger cables that are hung from the main cables. Most of the weight of the bridge is thus transferred to the cable anchors at either end of the bridge. The suspension concept has found its widest application in the 20th century. A variant type of suspension bridge is the cable-stayed span, in which the supporting cables run directly from tall vertical pylons to the horizontal deck. These three designs are sometimes combined, but the features of each are generally recognizable. Bridges are normally characterized by the length of their main spans; those of the suspension type are usually the longest. In the late 1990s the Akashi Kaiky Bridge in Japan was the world's longest, with a main span of 1,991 m (6,532 feet). Besides design, the other great challenge in building durable bridges has been in the methods and materials used in their construction. Roman engineers introduced two significant innovations, the cofferdam and cement. The widespread use of a third major innovation, ferrous-metal structural members, did not occur until the late 18th century. These and other technological advances have made possible the large modern bridges. The Romans surmounted the problem of building bridge foundations in water by using cofferdams, which were temporary enclosures made out of timbers that were driven into the river bottom, made watertight with clay, and then pumped dry to facilitate the setting of a foundation. This method was long used and only significantly improved with the development of the pneumatic caisson in the 19th and 20th centuries. Water is forced out of the caisson (a large concrete or steel cylinder that rests on the bottom) with compressed air, allowing workers to descend through an airlock and dig to bedrock. As material is removed the caisson sinks, and after bedrock is reached, it is filled with concrete to become the foundation of the bridge pier. In the early days of caisson use there were many casualties among workers who made a rapid transition from the compressed chambers to normal atmospheric pressure and developed caisson disease (decompression sickness, also called the bends). Caissons are also sunk by mechanically pounding them down to bedrock with powerful drivers, thus eliminating the need for manual excavation. Roman builders also came up with the first concretemade of volcanic rock, lime, sand, and waterwhich they poured into cofferdams. Often concrete is the main construction material in modern bridges. Reinforced concrete, to which emplanted steel bars add strength, was introduced in the 19th century; its first significant application was in an arched bridge built in France in 1898. Engineers later learned to apply tension to the bars before pouring the concrete. This process, called prestressing, significantly reduces the amount of structural material needed for a bridge, since the built-in stress counteracts much of the weight of the load. Prestressed concrete bridges often have a graceful look, which cannot be achieved by iron and steel bridges with their geometric configurations. Although iron chains had been used in suspension bridges for centuries, the world's first all-iron bridge was not built until 1779. Numerous cast- and wrought-iron structures were subsequently built, but, by the last quarter of the 19th century, steel had largely supplanted iron as the primary construction material. With its high strength-to-weight ratio and great malleability, steel is the ideal material to use for the increasingly sophisticated and ambitious projects of modern bridge designers. New alloys and alloying processes have vastly improved its tensile strength, ductility, and resistance to fatigue and corrosion, making possible bridges that incorporate innovative uses of steel in plates and box girders, cable bracing, and continuous trusses. In addition to providing a passage for traffic, bridges have often had other functions, including service as fortifications and as business districts. The Old London Bridge, for example, which stood from the 13th to the 19th century, was a crowded thoroughfare, full of shops and houses. The original functions of bridges have also been modified for new uses. The drawbridge, or bascule, originally used for protection against intruders, has been turned into a means for allowing the passage of water traffic. The classic drawbridge, in which the bridge leaf is hoisted by rope or chain, has been improved by the development of gears and counterweights. Design variations include leaves that roll back on tracks, bridges whose entire span is lifted vertically by cables run over tall towers to counterweights, and bridges that swing on a central turntable. in stringed musical instruments, piece of elastic wood that transmits the vibrations of the string to the resonating body. Bridges are of two kinds. In the pressure bridge, the string is fastened at one end to a tuning peg or a wrest pin and at the other to a pin or a tailpiece; it passes over the bridge (or bridges), which may be glued to the soundboard (as in the piano) or held in position solely by the pressure of the strings (as in the violin). In the tension bridge, one end of the string is fastened to a tuning peg or wrest pin and the other to the bridge itself, which is glued to the soundboard (as in the guitar and the lute). Additional reading David P. Billington, The Tower and the Bridge: The New Art of Structural Engineering (1983), traces the history of structures since the Industrial Revolution and emphasizes a series of individual engineers who were structural artists. David J. Brown, Bridges (1993), chronicles bridge building from its beginnings to future projects, with discussions of the world's most important bridges, some of which were failures. Eric DeLony (ed.), Landmark American Bridges (1993), contains elegantly illustrated presentations of large and small bridges built in the United States from the late 18th century to the post-World War II era, with a time line encapsulating the history of bridges from 1570 to the present. C.M. Woodward, A History of the St. Louis Bridge (1881), on the Eads Bridge, is still the most complete work ever published on one bridge, a classic work for both the nontechnical reader and the engineer. David McCullough, The Great Bridge (1972, reissued 1982), narrates the political and human history of the building of the Brooklyn Bridge. David P. Billington, Robert Maillart's Bridges (1979), gives substantial details of the major works of this bridge designer and builder.Fritz Leonhardt, Bridges: Aesthetics and Design (1984), provides a substantial general discussion and includes a wide selection of striking photographs of old and new bridges. A finely illustrated nontechnical text by Hans Wittfoht, Building Bridges (1984), centers on both the design and the construction of bridges. M.S. Troitsky, Planning and Design of Bridges (1994), focuses on the selection of bridge type and preliminary design. Walter Podolny, Jr., and John B. Scalzi, Construction and Design of Cable-Stayed Bridges, 2nd ed. (1986), treats the modern bridge form and provides substantial technical detail, although the nontechnical reader will understand much of the text. Christian Menn, Prestressed Concrete Bridges, trans. and ed. by Paul Gauvreau (1990; originally published in German, 1986), a technical work primarily for engineers, provides historical coverage and deals elegantly with the many complex issues in bridge design. Louis G. Silano (ed.), Bridge Inspection and Rehabilitation (1993), is a compendium of practical information.Early works on bridge design and construction still widely available include Henry Grattan Tyrrell, History of Bridge Engineering (1911), a fine, nontechnical sweep through history from ancient bridges to early reinforced concrete arches; Wilbur J. Watson, Bridge Architecture (1927), exploring the collaboration between engineers and architects and focusing on early 20th-century designs, and A Decade of Bridges, 19261936 (1937), with a narrower scope; Charles S. Whitney, Bridges: A Study in Their Art, Science, and Evolution (1929, reprinted as Bridges: Their Art, Science, and Evolution, 1983), a nontechnical work; and Elizabeth B. Mock (Elizabeth B. Kassler), The Architecture of Bridges (1949), the first major book on bridges to give a modern viewpoint. David P. Billington