Any supported horizontal structure that spans an open space may be termed a bridge. While some bridges are simple structures, others are masterpieces of engineering. Many of the majestic structures we see today span vast distances and support great weights, a tremendous technological advance in comparison to the simple bridges first devised by the earliest humans. The First BridgesWhen prehistoric people wanted to cross a stream, they probably looked for a tree that had been blown down over the water by the wind. Later, people imitated nature by chopping down trees to cross streams. Sometimes logs or stone slabs were placed from rock to rock in the water to serve as bridges. In tropical lands people were able to cross a river by climbing hand over hand on vines that hung from a tree on one bank to a tree on the opposite bank. Occasionally they could cross on a natural bridge carved by streams through clay or rock. Some of these still exist, including Natural Bridge, a limestone arch in Virginia, and Rainbow Bridge, a pink sandstone arch in Utah (see Earth). In time people learned to build their own bridges with materials other than those found in nature. Today, man-made bridges are found throughout the world. They are made from a great variety of materials and built in many shapes and sizes. The loads that bridges transfer to their vertical supports are of various kinds. Dead load is defined as the weight of the bridge. Live load is the weight of the traffic—human or otherwise—on it. Wind load is the pressure of the wind against the bridge. This pressure is usually assumed by engineers to be 30 pounds per square foot (146 kilograms per square meter) of exposed surface, including live load or 50 pounds per square foot (244 kilograms per square meter) without live load. Fixed and Movable BridgesBridges are divided into two general classes, according to the way that they provide clearance for navigation under them. A fixed bridge is sufficiently high to permit ships or vehicles to pass under it. A movable bridge swings up or sideways to allow passage. The two classes may be further divided into different types. The basic kinds of fixed bridges are beam, arch, and suspension. Movable bridges are bascule, swing span, vertical lift, floating, and transporter. The Simplest Bridge—The Beam - A beam bridge, with forces of tension represented by red lines and forces of compression by green …
The early human's log across the stream is a perfect example of the beam bridge. A horizontal member resting on a vertical support at each end is a simple beam. Plate girder bridges over highways are an excellent example of this. A plate girder is a built-up beam consisting of a steel plate to which angles are riveted or welded. A simple beam tends to bend down at its middle. Its lower part is subject to tension, or pull, while its upper part is being subjected to compression, or squeezing. To bridge wide streams, the beam may rest on more than one support. This is a continuous beam. Another beam bridge is the truss. It is made up of members forming rigid triangles. Members in compression are built as rectangles to resist buckling. Those subject to tension may be slender bars. A variation of the beam principle is used in the cantilever bridge. A cantilever is a beam that extends beyond its support. Sometimes two cantilever arms meet at mid-span. Usually they do not meet but are connected by a light suspended span. The Graceful Arch Bridge - An arch bridge, with forces of compression represented by the green line.
Based on a different principle than the beam is the arch bridge. In the beam bridge the load is transmitted vertically to the supports. The arch bridge pushes outward against its supports. They must be heavy to resist the horizontal thrust of the arch. The arch may be fixed, with each end rigid, two-hinged, with a hinge at each support, or three-hinged, with a third hinge at its crown. The hinges permit movement because of loads or temperature changes. Arch bridges differ according to the way in which they carry the horizontal roadway. In the deck arch the road is carried above the arch that supports it. In the half-through arch the road cuts through the arch. Its middle section is hung from the arch, and its outer sections are supported by the arch. In the through arch the road is suspended from the arch. An arch bridge of rectangular shape is the rigid frame. It is used for shorter spans, such as in highway and railway grade separations. Its horizontal span is made in one piece with the vertical supports. The rigid frame is made of steel or reinforced or prestressed concrete (see concrete). Spidery Cables of the Suspension BridgeIn the suspension bridge huge cables are hung over two high towers. The cable ends are fastened to heavy concrete or masonry anchorages. Suspender cables hanging from the main cables support the roadway. As in the arch, the thrust on the suspension bridge is horizontal. Instead of horizontal compression, or push, however, there is horizontal tension, or pull, upon the anchorages. Cables are parallel wire, wire rope strand, or eyebar (chain). The parallel wire cable is best for long spans. Thousands of wires are laid side by side, then squeezed together and wrapped with wire to protect them. Each cable of the Golden Gate Bridge, at San Francisco, is 36.5 inches (92.7 centimeters) in diameter and contains 27,572 wires; each wire measures 0.2 inch (0.5 centimeter) in diameter. The wire rope strand cable consists of prestressed twisted wire strands. Its great advantage is that it is ready for erection without spinning. Each cable of the St. Johns Bridge, at Portland, Ore., has 91 wire rope strands, each measuring 1.25 inches (3.18 centimeters) in diameter. An eyebar is a flat piece of steel with a circular head at each end in which there is a hole. Pins connect the eyebars. The eyebar suspension Florianopolis Bridge, in Brazil, has a main span of 1,114 feet (340 meters). The Bascule, or Seesaw, BridgeA good example of a type of movable bridge is the drawbridge over the moat of a medieval castle (see castle). The bascule bridge is the modern-day version. Bascule in French means “seesaw.” A counterweight balances the span in every open position. The bascule may be single-leaf or double-leaf. The Swing Span Bridge RotatesThe swing span bridge turns on a vertical axis to allow ships to pass. It is balanced on a pivot pier, usually in its center. Its span is measured by including the length of both arms. The Vertical Lift BridgeThe vertical lift bridge has a tower at each end of its span. Cables attached to ends of the span pass over pulleys at the top of the towers and are fastened to counterweights that equal the weight of the span. The span moves up and down like an elevator. The Bridge That FloatsIn the floating bridge, boats or pontoons support the road. The bridge retracts or swings aside to allow ships to pass. In the 9th century BC, Homer spoke of the use of pontoon bridges by soldiers. The Allies crossed the Rhine on them in World War II. The Transporter Bridge—An Aerial FerryThe transporter bridge has two towers supporting a fixed span from which a moving platform or car is hung. The transporter of the Sky Ride at the Century of Progress Exposition at Chicago in 1933–34 had an 1,850-foot (564-meter) span. Suspended from it, 215 feet (66 meters) above the ground, were 10 cars, each carrying 36 passengers. How Bridges Are BuiltThe design that engineers select depends upon many considerations. The location is important. Is it a high, narrow canyon, a wide marshy bay, or an earthquake area? A higher bridge is needed if ocean liners pass under it than is needed if only river barges do. The foundations may rest on bedrock close to the surface or under many feet of soft mud. Other factors are appearance and construction, operation, and maintenance costs. The method of erection is also considered. Bridge construction begins with the substructure, or foundation. Then the superstructure, or span, is erected. The foundation consists of abutments and piers. An abutment is support at the end of a bridge. A pier is an intermediate support. Timber or concrete piles may be used to hold up light bridges. For heavier bridges concrete or masonry supports may be built. Planks or steel pilings are used to form a cofferdam, or watertight enclosure. After the water is pumped out, the ground is excavated and concrete is poured. If bedrock is deep or quicksand is encountered, a caisson may be used. This is a large bottomless box or cylinder made of timber or concrete. Weights force it down through the soil. Workers dig out the ground inside. Compressed air may be used to keep out the water. The excavated caisson is filled with concrete. A pier of the Transbay Bridge, in California, is 240 feet (73 meters) below water, a world record. (See also caisson.) Many methods for erecting the superstructure have been developed for the different types of bridges and site conditions. The six main methods are by falsework, flotation, hoisting, cantilevering, suspension erection, and rolling (or sliding). FalseworkThe temporary scaffolding built of timber or steel that holds the bridge up until it is self-supporting is called falsework. It is well suited to truss bridges because many pieces have to be assembled. The riverbed must be suitable for driving piles and the river must be free of traffic, floods, swift currents, and ice. FlotationFlotation consists of constructing the span on barges or on shore and transferring it to barges. It is then floated into position. The span is lowered into place by flooding the barges or waiting until the tide goes out. HoistingHoisting is generally used to erect girder spans. A crane mounted on the bridge, on shore, or on a barge lifts the span into position. CantileveringSeveral instances in bridgebuilding require cantilevering. For cantilever bridges, the arms are built out piece by piece. Then the suspended span is raised into place from barges by jacks or cables or built out until the halves meet. For steel arches tie-back cables from the shore to the unfinished sections hold up the bridge until it is finished. Cantilevering does not halt navigation. Suspension ErectionWhen the towers of a suspension bridge are built first, this method is called suspension erection. The towers of the Golden Gate Bridge are 746 feet (227 meters) high—191 feet (58 meters) taller than the Washington Monument in Washington, D.C. After the towers are built the wires for the cables are strung over them from anchorage to anchorage by spinning wheels that travel back and forth. Suspender cables are hung from the main cables. The floor and stiffening trusses are fastened to them. RollingMilitary engineers commonly use rolling, or sliding, to construct bridges. The bridge is constructed on shore and rolled endwise into position. Sometimes a new span is built next to an old one and moved sidewise into place to reduce interruption of traffic. The portable Bailey bridge was invented in World War II by a British Engineer, Sir Donald Coleman Bailey. Its small truss sections look like a toy Erector set. It is moved out over the stream on rollers. One of Bailey's bridges spanned 4,000 feet (1,200 meters). The Long History of BridgebuildingThe earliest bridges were made from materials at hand. The Swiss lake dwellers built their timber houses by driving piles into the lake bed. From this evolved the timber pile and trestle railroad bridge. In warmer parts of the world bridgebuilders erected suspension bridges. In one Chinese type the traveler sat in a basket or saddle suspended from a cable and slid to the opposite bank. Bridgemakers in the Himalayas threw ropes across a chasm and from these hung thinner ropes to carry the road. This was the origin of the modern suspension bridge. The cantilever bridge also originated in India. Wooden planks, weighted down by abutment stones, were projected from the two banks until they met in the center. The principle of the true arch was discovered by the Babylonians in Mesopotamia before 5000 BC. The Chinese used the method for bridges after AD 100 and built the camel-back arch. In the conquest of Babylon the Persian army under Cyrus the Great in 537 BC built a bridge across the Euphrates River supported by inflated animal skins. This is the earliest pontoon bridge on record. The Romans were great bridgebuilders. Six of their masonry arch bridges over the Tiber River still stand in Rome. The most beautiful of the existing Roman bridges is the Ponte di Augusto, built at Rimini around 5 BC. The greatest Roman aqueduct is the Pont du Gard at Nîmes, France. It has three tiers of arches, which rise 155 feet (46 meters) above the Gard River. Monks Built Medieval BridgesDuring the Middle Ages the church became the chief builder of bridges. Churchmen formed the Brotherhood of Bridgebuilders in Italy and France at the end of the 12th century. St. Bénézet built a beautiful stone bridge over the Rhône River at Avignon, in southern France. Four arches still remain. Monks also built the old London Bridge, completed in 1209, over the Thames River, in London, England. This is the bridge of nursery-rhyme fame. By the 16th century more than a hundred shops and dwellings had been erected on it. It served London for 600 years. Another covered bridge, the Ponte Vecchio, still stands over the Arno River, at Florence, Italy. Built in 1345, it has shops along both sides. A major contribution of the Renaissance was the theory of the truss. Andrea Palladio, an Italian architect, published a treatise which applied four different trusses to bridgebuilding. Typical of the bridge design of this period is the stone arch Rialto Bridge over the Grand Canal in Venice, Italy. The marble Santa Trinita Bridge over the Arno at Florence, completed in 1569, was destroyed in World War II but rebuilt. A number of Renaissance stone bridges remain over the Seine in Paris, though most have been rebuilt. The Pont Neuf, completed in 1606, remains intact. At the time of its completion, it was the newest bridge in Paris, hence its name, New Bridge. Today however, it is the oldest bridge in the city. Bridgebuilding Becomes a ScienceIn the 18th century, bridge design became a science. Hubert Gautier, a French engineer, wrote the first treatise on bridgebuilding in 1717. The first engineering school was founded in Paris at about this time. Its director, Jean Perronet, is called the father of modern bridgebuilding. He perfected the masonry arch, using a flat arch and slender piers. One of his finest bridges is the Pont de la Concorde in Paris. A great English builder, John Rennie, designed the new London Bridge over the Thames. - A single-span truss bridge, with forces of tension represented by red lines and forces of …
Also in the 1700s the wooden truss bridge was rediscovered. The covered wooden bridge was probably invented in Switzerland. The Grubenmann brothers, Swiss carpenters, built a 200-foot (60-meter) span at Wettingen. The picturesque covered bridge was highly developed in the American Colonies. It looks more like a barn than a bridge. Colonel Enoch Hale built the first framed timber bridge in the United States, over the Connecticut River at Bellows Falls, Vt., in 1785. The greatest span built of wood was that of the “Colossus”—340 feet (104 meters) over the Schuylkill River at Fairmont, Pa. It was the work of Louis Wernwag in 1812. The timber bridges combined the arch and truss. Era of Iron and SteelThe invention of the steam locomotive changed bridgebuilding because stronger spans were needed. Iron was first used for chain cables of a suspension over the Tees River, in England, in 1741. The flooring was laid directly upon the cables. Abraham Darby and John Wilkinson built the first iron bridge over the Severn River at Coalbrookdale, England, in 1779. This 100-foot (30-meter) arch bridge is still in service. Thomas Telford built the first modern iron arch bridge in 1813. It is Craig Ellachie Bridge over the Spey at Banffshire, Scotland, with a 150-foot (46-meter) span. It was not built up of cast-iron blocks in imitation of masonry as were previous iron arch bridges but was the first to use an arch made up of iron trusses. In 1819–24 Telford built the forerunner of the modern suspension bridge—the 570-foot (174-meter) span over Menai Strait in Wales. It had wrought-iron chains for cables. The first to design railroad bridges was George Stephenson, who with his son Robert invented the Rocket, the first practical locomotive. Robert Stephenson built the Britannia Tubular Bridge over Menai Strait in 1846–50. Its two boxlike tubes were made of iron plates riveted together. Many truss designs were patented in the 1850s for railroad bridges. After numerous failures of cast-iron bridges, wrought iron was used, then steel. Modern Arch BridgesThe first bridge to use steel extensively was the triple-arched Eads Bridge over the Mississippi at St. Louis, Mo., in 1874. The first major use of pneumatic caissons for large piers was made in this bridge. It was an important link in the transcontinental railroad and made St. Louis a crossroads. James B. Eads became the first engineer in the Hall of Fame. The modern era of steel arch building began in the 20th century. The Bayonne Bridge, completed in 1931 over Kill van Kull between New York and New Jersey, has a 1,652-foot (504-meter) span. Australia's Sydney Harbor Bridge, finished in 1932, is only 2 feet (0.6 meter) shorter. At the turn of the 20th century, the construction of masonry arch bridges reached its peak. Then the more economical and easier to use concrete became common for arch bridges. Later, reinforced concrete and then prestressed concrete were used. Cantilever BridgesThe first modern cantilever was built in 1867 by Heinrich Gerber over the Main River at Hassfurt, Germany. It had a main span of 425 feet (130 meters). The first major example of the cantilever, however, was the Firth of Forth Bridge in Scotland. It was built in 1882–90 with two 1,700-foot (518-meter) spans. Its steel truss members are tubular in shape. Even greater in main span length is the Quebec Bridge over the St. Lawrence River in Canada. Its main span is 1,800 feet (549 meters) long. It was completed in 1917. Giant Suspension Bridges - A suspension bridge, with forces of tension represented by red lines and forces of compression by …
The greatest triumph of bridge engineering is the suspension bridge. As the suspension bridge replaced the cantilever, the United States became the world leader in this new type of long-span bridgebuilding. One reason was the peninsula sites of two of its greatest cities—New York and San Francisco. John A. Roebling perfected the suspension bridge. He developed the modern method of stringing parallel wire cables and of stiffening suspension bridges. Roebling's greatest achievement was his design of the Brooklyn Bridge over the East River in New York City. It has a span of 1,595 feet (486 meters). While making his final surveys, Roebling had an accident and died. His son, Washington A. Roebling, completed the bridge. He was paralyzed during the sinking of a pneumatic caisson but continued to direct construction from his room. The bridge was finished in 1883. For 20 years, the Brooklyn Bridge span was the longest in the United States. Then the Williamsburg Bridge over the East River in New York was completed in 1903 with a span only about 5 feet (1.5 meters) longer. In the 1920s and 1930s even longer bridges were built. The George Washington Bridge over the Hudson linking New York and New Jersey was completed in 1931 with a span of 3,500 feet (1,067 meters). This was almost double the former record. In 1937 San Francisco's Golden Gate Bridge was completed with a span of 4,200 feet (1,280 meters). This record span went unchallenged until 1964, when the Verrazano-Narrows Bridge was completed. Its two 690-foot (210-meter) towers support a 4,260-foot (1,298-meter) center span over New York Harbor, linking Brooklyn with Staten Island (Richmond). This bridge cost some 325 million dollars to build. It was designed by Othmar H. Ammann, who also participated in the building or planning of the George Washington Bridge, the Golden Gate Bridge, and the Bayonne Bridge. The San Francisco–Oakland Bay Bridge (Transbay) was completed in 1936. It is more than 8 miles (12.9 kilometers) long and includes two suspension bridges, each with a 2,310-foot (704-meter) main span, and a cantilever bridge with a 1,400-foot (427-meter) main span. Engineers compare the lengths of bridges by their main spans—not by the distance between anchorages or by the total length including approaches. Building the longer main span requires greater skill. In 1940 the Tacoma Narrows Bridge—then the world's third longest bridge—was built across the Narrows of Puget Sound, in Washington. Four months later a 42-mile-per-hour wind caused the flexible 2,800-foot (853-meter) span to twist until it tore loose. Its failure spurred research in aerodynamic design. “Galloping Gertie,” as it was nicknamed, was replaced in 1950 with a structure that incorporated stiffening girders. These components reduce the vertical and torsional, or twisting, movements that can result from high winds or other forces. The Ponte 25 de Abril (originally Salazar Bridge), over Portugal's Tagus River, was completed in 1966. Its main span of 3,323 feet (1,013 meters) is 23 feet (7 meters) longer than Scotland's Firth of Forth suspension bridge, completed in 1964. The Tagus bridge was the longest suspension bridge in Europe until the Humber Bridge in England, with a main span of 4,626 feet (1,410 meters), was opened in 1981. One of the world's longest suspension bridges in total length is the Mackinac Bridge, linking the upper and lower peninsulas of Michigan. From one anchor block to the other, it is about 1.6 miles long. It has a center span of 3,800 feet (1,158 meters). It was designed by David B. Steinman and completed in 1957. The Akashi Kaikyo Bridge, which was completed in 1998, is the longest and tallest suspension bridge in the world. Measuring approximately 12,828 feet (3,910 meters) in total length and with a center span of roughly 6,532 feet (1,991 meters), the bridge links the city of Kobe, on Honshu Island, with Matsuho, on Awaji Island. Its towers soar 928 feet (283 meters) in height. The bridge is part of an extensive project involving some 17 bridges forming three separate routes to link the islands of Honshu and Shikoku. Other Bridge AchievementsCompleted in 1962 over the Panama Canal was the Thatcher Ferry Bridge, a high-level bridge of unusual tied arch design. It has a main span of 1,128 feet (344 meters). Europe's highest bridge is the Europabrücke, opened in 1965, a plate-girder bridge that towers 623 feet (190 meters) over the Austrian Sill River. The San Mateo–Hayward Bridge across San Francisco Bay incorporates 5,500 feet (1,676 meters) of orthotropic construction. The 750-foot (229-meter) central section of the plate-girder bridge is the longest span of this type in the United States and the third longest in the world. In orthotropic construction a stiffened steel plate acts as both the road deck and the top flange of the main lengthwise girders. This design, first used in Europe, cuts costs and reduces the weight of the bridge. |