INTRODUCTION

A bridge is a structure designed to provide continuous passage over an obstacle. Bridges commonly carry highways, railroad lines, and pathways over obstacles such as waterways, deep valleys, and other transportation routes. Bridges may also carry water, support power cables, or house telecommunications lines.

Some special types of bridges are defined according to function. An overpass allows one transportation route, such as a highway or railroad line, to cross over another without traffic interference between the two routes. The overpass elevates one route to provide clearance to traffic on the lower level. An aqueduct transports water. Aqueducts have historically been used to supply drinking water to densely populated areas. A viaduct carries a railroad or highway over a land obstruction, such as a valley.

The earliest bridges were simple structures created by spanning a gap with timber or rope. Designs became more complex as builders developed new construction methods and discovered better materials. The stone arch was the first major advance in bridge design. It was used by the ancient Greeks, Etruscans, and Chinese. The Romans perfected arch design, using arches to build massive stone bridges throughout the Roman Empire. Stone arch construction remained the premier bridge design until the introduction of the steam locomotive in the early 19th century.

Between 1830 and 1880, as railroad building expanded throughout the world, bridge design and construction also evolved to carry these heavy vehicles over new obstacles. Designers experimented with a wide variety of bridge types and materials to meet the demand for greater heights, spans, and strength. Locomotives were heavier and moved faster than anything before, requiring stronger bridges. The basic beam bridge, a simple beam over a span, was strengthened by adding support piers underneath and by reinforcing the structure with elaborate scaffolding called a truss. During the period of railroad expansion, iron trusses replaced stone arches as the preferred design for large bridges.

In 1855 British inventor Sir Henry Bessemer developed a practical process for converting cast iron into steel. This process increased the availability of steel and lowered production costs considerably. The strength and lightness of steel revolutionized bridge building. In the late 19th century and the first half of the 20th century, many large-scale steel suspension bridges were constructed over major waterways. Also in the late 19th century, engineers began to experiment with concrete reinforced with steel bars for added strength. More recently, reinforced concrete has been combined with steel girders, which are solid beams that extend across a span. When the Interstate Highway System in the United States and similar road systems in other countries were constructed in the mid- to late 20th century, the steel-and-concrete girder bridge was one of the most commonly used bridge designs. The last three decades of the 20th century saw a period of large-scale bridge building in Europe and Asia. Current research focuses on using computers, instrumentation, automation, and new materials to improve bridge design, construction, and maintenance.

BRIDGE PLANNING AND CONSTRUCTION

New bridges are built either to replace old structures that no longer meet the demands of modern traffic or to cross obstacles on a new transportation route. Old bridges are replaced when repairs cannot be made economically or when traffic becomes too heavy for the old bridge. New transportation routes are built when traffic levels have outgrown the capacity of existing routes or simply to make it faster to get from one busy place to another. Often, new transportation routes are part of government programs to promote regional economic development.

In the United States, state and local transportation agencies determine where new bridges are needed and pay a small portion of the cost. The federal government usually pays for most of the construction expense, using money generated from taxes. Bridges funded by tax dollars are used free of charge. The few bridges for which a toll is charged to drivers for use are funded through the sale of bonds to raise money for construction. The money collected from the toll is used to pay back the bonds. The use of tolls and borrowing to finance bridge construction was more widespread in the past than it is today.

Design Selection...

Engineers must consider several factors when designing a bridge. They consider the distance to be crossed and the feature, such as a river, bay, or canyon, to be crossed. Engineers must anticipate the type of traffic and the amount of load the bridge will have to carry and the minimum span and height required for traffic traveling across and under the bridge. Temperature, environmental conditions, and the physical nature of the building site (such as the geometry of the approaches, the strength of the ground, and the depth to firm bedrock) also determine the best bridge design for a particular situation.

Once engineers have the data they need in order to design a bridge, they create a work plan for constructing it. Factors to be considered include availability of materials, equipment, and trained labor; availability of workshop facilities; and local transportation to the site. These factors, in combination with the funding and time available for bridge design and construction, are the major requirements and constraints on design decisions for a particular site.

Design Decisions...

There are four basic categories of design decisions: the type of bridge, the materials of which it will be made, the type of foundations that will support the structure, and the construction method to be used. Typically, several feasible choices exist in each category, and each option is evaluated in terms of convenience, appearance, endurance, and cost. Bridges must be convenient to build, use, and maintain. Appearance is important in gaining public approval, which is particularly critical for taxpayer-funded projects. Bridges must be designed to endure, as most structures can be expected to provide service for at least 50 to 100 years. Durability of materials and maintenance requirements are important considerations, as the true cost of a bridge is not simply the initial construction expense but the total cost of constructing and maintaining the structure throughout its service life. Good designs minimize total cost.

Bridge Type...

The bridge type (such as beam, arch, truss, and others) depends largely on the required dimensions for the bridge and the type of traffic to be carried. The required length and clearances needed by traffic are major considerations in bridge design. Many bridges are long enough to require several intermediate supports, or piers. The location of piers is usually a crucial factor, whether in water or on land.

Materials...

Materials historically used for bridge building include rope and other fibers, wood, stone and masonry, iron, steel, and concrete. Fiber, timber, stone, and masonry are still used occasionally, but steel and concrete are the materials used for most modern bridge building. Fiber rope is occasionally used for short pedestrian bridges. Timber is perceived as a rustic material and is sometimes used in public parks, on private property, or in other situations in which a natural or historic appearance is desirable. The strength and durability of timber are quite limited compared to those of steel and concrete. Therefore, timber is suitable only for short spans that carry minimal traffic loads. Stone and masonry are sometimes used as facing materials on concrete and steel bridges, if appearance is important enough to justify the additional expense.

When deciding between steel and concrete, designers evaluate the tradeoffs among weight, strength, and expense to determine which material is best for a particular bridge. Concrete is heavier than steel, but steel is much stronger. The major advantages of concrete are that it is considerably cheaper than steel and can be formed into a greater variety of shapes. For short bridges, the weight of material is not an important concern, and so concrete is an economical choice. However, as span increases, the weight of the structure grows substantially, and greater strength is needed to support the overall structure. Steel tends to be preferred for large bridges because less material has to be handled and supported during the construction process.

The distinction between steel and concrete is not absolute, as most steel bridges have concrete decks, and all concrete is reinforced with steel to provide greater tensile strength (resistance to pulling). Reinforced concrete is made by pouring concrete mix over steel bars or mesh. The concrete and metal bond as the mix hardens, producing a material in which the high tensile strength of steel is combined with the great compressive strength (ability to resist pushing or squeezing) of concrete. An alternative method of reinforcing concrete is prestressing. Prestressed concrete is made by pouring concrete over stretched and anchored steel strands. After the concrete has set, the anchors are released. As the steel tries to return to its original length, it compresses the concrete, resulting in a relatively lightweight, extremely strong material.

Foundations...

All bridge piers rest on foundations that transfer loads from the bridge structure into the ground. The foundations support the bridge, and their design is critical. Difficult conditions, such as deep water or soft ground, can make foundation construction complicated and expensive. In such circumstances designers may choose to decrease the number of piers by increasing span length. Of course, greater span lengths often require a more expensive bridge type, and therefore the tradeoffs must be evaluated carefully.

If the ground is very strong at a bridge site, a foundation is formed by pouring a simple concrete mat beneath each of the piers. If the soil is weak, it may be excavated down to bedrock, and the piers can then be built directly on the solid rock. Alternatively, a group of vertical posts, or piles, can be driven through the soil to bedrock, and piers can be built on top of the piles.

Construction Methods...

Bridges are erected using a variety of construction methods. Some techniques are associated with a particular bridge type, and care must be taken not to select a design that requires construction methods unsuitable for the site. Concrete and steel bridges are generally built using similar techniques, although concrete bridges are built in shorter sections than are steel bridges because of the greater weight of the material.

One of the simplest construction methods for bridges is to assemble a span away from the bridge site and then transport it to the site. The span can then be lifted into position as one piece. This method is most often suitable for small truss bridges or for the suspended span of a cantilever truss. Another approach is to use falsework, or temporary scaffolding, to support the incomplete parts of a bridge before they are joined and able to support themselves. The use of falsework is not always possible, owing to strong river currents, interference with river traffic, or great distances to the ground. If falsework is impractical, bridges can be constructed by the cantilever method. With this technique, a bridge is built piece by piece, with the entire structure supported from the section previously completed. Thus, the structure is self-supported throughout the entire construction process. The use of cantilever construction methods saves material and therefore expense, but it is very complex, as great care must be taken not to unbalance the structure during construction. Most arch bridges, and of course cantilever bridges, are built using cantilever methods.

The large towers and cable anchorages of suspension bridges are built without the use of falsework, and then the suspension cables are spun. Many individual wires are draped over the towers and are then squeezed together into a circular shape and clamped at intervals to create a main cable. Suspension wires are dropped from the cables to support the roadway, and the roadway is completed.

For all bridge types, underwater foundations require unique construction methods. Builders use cofferdams and caissons to obtain access to ground that is normally under water. A cofferdam is a temporary watertight enclosure constructed on the spot where a pier is to be built. A cofferdam usually consists of sheets of steel driven into the ground to create a walled chamber. The cofferdam is then pumped dry to expose the riverbed. Soil can be excavated to bedrock, or piles can be driven to create the pier foundation. The cofferdam is removed after the foundation and pier are constructed. A caisson is a large cylinder or box chamber that is sunk into the riverbed. The excavation and foundation work takes place within the submerged caisson. Some caissons are removed after construction, while others are left in place, filled with concrete, and used as part of a permanent foundation.

Safety ...

In bridge design, engineers strive to plan an economical structure that will safely transmit loads to the ground without collapsing or deforming excessively. Since it is difficult to predict the exact loading and circumstances that a bridge must withstand, all bridge designs include a substantial margin of safety. Design standards vary throughout the world, but all aim at ensuring that new bridges will provide many years of service and will maintain an adequate margin of safety against failure. Of course, the safety of a structure when it is first erected does not ensure that it will remain safe for all time. All structures require both periodic inspection and proper maintenance to keep them safe. Notable bridge failures include the collapse of the Firth of Tay Bridge in Scotland in 1879, the collapse of the QuÈbec Bridge in Canada while under construction in 1907, and the collapse of the Tacoma Narrows Bridge, nicknamed Galloping Gertie, in Washington State in 1940.

HISTORY

The different sizes and shapes of bridges encountered today reflect thousands of years of progress in engineering, technology, and building materials.

Early and Medieval Bridges...

In ancient times, builders would throw a log across a stream or use two vines or ropes (the upper for a handhold and the lower for a foothold) to create a crossing. The earliest rudimentary arches (built from 4000 to 2000 BC) consisted of stones balanced on top of one another. The ancient Romans perfected stone arch design and were the first to build large-scale bridges, many of which still stand today. The largest remaining ancient Roman aqueduct, the Pont du Gard in southern France, is 270 m (886 ft) long and consists of three tiers of semicircular stone arches. The Romans built the Pont du Gard in the late 1st century BC or the early 1st century AD. The aqueduct stands approximately 47 m (155 ft) tall, and the longest arch spans 24 m (80 ft).

The ancient Chinese also built many notable bridges. In the 7th century AD, bridge designer Li Chun built the Anji Bridge south of Beijing using a stone arch built of massive limestone wedges reinforced with iron. The innovative main arch of the Anji curves in a shallow arc or segment of a circle, rather than the half circle preferred by Roman engineers at the time. The Anji Bridge, which spans 37 m (121 ft), predates any comparable development in Europe by several hundred years.

From ancient times through the 16th century, designers made few engineering advances. Masonry arch construction remained the premier choice for bridge design. In the Middle Ages in Europe, religious orders administered bridge construction. Considered pious works, bridges often had chapels and were decorated with effigies of saints. Inhabited bridges were developed during the Renaissance. These bridges were exemplified by the Ponte Vecchio, 100 m (330 ft) long, in Florence, Italy, designed by Taddeo Gaddi in 1345; and by the Rialto Bridge, 27 m (89 ft) long, in Venice, Italy, designed by Antonio da Ponte in 1591. Shops were built directly on the roadway of these bridges, and rents were used to finance new public works.

Iron and Steel Bridges ...

The widespread use of iron in the 18th century and the introduction of the steam locomotive in the 19th century encouraged rapid innovation in bridge design. Engineers made more advances in the first half of the 19th century than they had in the previous 1,800 years. As the railroad industry developed, bridges rapidly increased in height, span, strength, and numbers. Iron was plentiful, cheap, much stronger than wood, and more flexible than stone. The Ironbridge at Coalbrookdale, England, completed in 1779, was the first major structure to be constructed entirely of iron. Designed by Abraham Darby III and Thomas Pritchard, the arched structure spans about 30 m (about 100 ft). Scottish engineer Thomas Telford used wrought iron and limestone to design the Menai Suspension Bridge in Wales in 1826. This bridge was the worldís first major suspension bridge. It spanned 176 m (579 ft). During the first half of the 19th century, iron became the premier building material. In addition, truss designs were developed to provide the additional strength needed to bear the massive weight of railroad trains.

The major disadvantage of iron, namely, low tensile strength, was overcome in the mid-1850s, when the Bessemer process of making steel (an alloy of iron and carbon) was developed. The first major structure built entirely of steel was the cantilevered Forth Bridge in Scotland, completed in 1890. Its two record-setting spans of 521 m (1,710 ft) were the longest in existence until 1917. The arched Eads Bridge over the Mississippi River at St. Louis, Missouri, designed by James Eads and completed in 1874, was the first steel bridge in the United States. The Eads Bridge has three main spans. The center span is 160 m (520 ft) long, and the spans on either side are each 153 m (502 ft) in length. At the time the Eads Bridge was built, it was the longest structure in the United States. By 1890 the strength and lightness of steel had made it the material of choice for bridge building.

Suspension Bridges...

The Roebling family pioneered the use of steel in suspension bridges. John Augustus Roebling, a German-born engineer who emigrated to the United States in 1831, is considered the father of modern suspension-bridge design. His major contribution was the development of construction techniques to spin wire cables. He was the first to use cables and stiffening trusses in suspension bridges. Roebling designed the Cincinnati Bridge, over the Ohio River at Cincinnati, Ohio. It was built by his son Washington Roebling in 1866. The Cincinnati Bridge has a span of 322 m (1,057 ft).

John and Washington Roebling also designed and built the Brooklyn Bridge, which was the worldís longest suspension bridge at the time of its completion in 1883, having a main span of 486 m 31 cm (1,595 ft 6 in). The completion of the Brooklyn Bridge marked the beginning of an 80-year period of large-scale suspension-bridge design in the United States. That period ended in 1964 with the completion of the Verrazano-Narrows Bridge in New York City. The Golden Gate Bridge in San Francisco, California, is perhaps the best-known landmark of this remarkable era in bridge building. Completed in 1937, the Golden Gate Bridge has a main span of 1,280 m (4,200 ft).

Introduction of Concrete ...

The introduction of concrete as a building material represented a major chapter in the history of bridge building. Although the ancient Romans had used concrete, the knowledge of this material virtually disappeared during the Middle Ages and was not rediscovered until the late 18th century. The first modern concrete bridge was a solid concrete bridge, 12 m (39 ft) long, built over the Garonne Canal at Grisoles, France, in 1840.

All early concrete bridges used arched designs by necessity because concrete has great compressive strength but is very weak in tension. Until the invention of metal reinforcement, which adds strength in tension, the arch was the only feasible shape for structures made entirely of concrete. Reinforced concrete emerged simultaneously in Germany, the United States, England, and France between 1870 and 1900. Swiss engineer Robert Maillart became a celebrated designer of reinforced-concrete bridges in the first half of the 20th century, producing extremely innovative designs based on the unique engineering properties of reinforced concrete. The last half of the 20th century saw the construction of major reinforced-concrete structures, such as the Lake Maracaibo Bridge in Venezuela, designed by Italian engineer Riccardo Morandi. This prestressed-concrete cable-stayed bridge is 8 km (5 mi) long.

Recent Designs...

The first modern cable-stayed bridge featured a span of 183 m (600 ft). German engineers constructed this bridge in Sweden in 1956. Thereafter, Germany led the field in developing this type of bridge. Cable-stayed bridges can be constructed in an infinite variety of shapes and are one of the most popular long-span bridge designs. Notable examples include the Severn II Bridge in Bristol, England, completed in 1996 with a span of 456 m (1,496 ft), and the Sunshine Skyway Bridge in Tampa, Florida, completed in 1987 with a span of 366 m (1,200 ft).

At the end of the 20th century, construction was under way on a number of monumental bridges, most notably in Northern Europe and in Asia. Engineers are combining aspects of both suspension and cable-stayed bridges into new bridge designs that will someday span once-impossible distances of more than 3,000 m (10,000 ft). At the same time, researchers are investigating new methods of preserving and renewing bridges, so that existing structures will last longer and provide greater service and fewer new structures will have to be built in the future.