Nearly 590,000 roadway bridges span waterways, dryland depressions, other roads, and railroads throughout the United States. The most dramatic bridges use complex systems like arches, cables, or triangle-filled trusses to carry the roadway between majestic columns or towers. However, the work-horse of the highway bridge system is the relatively simple and inexpensive concrete beam bridge.
Also known as a girder bridge, a beam bridge consists of a horizontal slab supported at each end. Because all of the weight of the slab (and any objects on the slab) is transferred vertically to the support columns, the columns can be less massive than supports for arch or suspension bridges, which transfer part of the weight horizontally.
A simple beam bridge is generally used to span a distance of 250 ft (76.2 m) or less. Longer distances can be spanned by connecting a series of simple beam bridges into what is known as a continuous span. In fact, the world's longest bridge, the Lake Pontchartrain Causeway in Louisiana, is a pair of parallel, two-lane continuous span bridges almost 24 mi (38.4 km) long. The first of the two bridges was completed in 1956 and consists of more than 2,000 individual spans. The sister bridge (now carrying the north-bound traffic) was completed 13 years later; although it is 228 ft longer than the first bridge, it contains only 1,500 spans.
A bridge has three main elements. First, the substructure (foundation) transfers the loaded weight of the bridge to the ground; it consists of components such as columns (also called piers) and abutments. An abutment is the connection between the end of the bridge and the earth; it provides support for the end sections of the bridge. Second, the superstructure of the bridge is the horizontal platform that spans the space between columns. Finally, the deck of the bridge is the traffic-carrying surface added to the superstructure.
Prehistoric man began building bridges by imitating nature. Finding it useful to walk on a tree that had fallen across a stream, he started to place tree trunks or stone slabs where he wanted to cross streams. When he wanted to bridge a wider stream, he figured out how to pile stones in the water and lay beams of wood or stone between these columns and the bank.
The first bridge to be documented was described by Herodotus in 484 B.C. It consisted of timbers supported by stone columns, and it had been built across the Euphrates River some 300 years earlier.
Most famous for their arch bridges of stone and concrete, the Romans also built beam bridges. In fact, the earliest known Roman bridge, constructed across the Tiber River in 620 B.C. , was called the Pons Sublicius because it was made of wooden beams (sublicae). Roman bridge building techniques included the use of cofferdams while constructing columns. They did this by driving a circular arrangement of wooden poles into the ground around the intended column location. After lining the wooden ring with clay to make it watertight, they pumped the water out of the enclosure. This
Bridge building began the transition from art to science in 1717 when French engineer Hubert Gautier wrote a treatise on bridge building. In 1847, an American named Squire Whipple wrote A Work on Bridge Building, which contained the first analytical methods for calculating the stresses and strains in a bridge. "Consulting bridge engineering" was established as a specialty within civil engineering in the 1880s.
Further advances in beam bridge construction would come primarily from improvements in building materials.
Most highway beam bridges are built of concrete and steel. The Romans used concrete made of lime and pozzalana (a red, volcanic powder) in their bridges. This material set quickly, even under water, and it was strong and waterproof. During the Middle Ages in Europe, lime mortar was used instead, but it was water soluble. Today's popular Portland cement, a particular mixture of limestone and clay, was invented in 1824 by an English bricklayer named Joseph Aspdin, but it was not widely used as a foundation material until the early 1900s.
Concrete has good strength to withstand compression (pressing force), but is not as strong under tension (pulling force). There were several attempts in Europe and the United States during the nineteenth century to strengthen concrete by embedding tension-resisting iron in it. A superior version was developed in France during the 1880s by Francois Hennebique, who used reinforcing bars made of steel. The first significant use of reinforced concrete in a bridge in the United States was in the Alvord Lake Bridge in San Francisco's Golden Gate Park; completed in 1889 and still in use today, it was built with reinforcing bars of twisted steel devised by designer Ernest L. Ransome.
The next significant advance in concrete construction was the development of prestressing. A concrete beam is prestressed by pulling on steel rods running through the beam and then anchoring the ends of the rods to the ends of the beam. This exerts a compressive force on the concrete, offsetting tensile forces that are exerted on the beam when a load is placed on it. (A weight pressing down on a horizontal beam tends to bend the beam downward in the middle, creating compressive forces along the top of the beam and tensile forces along the bottom of the beam.)
Prestressing can be applied to a concrete beam that is precast at a factory, brought to the construction site, and lifted into place by a crane; or it can be applied to cast-in-place concrete that is poured in the beam's final location. Tension can be applied to the steel wires or rods before the concrete is poured (pretensioning), or the concrete can be poured around tubes containing untensioned steel to which tension is applied after the concrete has hardened (postensioning).
Each bridge must be designed individually before it is built. The designer must take into account a number of factors, including the local topography, water currents, river ice formation possibilities, wind patterns, earthquake potential, soil conditions, projected traffic volumes, esthetics, and cost limitations.
In addition, the bridge must be designed to be structurally sound. This involves analyzing
Since the late 1960s, the value of redundancy in design has been widely accepted. This means that a bridge is designed so the failure of any one member will not cause an immediate collapse of the entire structure. This is accomplished by making other members strong enough to compensate for a damaged member.
Because each bridge is uniquely designed for a specific site and function, the construction process also varies from one bridge to another. The process described below represents the major steps in constructing a fairly typical reinforced concrete bridge spanning a shallow river, with intermediate concrete column supports located in the river.
Example sizes for many of the bridge components are included in the following description as an aid to visualization. Some have been taken from suppliers' brochures or industry standard specifications. Others are details of a freeway bridge that was built across the Rio Grande in Albuquerque, New Mexico, in 1993. The 1,245-ft long, 10-lane wide bridge is supported by 88 columns. It contains 11,456 cubic yards of concrete in the structure and an additional 8,000 cubic yards in the pavement. It also contains 6.2 million pounds of reinforcing steel.
The design and construction of a bridge must meet standards developed by several agencies including the American Association of State Highway and Transportation Officials, the American Society for Testing and Materials, and the American Concrete Institute. Various materials (e.g., concrete batches) and structural components (e.g., beams and connections) are tested as construction proceeds. As a further example, on the Albuquerque bridge project, static and dynamic strength tests were conducted on a sample column foundation that was constructed at the site, and on two of the production shafts.
Numerous government agencies and industry associations sponsor and conduct research to improve materials and construction techniques. A major goal is the development of lighter, stronger, more durable materials such as reformulated, high-performance concrete; fiber-reinforced, polymer composite materials to replace concrete for some components; epoxy coatings and electro-chemical protection systems to prevent corrosion of steel rebar; alternative synthetic reinforcing fibers; and faster, more accurate testing techniques.
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Hardesty, E. R., H. W. Fischer, R. W. Christie, and B. Haber. "Bridge." In McGraw-Hill Encyclopedia of Science & Technology. New York: McGraw-Hill Book Company, 1987, pp. 49-58.
Troitsky, M.S. Planning and Design of Bridges. New York: John Wiley & Sons, Inc., 1994.
"General Information About Concrete Pavement." American Concrete Pavement Association. http://www.pavement.com/general/conc-info.html (24 Feb. 1998).
"Beam Bridge." Nova Online "Super Bridge." November 1997. http://www.pbs.org/wghb/nova/bridge/meetbeam.html (24 Feb. 1998).
— Loretta Hall