II.

Types of Bridges

Bridge designs differ in the way they support loads. These loads include the weight of the bridges themselves, the weight of the material used to build the bridges, and the weight and stresses of the vehicles crossing them. There are basically eight common bridge designs: beam, cantilever, arch, truss, suspension, cable-stayed, movable, and floating bridges. Combination bridges may incorporate two or more of the above designs into a bridge. Each design differs in appearance, construction methods and materials used, and overall expense. Some designs are better for long spans. Beam bridges typically span the shortest distances, while suspension and cable-stayed bridges span the greatest distances.

A.

Beam Bridges

Beam bridges represent the simplest of all bridge designs. A beam bridge consists of a rigid horizontal member called a beam that is supported at both ends, either by a natural land structure, such as the banks of a river, or by vertical posts called piers. Beam bridges are the most commonly used bridges in highway construction. Single-piece, rolled-steel beams can support spans of 15 to 30 m (50 to 100 ft). Heavier, reinforced beams and girders are used for longer spans.

B.

Cantilever Bridges

Cantilever bridges are a more complex version of the beam-bridge design. In a cantilever design, a tower is built on each side of the obstacle to be crossed, and the bridge is built outward, or cantilevered, from each tower. The towers support the entire load of the cantilevered arms. The arms are spaced so that a small suspended span can be inserted between them. The cantilevered arms support the suspended span, and the downward force of the span is absorbed by the towers.

Cantilever bridges are self-supporting during construction. They are often used in situations in which the use of scaffolding or other temporary supports would be difficult. The Forth Bridge, a railway bridge across the Firth of Forth in Queensferry, Scotland, has two main spans of 521 m (1,710 ft) each. The Hāora (Howrah) Bridge in Calcutta (now Kolkata), India, was opened in 1943, with a main span of 457 m (1,500 ft). The Québec Bridge across the St. Lawrence River in Canada has a span of 549 m (1,800 ft).

C.

Arch Bridges

Arch bridges are characterized by their stability. In an arch, the force of the load is carried outward from the top to the ends of the arch, where abutments keep the arch ends from spreading apart. Arch bridges have been constructed of stone, brick, timber, cast iron, steel, and reinforced concrete.

Steel and concrete arches are particularly well suited for bridging ravines or chasms with steep, solid walls. The New River Gorge Bridge in West Virginia is the longest arch bridge, spanning a gap of 518 m (1,700 ft). Other long arch bridges include the Bayonne Bridge between New York and New Jersey, and the Sydney Harbour Bridge in Australia, with main spans of 504 m (1,652 ft) and 503 m (1,650 ft), respectively.

D.

Truss Bridges

Truss bridges utilize strong, rigid frameworks that support these bridges over a span. Trusses are created by fastening beams together in a triangular configuration. The truss framework distributes the load of the bridge so that each beam shares a portion of the load. Beam, cantilever, and arch bridges may be constructed of trusses. Truss bridges can carry heavy loads and are relatively lightweight. They are also inexpensive to build. The Astoria Bridge over the Columbia River in Oregon has a span of 376 m (1,232 ft).

E.

Suspension Bridges

Suspension bridges consist of two large, or main, cables that are hung (suspended) from towers. The main cables of a suspension bridge drape over two towers, with the cable ends buried in enormous concrete blocks known as anchorages. The roadway is suspended from smaller vertical cables that hang down from the main cables. In some cases, diagonal cables run from the towers to the roadway and add rigidity to the structure. The main cables support the weight of the bridge and transfer the load to the anchorages and the towers.

Suspension bridges are used for the longest spans. The Brooklyn Bridge, which was the world’s longest suspension bridge at the time of its completion in 1883, crosses the East River in New York City and has a main span of 486 m 31 cm (1,595 ft 6 in). The Akashi Kaikyo Bridge between Honshū and Awaji Island in Japan was completed in 1998, with a span of 1,991 m (6,532 ft). While suspension bridges can span long distances, this design has a serious drawback: It is very flexible, and traffic loading may cause large deflections, or bending, in the bridge roadway. Suspension design is rarely used for railroad bridges, because trains are heavier and can travel faster than highway traffic.

F.

Cable-Stayed Bridges

Cable-stayed bridges represent a variation of the suspension bridge. Cable-stayed bridges have tall towers like suspension bridges, but in a cable-stayed bridge, the roadway is attached directly to the towers by a series of diagonal cables. A cable-stayed bridge is constructed in much the same way as a suspension bridge is, but without the main cables.

Cable-stayed designs are used for intermediate-length spans. Advantages a cable-stayed bridge has over a standard suspension bridge include speed of construction and lower cost, since anchorages are not necessary. There are no massive cables, as with suspension bridges, making cable repair or replacement simpler. The Pont de Normandie (Normandy Bridge) over the Seine River near Le Havre in France opened in 1995, with a span length of 856 m (2,808 ft).

G.

Movable Bridges

Movable bridges make up a class of bridge in which a portion of the bridge moves up or swings out to provide additional clearance beneath the bridge. Movable bridges are usually found over heavily traveled waterways. The three most common types of movable bridge are the bascule (drawbridge), vertical-lift, and swing bridges. Modern bascule bridges usually have two movable spans that rise upward, opening in the middle. A vertical-lift bridge consists of a rigid deck frame held between two tall towers. The bridge opens by hoisting the entire bridge roadway upward between the towers in an elevator-like fashion. Swing bridges are mounted on a central pier and open by swinging to one side, allowing ships to pass.

Movable bridges are generally constructed over waterways where it is either impractical or too costly to build bridges with high enough clearances for water traffic to pass underneath. Bascule bridges are used for short spans. A bascule bridge over the Black River in Lorain, Ohio, has a length of 102 m (333 ft). Vertical-lift bridges are useful for longer spans, but they must be built so they can be lifted high enough for tall ships to pass underneath. The vertical-lift bridge over Arthur Kill between Staten Island in New York City and New Jersey has a span of 170 m (558 ft) and can be raised 41 m (135 ft) above the water. Swing bridges have the advantage of not limiting the height of passing vessels, but they do restrict the horizontal clearance, or width, of passing ships. The longest swing-bridge span is that of a railroad and highway bridge crossing the Mississippi River at Fort Madison, Iowa. This bridge has a span of 166 m (545 ft).

H.

Floating Bridges

Floating bridges are formed by fastening together sealed, floating containers called pontoons and placing a roadbed on top of them. A pontoon typically contains many compartments so that if a leak occurs in one compartment, the pontoon will not sink. Some floating bridges are constructed using boats or other floating devices rather than pontoons.

Floating bridges were originally developed and are most widely used as temporary structures for military operations. For everyday use, floating bridges are popular when deep water, bad riverbed conditions, or other conditions make it difficult to construct traditional bridge piers and foundations. A concrete-pontoon bridge carries a highway across Lake Washington, near Seattle, Washington. It consists of 25 floating sections bolted together and anchored in place and a span that can be opened to permit the passage of large ships. The floating section of the bridge is 2.3 km (1.4 mi) long.

I.

Combination Bridges

Combination bridges include crossings consisting of several types of bridges or both bridges and tunnels. For example, the Chesapeake Bay Bridge-Tunnel in Virginia includes two tunnels that are each 1.6 km (1.0 mi) long along its 28 km (17 mi) length from shore to shore. The Triborough Bridge in New York City is actually a network of bridges connecting the boroughs of Queens, Manhattan, and the Bronx. These bridges meet over Randall’s Island. Seven truss spans stretch over Bronx Kills, and three truss spans and a vertical lift extend over the Harlem River. A viaduct and a suspension bridge also make up part of the Triborough Bridge.