Bridge

I

Introduction

Bridge, structure providing continuous passage over a body of water, roadway, or valley. Generally a bridge carries a pathway, road, or railway, but it may carry power-transmission lines or pipelines. Bridges that carry canals or water conduits are called aqueducts. A bridge built over dry land or over a wide valley and consisting of a number of small spans is usually referred to as a viaduct; the term overpass is applied to relatively short bridges crossing highways and railways. A low road bridge over a swamp, shallow lake, or bay and consisting of many short spans is frequently referred to as a causeway.

II

Early Bridges

The earliest bridges were probably formed by laying one or more logs across a brook or by stretching ropes or cables across a narrow valley. Such bridges are still employed. The single-span bridge is an outgrowth of these elementary forms. The stepping-stone crossing, improved by logs laid on the stones to connect them, is the prototype of the multiple-span bridge. Wooden piles driven into a river bottom to form bridge supports made it possible for the log or beam structure to span wider streams. Such trestle bridges are still used extensively to cross valleys or streams where they will not interfere with shipping. Use of stone piers as intermediate supports for wooden members marked another advance in the construction of wooden-beam bridges. The use of boats instead of fixed piers produced the pontoon bridge. Wooden-beam bridges appear to have been the most common type known to the ancients, although according to tradition a brick-arch bridge was built about 1800 bc in Babylon. Other forms, such as simple suspension and cantilever bridges, are known to have been used in ancient India, China, and Tibet. Pontoon bridges were used in the military expeditions of the Persian monarchs Darius I and Xerxes I.

The Romans built many timber-trestle bridges, one of which is described in detail in the Commentaries of Julius Caesar. Surviving Roman bridges, however, usually have a level road supported on one or more semicircular stone arches. Early examples are the bridge at Martorell near Barcelona, Spain, built about 219 bc, and the Ponte di Augusto at Rimini, Italy, from the 1st century bc. The Pont du Gard at Nîmes, France, which has three tiers of arches rising 47.2 m (155 ft) above the Gard River, spans a distance of 260.6 m (855 ft) and is probably the best-preserved example of a great Roman aqueduct; it was built in the 1st century bc. Semicircular arches were succeeded by pointed arches, generally of small span. Modern arches are often segmental or semielliptical forms that afford wide span and free waterway with moderate elevation. The bridge (1803) across the River Tweed at Kelso, Scotland, was an important semielliptical-arch bridge designed by the British engineer John Rennie.

A beam, or girder, bridge is limited in span by the strength of its girders. This limitation is overcome by assembling a system of supporting members in triangles above the horizontal span girders to form trusses. Leonardo da Vinci sketched truss bridges, and the Italian architect Andrea Palladio probably built several. Two truss bridges were built in Switzerland about 1760. Truss-bridge construction, however, did not develop on a large scale until after 1840. In the United States the use of wooden trusses with iron tie-rods led to a combination cast- and wrought-iron construction about 1850 and, later, to steel trusses.

Suspension bridges, with chains laid over towers on the banks, supporting a level roadway from the chains by hanger rods, were built around 1800 in the United States and about ten years later in Britain. In 1816 the first wire-cable suspension bridge was built across the Schuylkill River at Philadelphia.

III

Modern Bridges

Modern bridges are usually identified by the construction principle used, such as cantilever, suspension, steel arch, concrete arch, masonry arch, steel truss, steel girder, or pontoon. Where provision must be made for the passage of shipping under the bridge and where it is impracticable to build the bridge high enough for complete clearance, a movable span is constructed. Notable examples of the various types of bridges are listed below.

A

Cantilever Bridges

The cantilever bridge is characterized by spans that are supported not at the ends but near the centre of the girder or truss. The Forth Bridge, across the Firth of Forth at Queensferry, Scotland, is a steel railway bridge with two main spans of 521.2 m (1,710 ft) each, erected between 1882 and 1890 by engineers John Fowler and Benjamin Baker. Total length of the bridge is more than 1.6 km (1 mi). The Quebec Bridge across the St Lawrence River, completed in 1917, has a main span of 548.6 m (1,800 ft); it carries a double-track railway and a roadway. The Carquinez Strait Bridge, near San Francisco, was completed in 1927; it has two 335.3-m (1,100-ft) spans plus 152.4 m (500 ft) anchor spans and was designed to resist damage from earthquakes. The Haora Bridge over the Hugli River at Kolkata, with a main span of 457.2 m (1,500 ft), was opened in 1943. The Greater New Orleans Bridge (1958) over the Mississippi River has a span of 480.1 m (1,575 ft).