Bridge

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Bridge, structure that spans horizontally between supports, whose function is to carry vertical loads. The prototypical bridge is quite simple—two supports holding up a beam—yet the engineering problems that must be overcome even in this simple form are inherent in every bridge: the supports must be strong enough to hold the structure up, and thespan between supports must be strong enough to carry the loads. Spans are generally made as short as possible; long spans are justified where good foundations are limited—for example, over estuaries with deep water.

Park Bridge in Kicking Horse CanyonTime-lapse video of construction of the Park Bridge in Kicking Horse Canyon, British Columbia, Canada. The bridge, which is part of the Trans-Canada Highway, opened in 2007.EarthCam

The New Castle, built by Richard Trevithick in 1803, the first locomotive to do actual work.

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All major bridges are built with the public’s money. Therefore, bridge design that best serves the public interest has a threefold goal: to be as efficient, as economical, and as elegant as is safely possible.Efficiency is a scientific principle that puts a value on reducing materials while increasing performance. Economy is a social principle that puts value on reducing the costs of construction and maintenance while retaining efficiency. Finally, elegance is a symbolic or visual principle that puts value on the personal expression of the designer without compromising performance or economy. There is little disagreement over whatconstitutes efficiency and economy, but the definition of elegance has always been controversial.

Modern designers have written about elegance oraesthetics since the early 19th century, beginning with the Scottish engineerThomas Telford. Bridges ultimately belong to the general public, which is the final arbiter of this issue, but in general there are three positions taken by professionals. The first principle holds that the structure of a bridge is the province of theengineer and that beauty is fully achieved only by the addition ofarchitecture. The second idea, arguing from the standpoint of pure engineering, insists that bridges making the most efficient possible use of materials are by definition beautiful. The third case holds that architecture is not needed but that engineers must think about how to make the structure beautiful. This last principle recognizes the fact that engineers have many possible choices of roughly equal efficiency and economy and can therefore express their ownaesthetic ideas without adding significantly to materials or cost.

Thomas Telford's suspension bridge across the River Conwy, leading to Conwy Castle, Conwy county borough, Wales.J. Allan Cash Photolibrary/Encyclopædia Britannica, Inc.

Generally speaking, bridges can be divided into two categories: standard overpass bridges or unique-design bridges over rivers, chasms, or estuaries. This article describes features common to both types, but it concentrates on the unique bridges because of their greater technical, economic, and aesthetic interest.

The elements of bridge design

Basic forms

There are six basic bridge forms: the beam, the truss, the arch, the suspension, the cantilever, and the cable-stay.

Beam

Thebeam bridge is the most common bridge form. Abeam carries vertical loads by bending. As the beam bridge bends, it undergoes horizontal compression on the top. At the same time, the bottom of the beam is subjected to horizontaltension. The supports carry the loads from the beam bycompression vertically to thefoundations.

When a bridge is made up of beams spanning between only two supports, it is called asimply supported beam bridge. If two or more beams are joined rigidly together over supports, the bridge becomescontinuous.

Truss

A single-spantruss bridge is like a simply supported beam because it carries vertical loads by bending. Bending leads to compression in the top chords (or horizontal members), tension in the bottom chords, and either tension or compression in the vertical and diagonal members, depending on their orientation. Trusses are popular because they use a relatively small amount of material to carry relatively large loads.

Arch

Thearch bridge carries loads primarily by compression, which exerts on the foundation both vertical and horizontal forces. Arch foundations must therefore prevent both vertical settling and horizontal sliding. In spite of the more complicated foundation design, the structure itself normally requires less material than a beam bridge of the same span.

Suspension

Asuspension bridge carries vertical loads through curved cables in tension. These loads are transferred both to thetowers, which carry them by vertical compression to the ground, and to the anchorages, which must resist the inward and sometimes vertical pull of the cables. The suspension bridge can be viewed as an upside-down arch in tension with only the towers in compression. Because the deck is hung in the air, care must be taken to ensure that it does not move excessively under loading. The deck therefore must be either heavy or stiff or both.

Cantilever

A beam is said to be cantilevered when it projects outward, supported only at one end. Acantilever bridge is generally made with three spans, of which the outer spans are both anchored down at the shore and cantilever out over the channel to be crossed. The central span rests on thecantilevered arms extending from the outer spans; it carries vertical loads like a simply supported beam or a truss—that is, by tension forces in the lower chords and compression in the upper chords. The cantilevers carry their loads by tension in the upper chords and compression in the lower ones. Inner towers carry those forces by compression to the foundation, and outer towers carry the forces by tension to the far foundations.

Cable-stay

Cable-stayed bridges carry the vertical main-span loads by nearly straight diagonalcables in tension. The towers transfer the cable forces to the foundations through vertical compression. The tensile forces in the cables also put the deck into horizontal compression.

Materials

The four primary materials used for bridges have been wood, stone, iron, and concrete. Of these, iron has had the greatest effect on modern bridges. From iron, steel is made, and steel is used to make reinforced andprestressed concrete. Modern bridges are almost exclusively built with steel,reinforced concrete, and prestressed concrete.

Wood andstone

Wood is relatively weak in both compression and tension, but it has almost always been widely available and inexpensive. Wood has been used effectively for small bridges that carry light loads, such as footbridges. Engineers now incorporate laminated wooden beams and arches into some modern bridges.

Stone is strong in compression but weak in tension. Its primary application has been inarches,piers, and abutments.

Iron andsteel

The firstiron used during theIndustrial Revolution wascast iron, which is strong in compression but weak in tension.Wrought iron, on the other hand, is as strong in compression as cast iron, but it also has much greatertensile strength.Steel is an even further refinement of iron and is yet stronger, superior to any iron in both tension and compression. Steel can be made to varying strengths, some alloys being five times stronger than others. The engineer refers to these ashigh-strength steels.