Bridges

Last updated: July 8, 2010.

Bridges are massive static structures that carry roads and railroads over rivers, canyons, and other obstacles. Different types of bridges are chosen depending on what they will carry, what they will cross, and the geological nature of Earth beneath them. Often miles long and taking years to construct, bridges are triumphs of engineering over the environment—the true wonders of the modern world.

Photo: One of the world's greatest bridges. Over 150 years after it was completed in 1859, Isambard Kingdom Brunel's amazing Royal Albert Bridge still carries railroad trains 30m (100ft) over the River Tamar, separating Cornwall and Devon in England. You can probably just about see from this picture that it's a kind of suspension bridge with added support from those tubular trusses at the top. A modern suspension bridge was built alongside in the 1960s to ferry cars across too (see the photos below).

How bridges support forces

From an engineering viewpoint, the purpose of a bridge is not just to span some physical obstacle, but to distribute forces (loads) evenly through a few key points in a structure using tension (pulling forces) and compression (pushing forces). With cables, girders, and arches, and materials such as reinforced concrete and steel, engineers seek to channel the weight of the bridge and its contents onto abutments (the supports at either end) and piers (the supports in the middle) resting on firm bedrock or caissons (reinforced boxes filled with concrete) sunk deep into softer ground. You can read more about how forces are balanced in structures in our article on how buildings work.

The evolution of engineering techniques has allowed bridges to span greater and greater distances. Stone bridges, often built by monks, became popular in medieval times, but could only be made so big without collapsing under their own weight. The first iron bridge constructed at Coalbrookdale in Shropshire, England, in 1779 led to many great iron and steel bridges in the 19th century. One of the first modern suspension bridges, New York’s Brooklyn Bridge of 1883, was considered an engineering masterpiece with a span of 1595 ft (486 m). Today’s longest suspension bridge, the Akashi Kaikyo in Japan, completed in 1998, is eight times longer at 2.4 miles (3.9 km)

Types of bridges

Different bridges distribute loads in different ways and are suited to different kinds of environments. Simple beam bridges can span only limited distances without extra support. Once built from wood, today they are constructed from box girders (reinforced steel tubes that are hollow inside). Arch bridges are stronger than beams and are used in places where supporting piers cannot be easily constructed; the arch may be built either above or beneath the bridge deck. Cantilever bridges consist of two beams or girders (cantilevers) stretching from the abutments that support a central beam between them. The piers are built in shallow water, but the central beam may span a deep-water channel. Suspension bridges and cable-stayed bridges are more visually attractive than cantilevers and can span much longer distances. Other types of bridges include the permanently floating pontoon bridge, which can carry light traffic over water or soft ground.

Beam The weight of the bridge and everything that travels over it is supported by a beam (or steel girders in modern bream bridges) which, itself, is supported by vertical pillars at either side. Beams are explained further in our article on how buildings work. Photo: A beam bridge carrying a railway line over a road in Dorset, England.
Arch The arch is in compression and pushes its load out horizontally and vertically into the supporting ground. Photo: A stone arch bridge over the River Avon in Bath, England.
Cantilever A cantilever bridge is similar to a beam bridge. Loads are carried vertically to the piers and abutments and evened out by a framework of metal trusses. Photo: The Huey P. Long cantilevered bridge on the Mississippi River near New Orleans. Picture courtesy: Defense Visual Information Centre/US Navy.
Suspension The bridge deck is reinforced with girders and trusses underneath. It hangs from many, relatively thin, vertical suspender cables supported by the longer, thicker main cables that stretch between the vertical piers. At each end of the bridge, side cables stretch out to the abutments (where the bridge is built into the ground). All the cables are in tension. Photo: The Tamar Bridge, completed in 1961, spans the River Tamar, the boundary between Cornwall and Devon, England, alongside Brunel's 1859 rail bridge (from which this photo was taken).
Cable-stayed A cable-stayed bridge reduces the bending in the mid-section of the deck and, unlike a suspension bridge, can be supported from a single pier. What's the difference between a suspension bridge and a cable-stayed bridge? The deck of a suspension bridge is supported indirectly from the main cables via suspender cables that hang vertically downward. In a cable-stayed bridge, a fan of diagonal cables supports the deck directly from the piers. Photo: The Aurthur Ravenel Jr. Bridge cable-stayed bridge in Charleston S.C. Picture by Richard Rose courtesy of US Air Force and Defense Imagery.
Pontoon The load in a pontoon bridge pushes vertically down onto floating boats, which limits the weight this type of bridge can carry. Photo: A pontoon bridge laid across the Euphrates River in Iraq. Photo by Kevin C. Quihuis, Jr. courtesy of US Marine Corps and Defense Imagery.
Steel arch A steel-arch bridge is like a cross between an arch bridge and a suspension bridge, with the bridge deck suspended from an overhead steel arch. This is also known as a compression arch suspended-deck bridge or through-arch bridge. Photo: Sydney Harbor Bridge during the 2000 Olympics. Photo by Robert A. Whitehead courtesy of US Air Force and Defense Imagery.