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Prince Albert Railroad Bridge over the River Tamar, Cornwall, England, by Isambard Kingdom Brunel.

Bridges and tunnels

by Chris Woodford. Last updated: May 28, 2014.

Over, under, or straight through the middle? That's the simple-sounding question that's challenged every great engineer since ancient times. We like highways and railroads to be straight and level, but Earth's bumps and wiggles make that kind of construction an amazing challenge. How do you take a highway through a mountain range or a railroad over the sea? The simple answer is that you use a bridge or a tunnel. Sounds easy, perhaps, but which type of bridge do you use? If you're digging a tunnel, how do you keep it going in a perfectly straight line? What kind of cutting tools do you use? Let's take a closer look and find out more!

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 thick, tubular, "lenticular" trusses at the top. The vertical ties running from the top of the trusses down to the lower arches mean that the bridge does not push outward on its supporting towers. A modern suspension bridge was built alongside in the 1960s to ferry cars across too (see the photos below).

Bridges

Compression and tension forces on four different types of bridges: beam, arch, suspension, and cable-stayed

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.

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)

Load bearing

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.

Artwork: Balancing forces in a bridge: Different types of bridges carry loads through the forces of compression ("squeezing"—shown here by red lines) and tension ("stretching"—shown by blue lines): 1) A beam bridge has its beam partly in tension and partly in compression, with the abutments (side pillars) in compression; 2) An arch bridge supports loads through compression; 3) A suspension bridge has its piers (towers) in compression and the deck hangs from thick suspension cables by thinner cables, all of which are in tension. 4) A cable-stayed bridge is similar but the deck hangs directly from the piers from cables. The piers are in compression and the cables are in tension.

Environmental forces

Apart from the forces imposed by their own weight and the loads they carry, bridges also have to withstand huge forces from the environment. River valleys and estuaries tend to channel wind, and that's why many bridges are designed with a slender profile to offer as little wind-resistance as possible. Arched bridges over rivers sometimes have large holes cut into their abutments (side supports) to let flood waters flow through, reducing the risk of collapse. In some parts of the world, bridges have to be designed to cope with hurricanes and earthquakes.

The Akashi Kaikyo bridge

Akashi Kaikyo bridge by Flickr user Hyougushi published under a creative commons licence http://creativecommons.org/licenses/by-sa/2.0/deed.en_GB

Two 984-ft (300-m) high piers and 186,000 miles (300,000 km) of cable support the 100,000-ton (91,000 metric-ton) weight of this bridge.

Its tough construction enables it to withstand winds of 178 mph (288 km/h) and earthquakes up to 8.5 on the Richter scale.

The Akashi Kaikyo bridge cables are composed of 290 strands, each containing 127 separate steel wires less than ¼ inch (5 mm) in diameter. One of these cables can withstand a tensile (stretching) force of up to 62,500 tons (57,000 metric tones)—strong enough to support around 1500 fully-laden trucks!

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Photo: The Akashi Kaikyo bridge in Japan. Photo published on Flickr by Hyougushi (Hideyuki KAMON) under a Creative Commons (Attribution-Share Alike 2.0 Generic) license.

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 beam 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.

Beam bridge carrying a railway line

Arch

The arch is in compression and pushes its load out horizontally and vertically into the supporting ground. One of the really clever things about an arch bridge is that a load makes it stronger: an arch made of bricks or stones will pack together tightly when a load is placed on it, making a more solid structure.

Examples of arch bridges include the Mostar Bridge in Bosnia Herzegovina and the Charles Bridge in Prague.

Photo: The Palladian stone arch bridge in Prior Park, Bath, England. You can see that the lower part of the bridge is made up of five separate stone arches.

A stone arch bridge

Cantilever

A cantilever is a beam supported at only one end—and it works rather like a see-saw. One person on a see-saw can be balanced in mid-air providing another person is at the other end counterbalancing them over a pivot (or fulcrum). In a cantilever bridge, each section of the bridge deck is balanced across the piers by the sections on either side of it. Loads are carried vertically to the piers and abutments and evened out by a rigid framework of metal trusses (criss-cross beams).

Examples of cantilever bridges include the Queensboro Bridge in New York City, the Crescent City Connection in New Orleans, and the Forth Bridge in Scotland.

Photo: The Huey P. Long cantilevered bridge on the Mississippi River near New Orleans. Picture courtesy: Defense Imagery/US Navy.

The Huey P. Long Bridge on the Mississippi River near New Orleans

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.

Famous suspension bridges include the Humber Bridge and the Clifton Suspension Bridge in England, the Golden Gate Bridge in California, and the Brooklyn Bridge in Manhattan.

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).

Tamar Road suspension bridge

Cable-stayed

At first glance, a cable-stayed bridge looks much like a suspension bridge, so what's the difference? 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. Because the piers support the bridge, there's no need for the bridge to be firmly anchored onto land at either end in the same robust way as with a suspension bridge. 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 in the middle. However, it usually requires a stronger and bulkier deck than a suspension bridge.

Examples of cable-stayed bridges include the Vasco da Gama Bridge in Portugal, the Millau Viaduct in France, the Hangzou Bridge in China, and the Chords Bridge in Jerusalem.

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.

Aurthur Ravenel Jr. Bridge in Charleston S.C.

Pontoon

Pontoon bridges are improvised crossings often quickly built for military use. 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.

Pontoon bridge in Iraq

Bow-string (tied) arch

A bow-string (or tied) 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 (because the bridge passes through the arch). It's also called a tied-arch bridge, because the vertical bars running from the arch down to the bridge deck "tie" the arch rigidly in place, preventing it from pushing outward at the two ends (as happens with a conventional arch bridge).

Examples of tied-arch bridges include the Sydney Harbor Bridge in Australia (shown here), the Hell Gate Bridge in New York, and the Tyne Road Bridge in Newcastle, England.

Photo: Sydney Harbor Bridge during the 2000 Olympics. Photo by Robert A. Whitehead courtesy of US Air Force and Defense Imagery.

Sydney Harbor bridge

Tunnels

tunnel at Yucca Mountain

Tunnels are long underground passageways that carry highways, railroads, and pipelines under mountains, seas, and rivers. Digging a tunnel can be difficult and dangerous, but the benefits of being able to transport things in a straight line usually outweigh the costs of the massive engineering operation. Tunnels were once dug entirely by hand. Today, gigantic tunnel-boring machines make tunneling both quicker and safer.

Photo: A tunnel at Yucca Mountain, the proposed nuclear waste site in Nye County, United States. Photo courtesy of US Department of Energy.

Digging tunnels

Different types of tunnels are used in different types of terrains. Shallow tunnels are often dug by the "cut-and-cover" method, which involves excavating a long trench, adding reinforced sides, roof, and floor or preformed pipe-like tunnel sections, and then covering the whole thing over with spoil (the original excavated material). Where shallow cut-and-cover tunnels are not possible, deep tunnels must be bored using drills, explosives, or boring machines.

Tunneling can be very hazardous. Apart from the risk of earth collapsing before the roof has been properly reinforced, difficulties include water pouring into the tunnel from aquifers (natural underground water reservoirs), dust from blasting, trapped gas, and the heat and humidity of working deep beneath Earth's surface. All these things can kill construction workers.

The dangerous nature of tunneling has not prevented the construction of many remarkable tunnels. As long ago as 36 BCE, Romans built a road tunnel 4800 ft (1463 m) long between Naples and Pozzuoli entirely by hand. Today's tunnels are much longer. The world's longest rail tunnel, the Seikan Tonneru between mainland Japan and the island of Hokkaido, is 33.5 miles (53.6 km) long, and took 22 years to excavate. 3000 people helped construct the tunnel, 34 of whom were killed before it was completed in 1988.

The Channel Tunnel

Since 1991, England and France have been linked by an underground tunnel 31 miles (50 km) long. The tunnel actually consists of two parallel railroad tunnels and a smaller service tunnel running between them approximately 148 ft (45 m) below the sea. Cross-tunnels between the main tunnels provide maintenance and emergency access and ventilation. Work began in 1987 and tunnel-boring machines working simultaneously outward from Folkestone, England, and Calais, France, finally met in the middle in 1990.

tunnel boring machine or TBM

The tunnel-boring machines (also known as TBMs or "moles") used to dig the Channel Tunnel advanced up to 246 ft (75 m) a day. How do machines like this work? A rotating cutter chips away spoil, which is carried backward on a conveyor into railroad trucks. Powerful hydraulic rams shift the position of the cutter and a laser guidance system keeps it moving straight. Giant gripper pads hold up the tunnel as it is being excavated and, just behind them, reinforced concrete tunnel supports are lifted into place as the machine moves along.

The cutting head of each Channel Tunnel boring machine is 28.2 ft (8.6 m) in diameter and rotates 2-3 times per minute. As the cutter is pressed against the tunnel face, spoil is removed through the slots in the cutting head. Some 11 different moles were used, with cutting heads hard enough to excavate soft chalk marl and wide enough to cut a tunnel of exactly the right diameter.

Photo: Tunnel boring machines ("moles") like this were used to dig the Channel Tunnel. Photo courtesy of US Department of Energy.

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Text copyright © Chris Woodford 2000, 2011. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2000) Bridges and tunnels. Retrieved from http://www.explainthatstuff.com/bridges.html. [Accessed (Insert date here)]

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