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)
The Akashi Kaikyo bridge
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!
You can read more about the bridge in this great article
by bridge engineer James D. Cooper.
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 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.
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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.
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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.
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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).
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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.
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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.
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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.
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