A bridge is a structure built to span physical obstacles such as a body of
water, valley,
or road, for the
purpose of providing passage over the obstacle. There are many different
designs that all serve unique purposes and apply to different situations.
Designs of bridges vary depending on the function of the bridge, the nature of
the terrain
where the bridge is constructed and anchored, the material used to make it, and
the funds available to build it.
History
The first bridges were made by
nature itself—as simple as a log fallen across a stream or stones in
the river. The
first bridges made by humans were probably spans of cut wooden logs or planks
and eventually stones, using a simple support and crossbeam
arrangement. Some early Americans used trees or bamboo poles to
cross small caverns or wells to get from one place to another. A common form of
lashing sticks, logs, and deciduous branches together involved the use of long reeds
or other harvested fibers woven together to form a connective rope capable of
binding and holding together the materials used in early bridges.
Arkadiko BridgeGreece
The Arkadiko
Bridge is one of four Mycenaean corbel arch
bridges part of a former network of roads, designed to accommodate chariots, between
Tiryns to Epidauros in
the Peloponnese,
in Greece.
Dating to the Greek Bronze Age (13th century BC), it is one of the oldest
arch bridges still in existence and use. Several intact arched stone bridges
from the Hellenistic era can be found in the Peloponnese
in southern Greece
The greatest bridge builders of
antiquity were the ancient Romans. The Romans
built arch
bridges and aqueducts that could stand in conditions that
would damage or destroy earlier designs. Some stand today. An example
is the Alcántara Bridge, built over the river Tagus, in Spain. The Romans
also used cement,
which reduced the variation of strength found in natural stone. One type
of cement, called pozzolana, consisted of water, lime,
sand, and volcanic rock. Brick and mortar bridges were built after the Roman era, as
the technology for cement was lost then later rediscovered.
The Arthashastra
of Kautilya
mentions the construction of dams and bridges. A Mauryan bridge
near Girnar was
surveyed by James Princep. The
bridge was swept away during a flood, and later repaired by Puspagupta, the
chief architect of emperor Chandragupta I. The
bridge also fell under the care of the Yavana Tushaspa,
and the Satrap
Rudra Daman.
The use of stronger bridges using plaited bamboo and iron chain was visible in
India by about the 4th century. A number
of bridges, both for military and commercial purposes, were constructed by the Mughal
administration in India.
Although large Chinese bridges of
wooden construction existed at the time of the Warring
States, the oldest surviving stone bridge in China is the Zhaozhou
Bridge, built from 595 to 605 AD during the Sui Dynasty.
This bridge is also historically significant as it is the world's oldest open-spandrel stone
segmental arch bridge. European segmental arch bridges date back to at least
the Alconétar Bridge (approximately 2nd century AD),
while the enormous Roman era Trajan's
Bridge (105 AD) featured open-spandrel segmental arches in wooden
construction.
Rope
bridges, a simple type of suspension
bridge, were used by the Inca civilization in the Andes mountains of
South America, just prior to European colonization in the 16th century.
During the 18th century there were
many innovations in the design of timber bridges by Hans Ulrich, Johannes Grubenmann, and others. The first book
on bridge engineering was written by Hubert
Gautier in 1716. A major breakthrough in bridge technology came with the
erection of the Iron Bridge in Coalbrookdale,
England in 1779. It used cast iron for the first time as arches to cross the river
Severn.
With the Industrial Revolution in the 19th century, truss systems of wrought
iron were developed for larger bridges, but iron did not have the tensile
strength to support large loads. With the advent of steel, which has a high
tensile strength, much larger bridges were built, many using the ideas of Gustave
Eiffel.
In 1927 welding pioneer Stefan
Bryła designed the first welded road bridge in the world, the Maurzyce
Bridge which was later built across the river Słudwia
at Maurzyce near Łowicz, Poland in 1929. In 1995, the American Welding Society presented the
Historic Welded Structure Award for the bridge to Poland.
Types
of bridges
Bridges can be categorized in
several different ways. Common categories include the type of structural
elements used, by what they carry, whether they are fixed or movable, and by
the materials used.
Structure
type
Bridges may be classified by how the
forces of tension, compression, bending, torsion and shear
are distributed through their structure. Most bridges will employ all of the
principal forces to some degree, but only a few will predominate. The
separation of forces may be quite clear. In a suspension or cable-stayed span,
the elements in tension are distinct in shape and placement. In other cases the
forces may be distributed among a large number of members, as in a truss, or
not clearly discernible to a casual observer as in a box beam.
Beam bridges are horizontal beams supported at
each end by substructure units and can be either simply
supported when the beams only connect across a single span, or continuous
when the beams are connected across two or more spans. When there are multiple
spans, the intermediate supports are known as piers.
The earliest beam bridges were simple logs that sat across streams and similar
simple structures. In modern times, beam bridges can range from small, wooden
beams to large, steel boxes. The vertical force on the bridge becomes a shear
and flexural
load on the beam which is transferred down its length to the substructures on
either side
They are typically made of steel, concrete or wood. Beam bridge spans rarely
exceed 250 feet (76 m) long, as the flexural stresses increase
proportional to the square of the length (and deflection increases proportional
to the 4th power of the length). However,
the main span of the Rio-Niteroi Bridge, a box girder bridge, is 300
metres (980 ft).
The world's longest beam bridge is Lake Pontchartrain Causeway in southern
Louisiana
in the United States, at 23.83 miles (38.35 km), with individual spans of
56 feet (17 m). Beam
bridges are the most common bridge type in use today.
A truss bridge is
a bridge whose load-bearing superstructure is composed of a truss. This truss
is a structure of connected elements forming triangular units. The connected
elements (typically straight) may be stressed from tension, compression, or
sometimes both in response to dynamic loads. Truss bridges are one of the
oldest types of modern bridges. The basic types of truss bridges shown in this
article have simple designs which could be easily analyzed by nineteenth and
early twentieth century engineers. A truss bridge is economical to construct
owing to its efficient use of materials.
Cantilever bridges are built using cantilevers—horizontal
beams supported on only one end. Most cantilever bridges use a pair of continuous
spans that extend from opposite sides of the supporting piers to meet at
the center of the obstacle the bridge crosses. Cantilever bridges are
constructed using much the same materials & techniques as beam bridges. The
difference comes in the action of the forces through the bridge.
The largest cantilever bridge is the
549-metre (1,801 ft) Quebec Bridge in Quebec, Canada.\
Arch
bridges have abutments at
each end. The weight of the bridge is thrust into the abutments at
either side. The earliest known arch bridges were built by the Greeks, and
include the Arkadiko Bridge.
With the span of 220 metres
(720 ft), the Solkan Bridge over the Soča River at Solkan in Slovenia
is the second largest stone bridge in the world and the longest railroad stone
bridge. It was completed in 1905. Its arch, which was constructed from over
5,000 tonnes (4,900 long tons; 5,500 short tons) of stone blocks in just 18
days, is the second largest stone arch in the world, surpassed only by the Friedensbrücke
(Syratalviadukt) in Plauen,
and the largest railroad stone arch. The arch of the Friedensbrücke, which was
built in the same year, has the span of 90 m (300 ft) and crosses the
valley of the Syrabach
River. The difference between the two is that the Solkan Bridge was built from
stone blocks, whereas the Friedensbrücke was built from a mixture of crushed
stone and cement mortar.
The world's current largest arch
bridge is the Chaotianmen Bridge over the Yangtze
River with a length of 1,741 m (5,712 ft) and a span of 552 metres
(1,811 ft). The bridge was open April 29, 2009 in Chongqing,
China.
Tied arch bridges have an arch-shaped
superstructure, but differ from conventional arch bridges. Instead of
transferring the weight of the bridge and traffic loads into thrust forces into
the abutments, the ends of the arches are restrained by tension in the bottom
chord of the structure. They are also called bowstring arches.
Suspension
bridges are suspended from cables. The
earliest suspension bridges were made of ropes or vines covered with pieces of
bamboo. In modern bridges, the cables hang from towers that are attached to
caissons or cofferdams. The caissons or cofferdams are implanted deep into the
floor of a lake or river. Sub-types include the simple suspension bridge, the stressed ribbon bridge, the underspanned suspension bridge, the suspended-deck suspension bridge,
and the self-anchored suspension bridge.
The longest suspension bridge in the
world is the 3,909 m (12,825 ft) Akashi Kaikyō Bridge in Japan.
Cable-stayed bridges, like suspension bridges, are held up by cables. However, in
a cable-stayed bridge, less cable is required and the towers holding the cables
are proportionately higher.
The first known cable-stayed bridge was designed in 1784 by C. T. (or C. J.)
Löscher.
The longest cable-stayed bridge is
the Sutong
Bridge over the Yangtze River in China.
Fixed
or movable bridges
Most bridges are fixed bridges,
meaning they have no moving parts and stay in one place until they fail or are
demolished. Temporary bridges, such as Bailey
bridges, are designed to be assembled, and taken apart, transported to a
different site, and re-used. They are important in military engineering, and
are also used to carry traffic while an old bridge is being rebuilt. Movable
bridges are designed to move out of the way of boats or other kinds of
traffic, which would otherwise be too tall to fit. These are generally
electrically powered.
Double-decked
bridges
Double-decked (or double-decker)
bridges have two levels, such as the George Washington Bridge, connecting New
York City to Bergen County, New Jersey,
USA, as the world's busiest bridge, carrying 102 million vehicles annually; truss work between
the roadway levels provided stiffness to the roadways and reduced movement of
the upper level when the lower level was installed three decades following the
upper level. The Tsing Ma Bridge and Kap Shui Mun Bridge in Hong Kong
have six lanes on their upper decks, and on their lower decks there are two
lanes and a pair of tracks for MTR metro trains. Some double-decked bridges only use one level
for street traffic; the Washington Avenue Bridge in Minneapolis reserves its lower level for
automobile traffic and its upper level for pedestrian and bicycle traffic
(predominantly students at the University of Minnesota). Likewise, in Toronto, the Prince Edward Viaduct has five lanes of motor
traffic, bicycle lanes, and sidewalks on its upper deck; and a pair of tracks
for the Bloor–Danforth subway line on its lower deck. The western
span of the San Francisco – Oakland Bay Bridge
also has two levels.
Robert
Stephenson's High Level Bridge across the River Tyne
in Newcastle upon Tyne, completed in 1849, is an
early example of a double-decked bridge. The upper level carries a railway, and
the lower level is used for road traffic. Other examples include Britannia
Bridge over the Menai Strait and Craigavon
Bridge in Derry,
Northern
Ireland. The Oresund Bridge between Copenhagen
and Malmö
consists of a four-lane highway on the upper level and a pair of railway tracks
at the lower level. The Tower Bridge is different example of a double-decked
bridge, with the central section consisting of a low level bascule
span and a high level footbridge.
Viaducts
A viaduct is made up of multiple
bridges connected into one longer structure. The longest and some of the
highest bridges are viaducts, such as the Lake Pontchartrain Causeway and Millau
Viaduct.
Three-way
bridges
A three-way bridge has three
separate spans which meet near the center of the bridge. The bridge appears as
a "T" or "Y" when viewed from above. Three-way bridges are
extremely rare, with just 27 known to exist. The Tridge, Margaret
Bridge, and Zanesville Y-Bridge are examples.
Bridge
types by use
A bridge can be categorized by what
it is designed to carry, such as trains, pedestrian or road traffic, a pipeline
or waterway for water transport or barge traffic. An aqueduct is a bridge that carries water, resembling
a viaduct, which is a bridge that connects points of equal height. A road-rail
bridge carries both road and rail traffic. A bridge can carry overhead power
lines as does the Storstrøm Bridge.
Some bridges accommodate other
purposes, such as the tower of Nový Most
Bridge in Bratislava,
which features a restaurant, or a bridge-restaurant
which is a bridge built to serve as a restaurant. Other suspension bridge
towers carry transmission antennas.
Bridges are subject to unplanned
uses as well. The areas underneath some bridges have become makeshift shelters
and homes to homeless people, and the undersides of bridges all around the
world are spots of prevalent graffiti. Some bridges attract people attempting suicide, and
become known as suicide bridges.
Bridge
types by material
The Iron Bridge
The materials used to build the
structure are also used to categorize bridges. Until the end of the 18th
Century, bridges were made out of timber, stone and masonry. Modern bridges are
currently built in concrete, steel, fiber reinforced polymers (FRP), stainless
steel or combinations of those materials.
Aesthetics
Most bridges are utilitarian in
appearance, but in some cases, the appearance of the bridge can have great
importance. Often, this is the case with a large bridge that serves as an
entrance to a city, or crosses over a main harbor entrance. These are sometimes
known as signature bridges. Designers of bridges in parks and along
parkways often place more importance to aesthetics, as well. Examples include
the stone-faced bridges along the Taconic State Parkway in New York.
To create a beautiful image, some
bridges are built much taller than necessary. This type, often found in
east-Asian style gardens, is called a Moon bridge,
evoking a rising full moon. Other garden bridges may cross only a dry bed of
stream washed pebbles, intended only to convey an impression of a stream. Often
in palaces a bridge will be built over an artificial waterway as symbolic of a
passage to an important place or state of mind. A set of five bridges cross a
sinuous waterway in an important courtyard of the Forbidden
City in Beijing,
China. The central
bridge was reserved exclusively for the use of the Emperor, Empress, and their
attendants.
Bridge
maintenance
Bridge maintenance consisting of a
combination of structural health monitoring and testing. This is regulated in
country-specific engineer standards and includes e.g. an ongoing monitoring
every three to six months, a simple test or inspection every two to three years
and a major inspection every six to ten years. In Europe, the cost of
maintenance is higher than spending on new bridges. The lifetime of welded
steel bridges can be significantly extended by aftertreatment of the weld transitions
. This results in a potential high benefit, using existing bridges far beyond
the planned lifetime.
Bridge
failures
The failure of bridges is of special
concern for structural engineers in trying to learn
lessons vital to bridge design, construction and maintenance. The failure of
bridges first assumed national interest during the Victorian
era when many new designs were being built, often using new materials.
In the United States, the National Bridge Inventory tracks the
structural evaluations of all bridges, including designations such as
"structurally deficient" and "functionally obsolete".
Bridge
monitoring
There are several methods used to
monitor the stress on large structures like bridges. The most common method is
the use of an accelerometer, which is integrated into the bridge
while it is being built. This technology is used for long-term surveillance of
the bridge.
Another option for
structural-integrity monitoring is "non-contact monitoring", which
uses the Doppler effect (Doppler shift). A laser beam from a Laser Doppler Vibrometer is directed at
the point of interest, and the vibration amplitude and frequency are extracted
from the Doppler shift of the laser beam frequency due to the motion of the
surface.
The advantage of this method is that the setup time for the equipment is faster
and, unlike an accelerometer, this makes measurements possible on multiple
structures in as short a time as possible. Additionally, this method can
measure specific points on a bridge that might be difficult to access.
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