bridge is a structure built to span physical obstacles such as a body of water

8/7/2019 bridge is a structure built to span physical obstacles such as a body of water

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BRIDGESbridge is a structure built to span physical obstacles such as a body of water, valley, orroad, for

the purpose of providing passage over the obstacle. Designs of bridges vary depending on the

function of the bridge, the nature of the terrain where the bridge is constructed, the material usedto make it and the funds available to build it.

The first bridges were made by nature itself as simple as a log fallen across a stream or stonesin 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 earlyAmericans 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 theuse of long reeds or other harvested fibers woven together to form a connective rope which was

capable of binding and holding in place materials used in early bridges.

The Arkadiko Bridge is one of fourMycenaeancorbel arch bridges part of a former network ofroads, designed to accommodate chariots, between Tiryns to Epidauros in the Peloponnese, in

Greece. Dating to the GreekBronze Age (13th century BC), it is one of the oldest arch bridgesstill in existence and use. Several intact arched stone bridges from the Hellenistic era can be

found in the Peloponnese in southern GreeceThe greatest bridge builders of antiquity were theancient 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 Alcntara Bridge,built over the riverTagus, 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. Brickand mortarbridges were built after the Roman era, as the

technology for cement was lost then later rediscovered.

The Arthashastra ofKautilya mentions the construction of dams and bridges. A Mauryan bridge

nearGirnarwas surveyed by James Princep. The bridge was swept away during a flood, andlater repaired by Puspagupta, the chief architect of emperorChandragupta I. The bridge also fell

under the care of the Yavana Tushaspa, and the Satrap Rudra Daman. The use of strongerbridges 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 Mughaladministration 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 Alcontar Bridge (approximately 2nd century AD), while theenormous Roman era Trajan's Bridge (105 AD) featured open-spandrel segmental arches inwooden 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.


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During the 18th century there were many innovations in the design of timber bridges by HansUlrich, Johannes Grubenmann, and others. The first book on bridge engineering was written by

Hubert Gautierin 1716. A major breakthrough in bridge technology came with the erection ofthe 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 ofwrought iron were developedfor 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 theideas ofGustave Eiffel.

In 1927 welding pioneerStefan Brya designed the first welded road bridge in the world which

was later built across the riverSudwia Maurzyce nearowicz, Poland in 1929. In 1995, theAmerican Welding Society presented the Historic Welded Structure Award for the bridge to

Poland.

Types of bridges

There are six main types of bridges: beam bridges, cantilever bridges, arch bridges, suspension

bridges, cable-stayed bridges and truss bridges.

Beam bridges

Beam bridges are horizontal beams supported at each end by abutments, hence their structural

name ofsimply supported. When there is more than one span the intermediate supports areknown as piers. The earliest beam bridges were simple logs that sat across streams and similar

simple structures. In modern times, beam bridges are large box steel girder bridges. Weight on

top of the beam pushes straight down on the abutments at either end of the bridge. They aremade up mostly of wood or metal. Beam bridges typically do not exceed 250 feet (76 m) long.The longer the bridge, the weaker. 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 individualspans of 56 feet (17 m).

Cantilever bridges

Cantilever bridges are built using cantilevershorizontal beams that are supported on only one

end. Most cantilever bridges use a pair ofcontinuous spans extending from opposite sides of thesupporting piers, meeting at the center of the obstacle to be crossed. Cantilever bridges are

constructed using much the same materials & techniques as beam bridges. The difference comesin the action of the forces through the bridge. The largest cantilever bridge is the 549-metre

(1,801 ft) Quebec Bridge in Quebec, Canada.


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Arch bridges

Arch bridges have abutments at each end. The earliest known arch bridges were built by the

Greeks and include the Arkadiko Bridge. The weight of the bridge is thrust into the abutments ateither side. Dubai in the United Arab Emirates is currently building the Sheikh Rashid bin Saeed

Crossing which is scheduled for completion in 2012. When completed, it will be the largest archbridge in the world.

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Suspension bridges

Suspension bridges are suspended from cables. The earliest suspension bridges were made ofropes 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 intothe floor of a lake or river. The longest suspension bridge in the world is the 12,826 feet (3,909

m) Akashi Kaikyo Bridge in Japan. See simple suspension bridge, stressed ribbon bridge,underspanned suspension bridge, suspended-deck suspension bridge, and self-anchored

suspension bridge.

Cable-stayed bridges

Cable-stayed bridges, like suspension bridges, are held up by cables. However, in a cable-stayedbridge, less cable is required and the towers holding the cables are proportionately shorter. The

first known cable-stayed bridge was designed in 1784 by C.T. Loescher.]The longest cable-

stayed bridge is the Sutong Bridge over the Yangtze River in China.

Movable bridges

Movable bridges are designed to move out of the way of boats or other kinds of traffic, whichwould 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 San Francisco Oakland

Bay Bridge, with two road levels. Tsing Ma Bridge and Kap Shui Mun Bridge in Hong Konghave six lanes on their upper decks, and on their lower decks there are two lanes and a pair of

tracks forMTRmetro trains. Likewise, in Toronto, the Prince Edward Viaduct has four lanes ofmotor traffic on its upper deck and a pair of tracks for the BloorDanforthsubway line. Some

double-decker 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 andbicycle traffic (predominantly students at the University of Minnesota).

Robert Stephenson's High Level Bridge across the River Tyne in Newcastle upon Tyne,completed in 1849, is an early example of a double-deck bridge. The upper level carries a

railway, and the lower level is used for road traffic. Another example is Craigavon Bridge inDerry, Northern Ireland. The Oresund Bridge between Copenhagen and Malm consists of a

four-lane highway on the upper level and a pair of railway tracks at the lower level.


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The George Washington Bridge between New Jersey and New York has two roadway levels. Itwas built with only the upper roadway as traffic demands did not require more capacity. A truss

work between the roadway levels provides stiffness to the roadways and reduced movement ofthe upper level when installed. Tower Bridge is different example of a double-decker bridge,

with the central section consisting of a low level bascule span and a high level footbridge.

Old Yamuna Bridge(Delhi) or Bridge No. 249 in technical railway parlance, was constructed in1866 by the East India Railway at a cost of 16,16,335. It was built with a total length of 2,640

feet and consisted of 12 spans of 202.5 feet each. With the completion of this bridge, twoprincipal cities of North India, Kolkata and Delhi, were connected by the Railways; this being

the last link of the trunk line on this route. In 1913, this was converted into a double line byadding down line girders of 12 spans of 202 feet each and 2 end spans of 42 feet to the bridge.

For the movement of road traffic, two road bridges were provided below the lines. The entry oftrains into Delhi Junction Railway Station, in such close proximity to the Red Fort, never ceases

to impress the rail traveller, reminding all that after the Uprising of 1857, Delhi was a fortifiedcity. The old Yamuna Bridge has an identical twin, a bridge further downstream at Naini on the

Allahabad Mughalsarai section of the now North Central Railways

By use

A bridge is designed for trains, pedestrian or road traffic, a pipeline or waterway for watertransport 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 railtraffic.

Bridges are subject to unplanned uses as well. The areas underneath some bridges have becomemakeshift 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 becomeknown as suicide bridges.

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. Othergarden 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 assymbolic 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, the People'sRepublic of China. The central bridge was reserved exclusively for the use of the Emperor,

Empress, and their attendants.


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Structure

Bridges may be classified by how the forces oftension, compression, bending, torsion and shearare 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. Inother cases the forces may be distributed among a large number of members, as in a truss, or notclearly discernible to a casual observer as in a box beam. Bridges can also be classified by their

lineage, which is shown as the vertical axis on the diagram.

Efficiency

A bridge's structural efficiency may be considered to be the ratio of load carried to bridge mass,given a specific set of material types. In one common challenge students are divided into groups

and given a quantity of wood sticks, a distance to span, and glue, and then asked to construct a

bridge that will be tested to destruction by the progressive addition of load at the center of thespan. The bridge taking the greatest load is by this test the most structurally efficient. A morerefined measure for this exercise is to weigh the completed bridge rather than measure against a

fixed quantity of materials provided and determine the multiple of this weight that the bridge cancarry, a test that emphasizes economy of materials and efficient glue joints (see balsa wood

bridge).

A bridge's economic efficiency will be site and traffic dependent, the ratio of savings by having abridge (instead of, for example, a ferry, or a longer road route) compared to its cost. The lifetime

cost is composed of materials, labor, machinery, engineering, cost of money, insurance,maintenance, refurbishment, and ultimately, demolition and associated disposal, recycling, and

replacement, less the value of scrap and reuse of components. Bridges employing onlycompression are relatively inefficient structurally, but may be highly cost efficient where suitable

materials are available near the site and the cost of labor is low. For medium spans, trusses orbox beams are usually most economical, while in some cases, the appearance of the bridge may

be more important than its cost efficiency. The longest spans usually require suspension bridges.


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BRIDGES

Made by-Priyanka Vadnal

A Division

Roll no.33


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bridge is a structure built to span physical obstacles such as a body of water