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BRIDGES

INTRODUCTION

. A bridge is a structure built to span a valley, road, body of water, orother physical obstacle, for the purpose of providing passage over theobstacle. Designs of bridges vary depending on the function of the bridgeand the nature of the terrain where the bridge is constructed. The first

bridges were made by nature as simple as a log fallen across a stream.The first bridges made by humans were probably spans of wooden logs or

planks and eventually stones, using a simple support and crossbeamarrangement.

There are about 600,000 bridges in the country with 500,000 of themover water. This infrastructure represents an investment of some $300 billionif it had to be totally replaced today. So the average cost of a bridge is $0.5million. The countrys budget per year is $2500 billion,a few dozens at mostdie in a bridge collapse. According to these numbers, crossing a bridge isvery safe compared to many of our daily activities.

Types of bridges

There are six main types of bridges Beam bridges

Cantilever bridges

Arch bridges

Suspension bridges

Cable-stayed bridges and

Truss bridges.

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Beam bridges

Beam bridges are horizontal beams supported at each end by

piers. The earliest beam bridges were simple logs that sat across streams and

similar simple structures. In modern times, beam bridges are large box steelgirder bridges. Weight on top of the beam pushes straight down on the piersat either end of the bridge

Cantilever bridges

Cantilever bridges are built using cantilevershorizontalbeams that are supported on only one end. Most cantilever bridges use twocantilever arms extending from opposite sides of the obstacle to be crossed,meeting at the center. The largest cantilever bridge is the 549-metre (1,800ft) Quebec Bridge in Quebec, Canada.

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Arch bridges

Stone arch bridge in Shaharah, Yemen Arch bridges are

arch-shaped and have abutments at each end. The earliest known archbridges were built by the Greeks and include the Arkadiko Bridge. Theweight of the bridge is thrust into the abutments at either side. Dubai in theUnited Arab Emirates is currently building the Sheikh Rashid bin SaeedCrossing which is scheduled for completion in 2012. When completed, itwill be the largest arch bridge in the world

Suspension bridges

Suspension bridges are suspended from cables. The earliest

suspension bridges were made of ropes or vines covered with pieces of

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bamboo. In modern bridges, the cables hang from towers that are attached tocaissons or cofferdams. The caissons or cofferdams are implanted deep intothe floor of a lake or river. The longest suspension bridge in the world is the12,826 feet (3,909 m) Akashi Kaikyo Bridge in Japan.

Cable-stayed bridges

Like suspension bridges, cable-stayed bridges are held up by

cables. However, in a cable-stayed bridge, less cable is required and thetowers 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.

Truss bridges

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Truss bridges are composed of connected elements. Theyhave a solid deck and a lattice of pin-jointed or gusset-joined girders for thesides. Early truss bridges were made of wood, and later of wood with irontensile rods, but modern truss bridges are made completely of metals such aswrought iron and steel or sometimes of reinforced concrete. The QuebecBridge, mentioned above as a cantilever bridge, is also the world's longesttruss bridge

Purposes of Bridges

A bridge is designed for trains, pedestrian or road traffic, a pipelineor waterway for water transport or barge traffic. An aqueduct is a bridge thatcarries water, resembling a viaduct, which is a bridge that connects points ofequal height. A road-rail bridge carries both road and rail traffic.

Bridges are subject to unplanned uses as well. The areas underneath somebridges have become makeshift shelters and homes to homeless people, andthe undersides of bridges all around the world are spots of prevalent graffiti.Some bridges attract people attempting suicide, and become known assuicide bridges.

Differences and similarities in Bridge structure

A bridge taxonomy showing evolutionary relationships. Bridges

may be classified by how the forces of tension, compression, bending,torsion and shear are distributed through their structure. Most bridges willemploy 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 orcable-stayed span, the elements in tension are distinct in shape and

placement. In other cases the forces may be distributed among a largenumber of members, as in a truss, or not clearly discernible to a casualobserver as in a box beam.

Efficiency

The lifetime cost is composed of materials, labor, machinery, cost ofmoney, maintenance, demolition and associated disposal, recycling,and replacement, less the value of scrap and reuse of components.

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Bridges employing only compression are relatively inefficientstructurally, but may be highly cost efficient where suitable materialsare available near the site and the cost of labor is low.

For medium spans, the appearance of the bridge may be more

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

Bridge failures

1.

The condition or fact of not achieving the desired end or ends: thefailure of an experiment.

One that fails: a failure at one's career.

The condition or fact of being insufficient or falling short: a crop

failure. A cessation of proper functioning or performance: a power failure. Nonperformance of what is requested or expected; omission:failure

to report a change of address.

The act or fact of failing to pass a course, test, or assignment.

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A decline in strength or effectiveness. The act or fact of becomingbankrupt or insolvent.

Movable bridges

Movable bridge are not fixed crossings, but can move out of theway of boats or other kinds of traffic which, ideally, moves under them, butis sometimes too tall to fit. A moveable bridge is abridge that moves toallow passage for (usually) boats or barges. By making the bridge moveable

it may be made low, which avoids the expense of high piers and longapproaches, greatly reducing the cost of the bridge. The principaldisadvantage is that the traffic on the bridge must be halted when it isopened for passages. For seldom used railroad bridges over busy channelsthe bridge may be left open and then closed for train passages. For small

bridges bridge movement may be enabled without the need for an engine.Some bridges are operated by the users, especially those with a boat, others

http://en.wikipedia.org/wiki/Bridgehttp://en.wikipedia.org/wiki/Bridge

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by a bridgeman, sometimes remotely using video-cameras and loudspeakers.Generally the bridges are powered by electric motors, whether operatingwinches, gearing, or hydraulic pistons. While moveable bridges in theirentirety may be quite long, the length of the moveable portion is restricted

by engineering and cost considerations to a few hundred feet.