Basic Definition of a Bridge

8/6/2019 Basic Definition of a Bridge

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Basic definition of a

bridge:A bridge is a structure built to span a valley, road, body of water, or

other physical obstacle, for the purpose of providing passage over the

obstacle. Designs of bridges vary depending on the function of the

bridge and the nature of the terrain where the bridge is constructed.
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History: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 crossbeam arrangement

Roman bridge of Crdoba, Spain, built in the 1st century BC.[1]

Ponte di Pietra in Verona, Italy.

A log bridge in the French Alps nearVallorcine.
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An English 18th century example of a bridge in the Palladian style, with

shops on the span: Pulteney Bridge, Bath

A Han Dynasty (202 BC 220 AD) Chinese miniature model of two

residential towers joined by a bridge
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EVOLUTION OF

BRIDGES:Basically bridges evolved after man realized that the best way overcome

obstacles on the road is through bridges

Bridges evolved as centuries passed by like in

Pre-civilisation : Beams fallen across a stream. -> Rope bridges

2000 BC: According to Chinese tradition, stationary bridges

existed in this period.

493 BC: Timber (pontoon) bridges used during Persian military

campaign.

Early Roman: Stone arch bridges, wooden framework used to hold

the incomplete bridge. Use of concrete.

16th 18th Century: After the collapse of Roman empire, no

advance in bridge building. During the Renaissance advances in

architecture and engineering

18th 19th Century: Use of iron cables in suspension bridges and

iron beams also used for the first time.

19th - 20th Century: Mass production of steel begun. Firstsuspension bridge which used steel cables. Use of concrete begun.

20th Century: Steel replaced iron. Reinforced concrete bridges

became common. Development Cantilever construction and

Travelling formwork techniques.


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Design style ofbridges

The design of bridges has substantially changed with time and has some

remarkable changes like timber bridges were covered in order to protect

them from weather and use of stone came into picture.

Pattern of innovation

Different innovative methods came into existence while designing of

bridges like1) Increased spans

2) heavier loads could be carried

3) Reduction in wastage of materials and construction time

4) Giving preference to quality over quantity

5) And lastly ,introduction of stone


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Types of BridgesThere are six types of bridges:

Beam Bridges

Arch Bridges

Cantilever Bridges

Suspension Bridges

Cable stayed Bridges

Truss Bridges

Beam BridgesBeam 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 steel girder bridges. Weight on top of the beam pushesstraight down on the piers at either end of the bridge In the

beginning of the Industrial Revolution, beam-bridge construction

in the United States was developing rapidly. Designers were

coming up with many different truss designs and compositions.
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Wooden bridges were being replaced by all-iron or wood-and-iron

combinations.

Parts of Beam

Bridges

The force of compression supports itself on the top side of the

beam bridge's deck.This causes the upper portion of the deck to

shorten. The result of the compression on the upper portion of the
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deck causes tension in the lower portion of the deck. This tension

causes the lower portion of the beam to lengthen.

Many beam bridges that you find on highway overpasses use

concrete or steel beams to handle the load. By increasing the height

of the beam, the beam has more material to dissipate the

tension.To create very tall beams, bridge designers add supporting

lattice work, or a truss, to the bridge's beam. Once the beam begins

to compress, the force is dissipated through the truss

Arch bridges:

Stone arch bridge in Shaharah, Yemen
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Arch bridges are arch-shaped and 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 theabutments at either 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 arch bridge in the world. An arch bridge is a semicircular

structure with abutments on each end.

The design of the arch, the semicircle, naturally diverts the weight

from the bridge deck to the abutments.

Arch bridges are always under compression. The force ofcompression is pushed outward along the curve of the arch toward

the abutments.

Arch bridges have now evolved into compression arch suspended-

deck bridge enabling the use of light and strongly tensile materials

in their construction.
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The greater the degree of curvature, the greater the effects of

tension on the underside.

Thus the shape of the arch itself is all that is needed to effectively

dissipate the weight from the center of the deck to the abutments.

Arches are fascinating in that they are a truly natural form of bridge. It is

the shape of the structure that gives it its strength. An arch bridge doesn't

need additional supports or cables. In fact, an arch made of stone doesn't

even need mortar. Ancient Romans built arch bridges (and aqueducts)

that are still standing, and structurally sound, today. These bridges and

aqueducts are real testaments to the natural effectiveness of an arch as a

bridge structure.
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Example of an arch bridge


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Cantilever bridges Cantilever bridges are built using cantilevershorizontal beams

that are supported on only one end. Most cantilever bridges use

two cantilever arms extending from opposite sides of the obstacle

to be crossed, meeting at the center. The largest cantilever bridge is

the 549-metre (1,800 ft) Quebec Bridge in Quebec, Canada.A

cantilever bridge is a structures that project horizontally into

space, supported on only one end.

It is basically a structure or beam that is unsupported at one end

but supported at the other.

Cantilevers are the structures that project along the X-axis in

space.

For small footbridges, the cantilevers may be simple beams.

Most cantilever bridges use two cantilever arms extending from

opposite sides of the obstacle to be crossed, meeting at the center.

Cantilever bridges made longer spans possible and wider

clearance beneath.

Modern motorways have cantilever bridges stretching across them,

they have a cantilever coming out from each side and a beambridge in between them.
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The Suspension

BridgeA suspension bridge is one where cables (or ropes or chains) are strung

across the river (or whatever the obstacle happens to be) and the deck is

suspended from these cables. Modern suspension bridges have two tall

towers through which the cables are strung. Thus, the towers are

supporting the majority of the roadway's weight.

The force of compression pushes down on the suspension bridge's deck,

but because it is a suspended roadway, the cables transfer the

compression to the towers, which dissipate the compression directly into

the earth where they are firmly entrenched.

The supporting cables, running between the two anchorages, are the

lucky recipients of the tension forces. The cables are literally stretched
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from the weight of the bridge and its traffic as they run from anchorage

to anchorage. The anchorages are also under tension, but since they, like

the towers, are held firmly to the earth, the tension they experience is

dissipated.

Almost all suspension bridges have, in addition to the cables, a

supporting truss system beneath the bridge deck (a deck truss). This

helps to stiffen the deck and reduce the tendency of the roadway to swayand ripple.

A classic suspension bridge inNew York City
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Cable-stayed

Bridges A cable-stayed bridge is a bridge that consists of one or more

columns (normally referred to as towers or pylons), with cables

supporting the bridge deck.

The cable-stay design is the optimum bridge for a span length

between that ofcantilever bridges and suspension bridges. Withinthis range of span lengths a suspension bridge would require a

great deal more cable, while a full cantilever bridge would require

considerably more material and be substantially heavier..
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There are two major classes of cable-stayed bridges:

In a harp design, the cables are made nearly parallel by attaching cablesto various points on the tower(s) so that the height of attachment of each

cable on the tower is similar to the distance from the tower along the

roadway to its lower attachment.

In a fan design, the cables all connect to or pass over the top of the

tower(s).


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COMPARISON BETWEENSUSPENSION AND CABLE-

STAYED BRIDGES

A suspension bridge is a type ofbridge in which the deck (the

load-bearing portion) is hung below suspension cables on vertical

suspenders.The cables transfer the weight to the towers, which

transfer the weight to the ground.

Cable-stayed bridges have towers, but cables from the towers godirectly to the road deck, instead of spanning from tower to tower.

A bridge falling under this category is suspended from cables. The

suspension cables are anchored at each end of the bridge. The load

that the bridge bears converts into the tension in the cables. These

cables stretch beyond the pillars up to the dock-level supports

further to the anchors in the ground.

Modern suspension bridges have two tall towers through whichthe cables are strung. Thus, the towers are supporting the majority

of the roadway's weight.

The supporting cables, running between the two anchorages, are

the lucky recipients of the tension forces. The cables are literally

stretched from the weight of the bridge and its traffic as they run

from anchorage to anchorage. The anchorages are also under

tension, but since they, like the towers, are held firmly to the earth,the tension they experience is dissipated.

Cable-stayed bridges have towers, but cables from the towers go

directly to the road deck, instead of spanning from tower to tower.

A bridge falling under this category is suspended from cables. The
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suspension cables are anchored at each end of the bridge. The load

that the bridge bears converts into the tension in the cables. These

cables stretch beyond the pillars up to the dock-level supports

further to the anchors in the ground Structured similar to the

suspension bridges, the difference lies in the amount of cable used.

Less cable is required and consequently, the towers holding the

cables are shorter. Two variants of cable-stayed bridges exist. In

the harp design, cables are attached to multiple points of the tower

thus making them parallel. In the fan variant of design, all the

cables connect to the tower or pass over it. Like suspension

bridges, cable-stayed bridges are held up by cables. However, in a

cable-stayed bridge, less cable is required and the towers holdingthe cables are proportionately shorter.

Truss bridges

Continuous under-deck truss bridge

Over-deck truss bridge with steel girders and wooden carriageway

Truss bridges are composed of connected elements. They have a solid

deck and a lattice of pin-jointed or gusset-joined girders for the sides.
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Early truss bridges were made of wood, and later of wood with iron

tensile rods, but modern truss bridges are made completely of metals

such as wrought iron and steel or sometimes of reinforced concrete. The

Quebec Bridge, mentioned above as a cantilever bridge, is also the

world's longest truss bridge

For purposes of analysis, truss are assumed to be pin jointed where the straight

components meet

This assumption means that members of the truss (chords, verticals and

diagonals) will only act in tension or compression.

Vertical members are in tension, lower horizontal members in tension, shear,

and bending, outer diagonal and top members are in compression, while the

inner diagonals are in tension
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Additional Bridge

ForcesWe have so far touched on the two biggest forces in bridge design.

There are dozens of other forces that also must be taken into

consideration when designing a bridge. These forces are usually specific

to a particular location or bridge design.

Torsion, which is a rotational or twisting force, is one which has been

effectively eliminated in all but the largest suspension bridges. The

natural shape of the arch and the additional truss structure of the beam

bridge have eliminated the destructive effects of torsion on these

bridges. Suspension bridges, however, because of the very fact that they

are suspended (hanging from a pair of cables), are somewhat more

susceptible to torsion, especially in high winds.

All suspension bridges have deck-stiffening trusses which, as in the caseof beam bridges, effectively eliminate the effects of torsion; but in

suspension bridges of extreme length, the deck truss alone is not enough.

Wind-tunnel tests are generally conducted on models to determine the

bridge's resistance to torsional movements. Aerodynamic truss

structures, diagonal suspender cables, and an exaggerated ratio between

the depth of the stiffening truss to the length of the span are some of the

methods employed to mitigate the effects of torsion.
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Differences and

similarities in bridgestructureA 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 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. Bridges can also be classified by their

lineage, which is shown as the vertical axis on the diagram to the right

ResonanceResonance (a vibration in something caused by an external force that is

in harmony with the natural vibration of the original thing) is a force

which, unchecked, can be fatal to a bridge. Resonant vibrations will

travel through a bridge in the form of waves. A very famous example of

resonance waves destroying a bridge is the Tacoma Narrows bridge,

which fell apart in 1940 in a 40-mph (64-kph) wind. Close examination

of the situation suggested that the bridge's deck-stiffening truss wasinsufficient for the span, but that alone was not the cause of the bridge's

demise. The wind that day was at just the right speed, and hitting the

bridge at just the right angle, to start it vibrating. Continued winds

increased the vibrations until the waves grew so large and violent that

they broke the bridge apart.


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When an army marches across a bridge, the soldiers are often told to

"break step." This is to avoid the possibility that their rhythmic marching

will start resonating throughout the bridge. An army that is large enough

and marching at the right cadence could start a bridge swaying and

undulating until it broke apart.

In order to mitigate the resonance effect in a bridge, it is important to

build dampeners into the bridge design in order to interrupt the resonant

waves. Interrupting them is an effective way to prevent the growth of the

waves regardless of the duration or source of the vibrations. Dampening

techniques generally involve inertia. If a bridge has, for example, a solid

roadway, then a resonant wave can easily travel the length of the bridge.

If a bridge roadway is made up of different sections that haveoverlapping plates, then the movement of one section is transferred to

another via the plates, which, since they are overlapping, create a certain

amount of friction. The trick is to create enough friction to change the

frequency of the resonant wave. Changing the frequency prevents the

wave from building. Changing the wave effectively creates two different

waves, neither of which can build off the other into a destructive force.

WeatherThe force of nature, specifically weather, is by far the hardest to

combat. Rain, ice, wind and salt can each bring down a bridge on its

own, and in combination they most certainly will. Bridge designers have

learned their craft by studying the failures of the past. Iron has replaced

wood and steel has replaced iron. Pre-stressed concrete is used in many

highway bridges. Each new material or design technique builds off the

lessons of the past. Torsion, resonance and aerodynamics (after severalspectacular collapses) have been addressed in better designs. The

problems of weather, however, have yet to be completely conquered.

Cases of weather-related failure far outnumber those of design-related

failures. This can only suggest that we have yet to come up with an

effective solution. To this day, there is no specific construction material


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nor bridge design that will eliminate or even mitigate these forces. The

only deterrent is preventive maintenance.

Unlike many of nature's deadly forces, earthquakes almost always strike

without warning. These destructive and devastating forces can topplecities in seconds, leaving behind rubble and tragedy in their wakes.

Earthquakes are not limited to any one area of the world or any one

season of the year. Although most earthquakes are just small tremors, it

only takes one to cause millions of dollars in property damage and

thousands of deaths. For this reason, scientists continue to pursue new

technologies to limit the destruction that earthquakes can dish out.

At Lord Corporation's labs in Cary, N.C., researchers believe they have

developed, in cooperation with University of Notre Dame researchers,

the latest product that can reduce the damage caused by earthquakes.

Lord is one of the largest producers of a unique substance, called

magnetorheological fluid (MR fluid), which is being used inside large

dampers to stabilized buildings during earthquakes. MR fluid is a liquid

that changes to a near-solid when exposed to a magnetic force, then back

to liquid once the magnetic force is removed.
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Photo courtesy Lord Corp.

In the future, buildings might be built with hundreds of large

dampers filled with MR fluid to stabilize the structures duringearthquakes. This diagram shows how the dampers would

work during an earthquake.

During an earthquake, MR fluid inside the dampers will change from

solid to liquid and back as tremors activate a magnetic force inside the

damper. Using these dampers in buildings and on bridges will create

smart structures that automatically react to seismic activity. This will

limit the amount of damage caused by earthquakes. In this edition of

By useA bridge is designed for trains, pedestrian or road traffic, a pipeline or

waterway for water transport or barge traffic. An aqueduct is a bridge
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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.

Bridges are subject to unplanned uses as well. The areas underneathsome 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.

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, the People's

Republic of China. The central bridge was reserved exclusively for the

use of the Emperor, Empress, and their attendants.

EfficiencyA 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 the span. The bridge taking the greatest load is bythis test the most structurally efficient. A more refined 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 can carry, a test that emphasizes economy of

materials and efficient glue joints (seebalsa wood bridge).
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A bridge's economic efficiency will be site and traffic dependent, the

ratio of savings by having a bridge (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 associateddisposal, recycling, and replacement, less the value of scrap and reuse of

components. Bridges employing only compression 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 or box 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.

Documents

Basic Definition of a Bridge