A Bridge is a Structure Built

8/8/2019 A Bridge is a Structure Built

1/17

Abridgeisastructurebuilttospanavalley,road,bodyofwater,orotherphysicalobstacle,forthepurposeofprovidingpassageovertheobstacle.Designsofbridgesvarydependingon

the function of the bridge, the nature of the terrain where the bridge is constructed, the

materialusedtomakeitandthefundsavailabletobuildit.

HistoryThefirstbridgesweremadebynatureitselfassimpleasalogfallenacrossastreamorstones

intheriver.Thefirstbridgesmadebyhumanswereprobablyspansofcutwoodenlogsorplanks

and eventually stones, using a simple support and crossbeam arrangement. Some early

Americansusedtreesorbamboopolestocrosssmallcavernsorwellstogetfromoneplaceto

another.Acommonformoflashingsticks,logs,anddeciduousbranchestogetherinvolvedtheuseoflongreedsorotherharvestedfiberswoventogethertoformaconnectiveropewhichwas

capableofbindingandholdinginplacematerialsusedinearlybridges.

TheArkadikoBridgeisoneoffourMycenaeancorbelarchbridgespartofaformernetworkof

roads,designedtoaccommodatechariots,betweenTirynstoEpidaurosinthePeloponnese, in

Greece.DatingtotheGreekBronzeAge(13thcenturyBC), it isoneoftheoldestarchbridges

still inexistenceanduse.Several intactarched stonebridges from theHellenisticera canbe

foundinthePeloponneseinsouthernGreece

The greatest bridge builders of antiquitywere the ancient Romans. The Romans built arch

bridges and aqueducts that could stand in conditions thatwould damage or destroy earlier

designs.Some stand today.Anexample is theAlcntaraBridge,builtover the riverTagus, in

Spain.TheRomansalsousedcement,whichreducedthevariationofstrengthfoundinnatural

stone.Onetypeofcement,calledpozzolana,consistedofwater,lime,sand,andvolcanicrock.

BrickandmortarbridgeswerebuiltaftertheRomanera,asthetechnologyforcementwaslost

thenlaterrediscovered.

TheArthashastraofKautilyamentionstheconstructionofdamsandbridges.AMauryanbridge

nearGirnarwassurveyedbyJamesPrincep.Thebridgewassweptawayduringaflood,andlater

repairedbyPuspagupta, the chiefarchitectofemperorChandragupta I. Thebridgealso fell

under the care of the Yavana Tushaspa, and the Satrap RudraDaman. The use of stronger

bridgesusingplaitedbambooand ironchainwasvisible in Indiabyabout the4thcentury.A

numberofbridges,bothformilitaryandcommercialpurposes,wereconstructedbytheMughal

administrationinIndia.

Although large Chinese bridges ofwooden construction existed at the time of theWarring

States,theoldestsurvivingstonebridgeinChinaistheZhaozhouBridge,builtfrom595to605

ADduring theSuiDynasty.Thisbridge isalsohistoricallysignificantas it is theworld'soldest

openspandrelstonesegmentalarchbridge.Europeansegmentalarchbridgesdateback toat


2/17

least the Alcontar Bridge (approximately 2nd century AD),while the enormous Roman era

Trajan'sBridge(105AD)featuredopenspandrelsegmentalarchesinwoodenconstruction.

Duringthe18thcenturythereweremany innovations inthedesignoftimberbridgesbyHans

Ulrich,JohannesGrubenmann,andothers.Thefirstbookonbridgeengineeringwaswrittenby

HubertGautier in1716.Amajorbreakthrough inbridgetechnologycamewiththeerectionof

theIronBridgeinCoalbrookdale,Englandin1779.Itusedcastironforthefirsttimeasarchesto

crosstheriverSevern.

With the Industrial Revolution in the 19th century, truss systems of wrought iron were

developedfor largerbridges,butirondidnothavethetensilestrengthtosupport large loads.

Withtheadventofsteel,whichhasahightensilestrength,muchlargerbridgeswerebuilt,many

usingtheideasofGustaveEiffel.

In1927weldingpioneerStefanBryadesignedthefirstweldedroadbridgeintheworldwhich

was later built across the river SudwiaMaurzyce near owicz, Poland in 1929. In 1995, the

AmericanWelding Society presented theHistoricWelded Structure Award for the bridge to

Poland.


3/17

Types of bridges

ARCH BRIDGE

Arch bridges are one of the oldest types of bridges and have great naturalstrength. Instead of pushing straight down, the weight of an arch bridge is carriedoutward along the curve of the arch to the supports at each end. These supports,called the abutments, carry the load and keep the ends of the bridge fromspreading out.

When supporting its own weight and the weight of crossing traffic, every part ofthe arch is under compression. For this reason, arch bridges must be made ofmaterials that are strong under compression.

The Romans used stones. One of the most famous examples of their handiworkis the Pont du Gard aqueduct near Nmes, France. Built before the birth of Christ,the bridge is held together by mortar only in its top tier; the stones in the rest ofthe structure stay together by the sheer force of their own weight. Todaymaterials like steel and pre-stressed concrete have made it possible to buildlonger and more elegant arches, including a spectacular 1700 foot span in NewRiver Gorge, West Virginia. (More typically, modern arch bridges span between200-800 feet.)Constructing an arch bridge can be tricky, since the structure iscompletely unstable until the two spans meet in the middle.

One technique is to build elaborate scaffolding, or "centering," below the spans tosupport them until they meet. A newer method supports the spans using cablesanchored to the ground on either side of the bridge. In situations where there isan active water or road way below, this method allows contractors to buildwithout disrupting traffic.One of the most revolutionary arch bridges in recentyears is the Natchez TraceBridge in Franklin, Tennessee, which was opened totraffic in 1994.

It's the first American arch bridge to be constructed from segments of pre-castconcrete, a highly economical material. Two graceful arches support the roadwayabove. Usually arch bridges employ vertical supports called "spandrels" to

distribute the weight of the roadway to the arch below, but the Natchez TraceBridge was designed without spandrels to create a more open and aestheticallypleasing appearance. As a result, most of the live load is resting on the crowns ofthe two arches, which have been slightly flattened to better carry it. Already thewinner of many awards, the bridge is expected to influence bridge design foryears to come.


4/17

BEAM BRIDGEA beam or "girder" bridge is the simplest and most inexpensive kind ofbridge. According to Craig Finley of Finley/McNary Engineering, "they'rebasically the vanillas of the bridge world." In its most basic form, a beambridge consists of a horizontal beam that is supported at each end by piers.The weight of the beam pushes straight down on the piers. The beamitself must be strong so that it doesn't bend under its own weight and theadded weight of crossing traffic. When a load pushes down on the beam,the beam's top edge is pushed together (compression) while the bottomedge is stretched (tension).

Pre-stressed concrete is an ideal material for beam bridge construction;the concrete withstands the forces of compression well and the steel rodsimbedded within resist the forces of tension. Pre-stressed concrete alsotends to be one of the least expensive materials in construction. But eventhe best materials can't compensate for the beam bridge's biggestlimitation: its length.The farther apart its supports, the weaker a beambridge gets. As a result, beam bridges rarely span more than 250 feet.

This doesn't mean beam bridges aren't used to cross great distances -- itonly means that they must be daisy-chained together, creating what'sknown in the bridge world as a "continuous span." In fact, the world'slongest bridge is a continuous span beam bridge. Almost 24 miles long,the Lake Ponchartrain Causeway consists of two, two-lane sections thatrun parallel to one another. The Southbound Lane, completed in 1956, ismade up of 2243 separate spans, while the Northbound Lane, completedin 1969, is pieced together from 1500 longer spans. Seven cross-overlanes connect the two main sections and function as pull-over bays inemergencies. Although impressive, the Lake Ponchartrain Causeway

bridge underscores the drawback of continuous spans: they are not wellsuited for locations that require unobstructed clearance below.


5/17

SUSPENSION BRIDGEAesthetic, light, and strong, suspension bridges can span distances from 2,000 to7,000 feetfar longer than any other kind of bridge. They also tend to be themost expensive to build. True to its name, a suspension bridge suspends theroadway from huge main cables, which extend from one end of the bridge to theother. These cables rest on top of high towers and are secured at each end byanchorages. The towers enable the main cables to be draped over longdistances. Most of the weight of the bridge is carried by the cables to theanchorages, which are imbedded in either solid rock or massive concrete blocks.

Inside the anchorages, the cables are spread over a large area to evenlydistribute the load and to prevent the cables from breaking free. Some of the

earliest suspension bridge cables were made from twisted grass. In the earlynineteenth century, suspension bridges used iron chains for cables.Today, the cables are made of thousands of individual steel wires bound tightlytogether. Steel, which is very strong under tension, is an ideal material for cables;a single steel wire, only 0.1 inch thick, can support over half a ton withoutbreaking Currently, the Humber bridge in England has world's longest centerspan measuring 4,624 feet. But this record won't stand for long. In 1998, theJapanese will unveil the $7.6 billion Akashi Kaikyo Bridge, linking the islands ofHonshu and Shikoku.

The bridge's center section stretches a staggering 6,527 feet. To keep thestructure stable, engineers have added pendulum-like devices on the towers to

keep them from swaying and a stabilizing fin beneath the center deck to resisttyphoonstrength winds. Because suspension bridges are light and flexible, windis always a serious concernas the residents of Tacoma, Washington can surelyattest

THE HUMBER BRIDGE : MORE FACTSThe Humber Bridge is a suspension bridge with the north tower sited on the high water

line and the south tower founded in shallow water 500m from the shore. On the north

bank, a hard well-jointed bed of chalk comes close to the surface and is covered by a

tough layer of glacially deposited chalky boulder clay. The chalk has provided goodfoundations for both the anchorage and tower on this bank, on the south side, soft

alluvium is underlain by beds of boulder clay, sand and gravel.

Below these beds, at a depth of 30m, there is a deep bed of stiff,heavily fissured

kimmeridge clay, on which the tower and anchorage have been founded. Designed to

cross the last major unbridged estuary in Britain, the bridge comprises reinforced

concrete towers aerial-spun catenary cables and a continuously-welded, closed box

road deck supported by inclined hanger cables.


6/17

CABLE-STAYED BRIDGECable-stayed bridges may look similar to suspensions bridgesboth haveroadways that hang from cables and both have towers. But the twobridges support the load of the roadway in very different ways. Thedifference lies in how the cables are connected to the towers. Insuspension bridges, the cables ride freely across the towers, transmittingthe load to the anchorages at either end. In cable-stayed bridges,thecables are attached to the towers, which alone bear the load.The cablescan be attached to the roadway in a variety of ways. In a radial pattern,cables extend from several points on the road to a single point at the topof the tower. In a parallel pattern, cables are attached at different heights

along the tower, running parallel to one other.

Even though cable-stayed bridges look futuristic, the idea for them goesback a long way. The first known sketch of a cable-stayed bridge appearsin a book called Machinae Novaepublished in 1595, but it wasn't until thiscentury that engineers began to use them. In post-World War II Europe,where steel was scarce, the design was perfect for rebuilding bombed outbridges that still had standing foundations. Cable stay bridges have begunto be erected in the United States only recently, but the response hasbeen passionate

For medium length spans (those between 500 and 2,800 feet), cable-stayeds are fast becoming the bridge of choice. Compared to suspensionbridges, cable-stayeds require less cable, can be constructed out ofidentical pre-cast concrete sections, and are faster to build. The result is acost-effective bridge that is undeniably beautiful.

In 1988, the Sunshine Skyway bridge in Tampa, Florida won theprestigious Presidential Design Award from the National Endowment forthe Arts. Painted yellow to contrast with its marine surroundings, theSunshine Skyway is one of the first cable-stayed bridges to attach cablesto the center of its roadway as opposed to the outer edges, allowing

commuters an unobstructed view of the magnificent bay.Recently, inBoston, Massachusetts, a cable-stayed design was selected for a newbridge across the Charles Rivereven though cheaper options wereproposed. City officials simply liked the way it looked.


7/17

Construction of BridgesIt is one thing to design a bridge. It is another thing to buildit. Planning and executing the construction of a bridge is often verycomplicated, and in fact may be the most ingenious (ingnieur -engineer) parts of the entire enterprise. An incomplete structure isoften subjected to stresses and oscillations that would not arise aftercompletion. The construction work is potentially a grave hindrance toexisting traffic and to normal life in the area, especially when largelocal fabrication works have to be installed.

Even before any actual construction is done, substantial work may

needed in the form of tests. Boreholes will be made to check thecondition of the ground, in conjunction with any available geologicalmaps. Records of wind speed and direction will be consulted, and newmeasurements made if necessary. In the case of a river or seacrossing, records of water levels and velocities will be needed. Modelsof the bridge or of parts may be tested aerodynamically andhydrodynamically, and of course mechanical tests will bemade. Computer simulations will supplement these tests, enabling agreat variety of applied forces to be investigated. There may also beinvestigations into the effects on people and on the natural

environment. It may even be necessary to overcome opposition to theconstruction, from a variety of objectors. There was much opposition,for example, to the construction of the Skye bridge. In older times,ferry operators could be extremely vociferous about a bridge proposal.This website includes very little material about bridge construction,because the subject is so varied and so specialist. As soon as possible,some useful links will be provided, and if possible, some simpleexplanations as well.Meanwhile, let's look at the living world, to see how things are donethere. Starting at the beginning, an immature form is likely to bevulnerable. A half built medieval castle must have been hard todefend. In modern times, a half built tank or aircraft is vulnerable ifbombs can be dropped on the factory. In the animal world, there aremany solutions.One is to produce so many offspring that an enormous mortality rate isacceptable. All that matters is that on average, enough survive toproduce an equally large next generation.


8/17

Another way is to nourish and protect young until their probability ofsurvival is sufficiently high. From eagle chicks to tiger cubs to babycrocodiles, even the fiercest animals can be vulnerable whenyoung. Many animals, from earwigs to apes, protect theiryoung. Nevertheless, mortality can be very high once the young leavetheir parents. In the mammals, this method is taken furthest, byretaining the young inside until they are quite large. Question, whyare baby mammals more round and cuddly looking than adults?If the young have already some defence, as in vipers, they can be leftalone from the start. But even there, some species retain the eggsinside until they have developed into little versions of the adult.A common strategy to cope with immaturity is to undergometamorphosis. By adopting very different forms, behaviours andhabitats at different stages of growth, animals can inhabit ecologicalniches that are optimal at each stage. This is most easily seen inmany insect orders, though prevalent in other classes which are lesscommonly seen. Sometimes there is a passive pupal stage, duringwhich the tissues are so radically reorganized that the adult, or imago,is almost unrecognizably different from the larva. The larval stages,which are devoted mainly to eating and growing, are very often notvery mobile, whereas the adult stage, or imago, is very frequently

mobile, because males and females have to meet. Mating with nearbyexamples from the same brood is deleterious genetically, so theemerging adults make at least some attempt to move. There arespecies in which the females are wingless, leaving the males to do thetravelling.The bridge builder, like the mammals, has to nurse the embryostructure through difficult stages. Very often, the stresses differconsiderably from those of the complete structure, and can be moreconcentrated. The collapse of several box girder bridges in the 1960sdamaged the reputation of this type for a period, until the stresses

were better understood. Note the paradox that an apparently simplestructure, a set of boxes, can be hard to understand, while acomplicated looking truss can be solved, at least in principle, using aset of equations. The variation in stresses during construction may beso severe that jacking must be provided. The four legs of the TourEiffel were provided with jack which were adjusted from time to timeas the structure grew. Even after completion, a concrete structuremay be subject to creep, and the ground may settle; jacks aretherefore provided for later adjustment when this behaviour isforeseen.


9/17

The pillars of the towers of big suspension bridges may have to bestabilized by temporary cables until they are completed and joined atthe top.Arches generally need to be supported on falsework until they arecomplete. Perhaps this is the origin of the word "keystone", the lastblock without which the structure cannot hold up, though in thefinished arch, the keystone is no more important than any othervoussoir. So the value of the word keystone is to remind us that untilthe structure is complete, we have to keep thinking.

Completeness includes completeness of communication. The chain ofcommand and communication must be designed to cope with every

foreseeable situation, and it must include rules for dealing withemergencies and unforeseen problems. Designers and builders haveranged from those who have overly interfered in small details thatshould have been delegated, I K Brunel, for example, and those inwhich day to day disconnection has contributed to eventual failure,and indeed total disaster, as in the first attempt at the Quebec bridge.Some very large arches have been built by treating the halves ascantilevers until they meet in the middle. Whether or not this methodis used, joining the parts of any big structure is a very serious

matter. The stresses in the separate parts are different from thosethat will apply in the complete system. If the parts are just left to restagainst each other, or joined as they meet, the resulting stresses maybe far from those that are required. Some form of jacking will oftenbe required.

Furthermore, the temperature and wind may make life difficult for thebuilders. There have been occasions when heaters or ice-packs havebeen used. Spinning the cables of the Forth road bridge was notpossible on many days because of high winds.


10/17

Construction of Arches

This pictures show the corbels upon whichthe centring was erected. When thecentring has been removed, or struck, thearch will inevitably settle slightly. This isinevitable, because it can only generate therequired compressive forces by undergoing

some strain. All structures, in fact, mustdeflect when temporary support is removed.Bridge, which sank about ten inches (25 cm)at the crown. Telford had wanted to buildone of his standard 150 foot cast iron arches,which would have been a very economical

solution, but some important people in Gloucester decided that theydid not want iron, even though Over is not in the city, and not evenvisible from it. These people should have known better, because thefact that Gloucester is built entirely on the east side of the river Severn


11/17

is a constant reminder that the ground on the west side is soft andunsuitable for construction work. Apparently the information thatTelford was given about the soil conditions were faulty. Nevertheless,the bridge was in use until the second half of the 20th century, andstill stands as one of many testimonies to Telford's skill, and also asone of many testimonies to the results of powerful people makingdecisions on matters of which they know little or nothing. In this casethe result was a very heavy and very expensive bridge.

Concrete bridges may be supported in a similar way, except that thecentring will support the formwork in which the concrete ispoured. For large bridges, the centring will be a substantial structurein its own right, and will be expensive. For a multi-arch bridge, it isdesirable to re-use the centring for each span. In the case of a bridgeacross a river, the centring may be floated from one arch to the next,and will be in the form of a tied arch to maintain it integrity. The

centring for the illustrated arch would have been impressive.


12/17

The stresses in the arch during construction would be different fromthose expected after completion, and the tensions in the temporarystays need to be adjusted as new sections are added. An advancedversion of this method was used for the Tilos bridge on la Palma,which is based on a concrete arch. The verticals used during theconstruction were the same ones that form spandrel piers in thecompleted bridge. These were connected at the top by horizontalmembers during construction. Unlike the deck of the completed bridge,these members were clearly in tension, illustrating very clearly howstresses in completed and uncompleted structures may differ greatly.

Change of stress of course implies change in strain, and this influencedthe design of the formwork for the concrte arch. Over the monthsfollowing completion of construction, any concrete element incompression will be slightly reduced in size. Because the Tilos archhas no hinges, the change in length would induce bendingmoments. Massive jacks were provided to allow froadjustments. Because of the differing stresses in the various phases,this bridge was designed to be very light.

The bowstring or tied arch isexceptional in that it can be treatedas a unit which creates no outwardthrust. In principle such an arch canbe built away from the final site andthen lifted or slid into place. Thearch illustrated here was built closeto the M42 motorway and slid into

place during a single night, with minimal disruption of traffic.


13/17

Construction of Beam BridgesBeam bridges are generally in the form of plate girders, box girders ortrusses. In all cases, a common construction method is to build thebeam away from the final position and slide it or lift it into place as acomplete unit. Bowstring or tied arches may be built in the same way.There is nothing especially complicated about the lifting process, andthe stresses in the beam are more or less as they will be in the final

position. Nevertheless, a heavy object suspended in space ispotentially dangerous, and accidents do happen. During the lifting ofthe suspended span of the Quebec bridge, something broke, and thespan fell into the river and was destroyed. During the lifting of onespan of the Britannia bridge, a jack burst, and the end of the spanfell. Fortunately, Stephenson had given strict orders to insert packingafter every few inches of lifting. Nevertheless, a slight distortion of thebeam did occur. The incident shows the importance, not only ofcorrect instructions, but of good communications. Accidents havehappened because instructions were not received, or if received,modified or ignored.

An exception to the method of lifting may be adopted in the case ofthe suspended span of a cantilever bridge, as in the case of the Forthrailway bridge. Here, the beams were built out from the ends of thecantilever arms until they could be joined in the middle. Only thenwere the temporary rigid connections removed. Such an operation has


14/17

to be done with great care, because large stresses are released andcreated. The bending moment at the join completely disappears, forexample, while the stress in the top chord of the beam changes fromtension to compression. The faked picture shows the general idea.Some continuous box girder beams and concrete beams areconstructed as cantilevers until they are joined up. New parts may betaken along the existing part of the bridge, or they may be lifted upfrom below. Sections of a segmented prestressed concrete bridge maybe lifted by a crane which rests on previously attached segments, orthey may be manipulated by means of a launching girder which isslowly advanced along the already built part of the span. Anothermethod is to use moving formwork which progresses with the

construction. Because the cantilevered part span is subject to stressesfor which the complete span is not designed, temporary stays aresometimes attached between the segments and a temporary tower,turning the span into a cable-stayed one until closure is complete.Yet another technique, when the lower surface of the span ishorizontal, is to rest the formwork on a steel girder which reaches tothe next pier.

The casting of each segment is often done using the previous segment

as part of the formwork. This technique, called match-casting,ensures an almost perfect fit at assembly time, and greatly reducesthe tendency for misalignments to occur during erection. A film ofepoxy may be spread between segments.


15/17

Construction of Cantilever Bridges

A great advantage of a cantilever is that it can be built out from asupport without hindrance to traffic or navigation below. The

technique varies according to the type of bridge. It will also dependon whether the cantilever is attached rigidly to a free standingtower. Some towers are hinged at the base, in which case falseworkwill be needed. Even with fixed piers or towers the span may beattached only on a pivot rather than a fixed connection - falsework willbe needed here too.New parts may be taken along the existing part of the bridge, or theymay be lifted up from below. Sections of a segmented prestressedconcrete bridge may be lifted by crane, or they may be manipulated

by means of a launching girder which is slowly advanced along thespan. An alternative may be moving formwork which progresses withthe construction. The idea is explained in more detail in the sectionabout beam construction.


16/17

Construction of Cable-Stayed Bridges

One advantage of cable-stayed bridges over suspension bridges is thatthey can be built out symmetrically from the towers, though longspans are rather flexible until the time of closure.


17/17

Summary

Abridgeisastructurebuilttospanavalley,road,bodyofwater,orotherphysicalobstacle,forthepurposeofprovidingpassageovertheobstacle.At first,bridgearemadebynature itself.

After thathumans started tobuildabridgeby themselvesusing treesorbamboopoles,and

thenevolutiontostone,cement,stealandsoon.

There are 5 typesofbridgewhich are archbridge,beambridge, suspensionbridge,humber

bridgeandcablestayedbridge.Firstofall,archbridgesareoneoftheoldesttypesofbridges

andhavegreatnaturalstrength.Theweightofanarchbridgeiscarriedoutwardalongthecurve

of thearch to the supportsateachend.When supporting itsownweightand theweightof

crossingtraffic,everypartofthearchisundercompression.Forthisreason,archbridgesmust

bemadeofmaterialsthatarestrongundercompression.Second,abeamor"girder"bridge is

thesimplestandmostinexpensivekindofbridge.Abeambridgeconsistsofahorizontalbeam

that issupportedateachendbypiers.Theweightof thebeampushesstraightdownon the

piers. When a load pushes down on the beam, the beam's top edge is pushed together

(compression)whilethebottomedgeisstretched(tension).Soprestressedconcreteisanideal

materialforbeambridgeconstruction.Thirdlyasuspensionbridgesuspendstheroadwayfrom

hugemain cables. It is also tend tobe themost expensive to build.Next ishumber bridge.

Humberbridgeisasuspensionbridgewiththenorthtowersitedonthehighwaterlineandthe

southtower

founded

in

shallow

water

500m

fromthe

shore.

Last

but

not

least,

is

cable

stayed

bridge.Cablestayedbridgesmay looksimilartosuspensionsbridgesbutthedifference lies in

howthecablesareconnectedtothetowers.

To design a bridge n build a bridge is two different things. Planning and executing the

construction of a bridge is often very complicated, and in factmay be themost ingenious

(ingnieur engineer)partsoftheentireenterprise. Anincompletestructureisoftensubjected

to stresses and oscillations thatwould not arise after completion. The constructionwork is

potentiallyagravehindrancetoexistingtrafficandtonormal life inthearea,especiallywhen

largelocalfabricationworkshavetobeinstalled.

Documents

A Bridge is a Structure Built