Lecture 2- Bridge Superstructure (Deck Structural Forms & Behaviour)

8/13/2019 Lecture 2- Bridge Superstructure (Deck Structural Forms & Behaviour)

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ACEN3DAYTECHNICALTRAINING:INTRODUCTIONTOTHEDESIGNOFSHORTSPANBRIDGES

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LECTURE2

BRIDGESUPERSTRUCTURE

(Structural

Form,

Behaviour

and

Idealization)

OBJECTIVE

Outlininggeneralconsiderationsneedforconceptualplanningandpreliminarydesignofabridgesuperstructure.

Understandingthedifferentdeckformsandtheir loaddistributionbehaviour.

Alookatwaysofidealizingthedifferentbridgedeckbehaviourforanalysispurpose.

Andthesuitableanalysismethodsrequiredforeachdeckform.


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1.2. DifferencebetweenBuildingSlabandBridgeDeck Loading are conventionally uniform in building but complex inbridgesduetoconstantvariableactionsofheavymovingloads.

Bridgedecksareconstantlyexposedtosevereenvironmentalactionsi.e.temperaturechanges,effectsofwindetc.

Bridge deck is very load sensitive hence, longitudinal and lateraldistributionofstressesbecomevitalforeconomicdesign.


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2.0 DECKGENERALCONSIDERATIONS2.1. ConceptualDesign

Conceptualdesignforbridgesdependsontheexperienceandintuitionof

thedesigner.The term conceptualdesign ismore crucial for large/long

spanbridgeswhosestructuralcomplexitiesincrease innonlinearmanner

withrespecttotheirspanlengths.

DeckGeometricEfficiency

Forcast insitubridges,deckdesign istotallygovernbythespan length.

For

shorter

spans,

simpler

and

heavier

shapes

are

mostly

preferred.

And

as span length increases, deck shapesmust bemademore efficient to

reducetheweightinordertolimitbendingforces.Butwhenspanlength

becomesverylarge,prioritymustbegiventostructuralefficiencyandto

reduce weight. This is the reason why deck shapes progresses from

rectangularslabs(forsmallerspans)toclassicalboxgirders.

Geometricefficiencycoefficient:

Where I= flexural inertia,A thecrosssectionareaofdeck,yandyarethedistances from thecentreofgravity to theextreme fibres. Itsvalue

variesfrom0.333forrectangularslabtoabout0.550.65forboxgirder.

Evidentofefficientshape forchangingcrosssection iswhenpermanent

loads producemore than 75 to 80% of the bendingmoments at the

criticalcrosssection.


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ConstructionMethods

Decktypesbasedonbridgeconstruction:


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AestheticRequirement

Basedonthefunctionalityofthebridgestructuresomeinherentaesthetic

featuresmayberequiredi.e.inlongspanbridgeswhichareoftenusedas

nations landmark structures. Hence this must be considered in the

detaileddesignandthisissometimesdevelopedwiththecollaborationof

anarchitect.

2.2. BridgeDeckIdealizationIdealization is the rational process ofmaking simplifications about the

behaviourofstructuresforthepurposeofstructuralanalysisanddetailed

design.

PurposeofIdealization

Todistilthekeymodesofcomplexstructuralbehaviourintopracticalanalysis.

Toidentifytheloadtransmissionandtravelpaths. Toensure thatprimarymodesarecapturedby thechosenanalysismodelandthesignificanceofsecondarymodesondesignforcesare

evaluated.


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3.0 DECKSTRUCTURALFORMSPrincipal types of bridge deck are divided into beam, grid, solid slab,

beamandslabdeck,andcellular/boxdeck inordertodifferentiatetheir

individualgeometricandbehaviourcharacteristic.

3.1. BeamDecksThesearebridgedeckswith the lengthsexceeding theirwidthsby such

amountthatunderloadstheybendandtwistalongtheirlengths.

Figures(a)(d)belowshowtypicalformsofbeamdeck:


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Theyaremostcommoninfootbridges. Caneitherbesteel,reinforcedconcreteorprestressedconcrete. Theyareoftencontinuousovertwoormorespans.

3.2. ConcreteSolidSlabDeckThe use of solid slab bridge deck today, provides the simplest form of

concretebridgedecks. Easeofconstructionresultingfromthesimplicity

makesthisthemosteconomictypeforshortspanstructures.

Concreteslabdecksarecommonlyusedforshortspansandcanspanupto15mlong.

Theycanbesimplysupportedfromabutmenttopiersorcontinuousoverintermediatesupportstoendabutments.

Thetypicalslabthicknessisbetween1/25and1/35ofthespan.


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The slab depth increases as the span increase. And hence, thematerialweightof a solid slabwill start tobecome excessive as it

exceeds15m.

The ability for the reinforced solid slab to carry sagging (+ve) andhogging(ve)moments,makesitadequateforcontinuousmultispan

bridgesandallowsfortheuseoflongprestressingtendons.

(a)Solidslab;(b)Voidedslab;(c)Compositesolid;and(d)Compositevoid

3.3. ConcreteVoidedSlabDeckDue to the increasing and excessivematerialweight as solid slab span

exceeds 15m it comes necessary to lighten itsweight by incorporating

voids.Voided slabdecks are frequently constructed as cast insituwith

permanent void formers or of precast prestressed concrete boxbeams

posttensionedtransverselytoensuretransversecontinuity.

If the void sizes are less than 60% of the overall deck depth, then the


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effectofvoidsonthedeckstiffnessisnegligiblysmall,hencethedeckcan

be analysed effectively as solid plate. Butwhen the void sizes exceeds

60%of thedeckdepth, thedeckcangenerallybeconsideredascellular

constructionwithbehaviourdifferentfromvoidedslab.

3.4. GridDecksTheprimary structural form for thisdeck type isagridof twoormore

longitudinal beams with transverse beams which support the running

slab.

Because of the amount of workmanship needed for fabricating or

shuttering the transversebeams, thisdeck type is lesspopularnowand

hasbeen replacedby slabandbeamandslabdeckswithno transverse

diaphragms.

3.5. ConcreteBeamandSlabDeckThis deck type consists of a number of longitudinal beams connected

transversely across their tops by a thin continuous concrete slab. This

decktypecouldalsobesimplysupportedorcontinuousatsupports.

When transferring load longitudinally to the support, the slab acts in

concertwith thebeamsas top flanges.Andgreaterdeflection from the

most heavily loaded beamswill cause the slab tobend transversely so

thatittransfersandshareoutloadtotheadjacentneighbouringbeams.

Conventionalconcretebeamandslabcanbeconstructedas:

Concreteinsituslaboninsitubeams Concreteinsituslabonprecastbeams


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Maineconomicadvantagesare:

I. The use of Isection steel girder will reduce the weight of thesuperstructureandhence,theoverallcostofthefoundation.

II. Foragivenspanandloadingsystemasmallerdepthofbeamcanbeusedthanforaconcretebeamsolution,whichleadstoeconomiesin

theapproachembankments.

III. Thesystemallowsforspeedconstructionandhence,savestime.IV. Thecrosssectionalareaofthegirdertopflangecanbereducedsince

insituconcretecontributestothestiffness.

V.Transverse

stiffening

for

the

top

compression

flange

of

the

steel

girdercanbereducedsincerestraintagainstbuckling isprovidedby

theconcreteslab.

3.7. BoxgirderDeckTheircomplexstructuraldeckformsaremadeupofanumberofthinslab

andwebs.Thesedeckformsarepreferableforspanover40m.

Inadditiontothelessmaterialandlowweightadvantages,theyhavehigh

longitudinalbendingstiffnessandtorsionalstiffnessandthesegivethem

betterstabilityandloaddistributioncharacteristics.

Aboxgirdersectioncouldbemadeofasingleboxormultibox.Theyare

practicallythemost feasibledeck forms for longerspanbridgesandthis

placethemincompetitionwithothertypesofdecks.


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4.0 DECKSTRUCTURALBEHAVIOURANDIDEALIZATIONDeckbehaviour in termof loaddistribution is such that traffic loadson

bridge decks are distributed according to the stiffness, geometry and

boundaryconditionsofthedeck.

Methodofanalysismostappropriatetoaparticulardeckdependsonthe

complexityofthestructuralform.

4.1. BeamDeckandFrameBeam deck behaves as a beam thatwhen loads causes it to bend and

twist

along

its

length,

its

cross

section

displaces

bodily

and

do

not

change

shape.

Hence itsbehaviourcanbe idealizedusingsimplebeamelastictheoryof

bendingandtorsion.Inthesimplebeamelastictheory,itisassumedthat

thedecksectionremainsplaneandthatthebeamiscomposedofdiscrete

linearfibres inwhichthe longitudinalbendingstress isproportionalto

longitudinalstrain inthefibre.


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Flexureabouttheprincipalaxisofthesectioncanbeexpressedas:

Most longspanbridgesbehavesasbeamsbecause thedominate load is

concentrated so that thedistortionof thecrosssectionundereccentric

loadshasrelativelylittleinfluenceontheprincipalbendingstress.

Analysisofbeam deck can bedone asdeterminate beam structure for

simply supported case and as indeterminate beam structure for

continuouscases.

Beam decks can also exhibit frame action in which the stiffness and

geometryof the supportshave significant influenceon theirbehaviour.

Hence,theycanbeidealisedandanalysedeitherasplaneor3Dframe.

4.2. SlabDeckDeck slab is structurally continuous in twoorthogonaldirectionsof the

slabsothatanappliedloadissupportedby2dimensionaldistributionof

shears,momentandtorqueeffects.

Classicalplatetheorycanbeusedfordeterminingthebehaviourofsolid

slab but for simplicity, deck slab can be idealized and analysed using

similarassumptionforelastictheoryofbendingasbeams.


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(a)Prototypesoliddeckand(b)equivalentgrillageGrillage idealizationcanbesuitable forthepurposesofanalysisand the

dispersedbendingandtorsionalstiffnessesineveryregionoftheslabare

assumedtobeconcentratedinthenearestequivalentbeam.

Ideally thebeam stiffness shouldbe such thatwhen theprototype slab

and its equivalent grillage are subjected to identical loads the two

structuresshoulddeflectidentically.

4.3. BeamandSlabDeckBehaviourof a typicalbeamandslabdeckunder anaxle load is shown

below:

Oneofthedisadvantagesofthisdecktypeispoorloaddistributioninthe

transversedirectionwhich iswhymost timestransversediaphragmsare


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introducedatpointsalongthedeckspan.

Forasinglespanrightdeckonsimplesupportswithdifferentstiffnessin

two orthogonal directions, it ispossible,using classical plate theory, to

determinetheloaddistributedforeachmember.Butgrillageidealization

isfoundtobeeffectiveforthispurposeandiftheamountofloadcarried

by the most heavily loaded member can be found then the bending

momentcanbeeasilycalculated.

Equivalentgrillageidealizationsareshownbelow:

4.4. CellularDecksIt isoftenconvenientandacceptable tousecellularstructuresandbox

sectiondeckswhichdistortsundershearand torsional loadingsand it is

then necessary to take into account this distortion in the method of

analysis.

Thedisplacementof thisdeckcrosssectionunder loads fallsunder four


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principalmodes.

Longitudinalbending Transversebending Torsion Distortion

The behaviour of cellular decks can be analysed with shearflexible

grillage. The shear flexibility in the grillage idealization reproduces the

distortionbehaviourofthecell.

In flexible grillage, the deck is simulated by a grid of beamswhich are

given high torsional stiffnesses of the cellular deck and the slope

deflection equation takes into account the shear deformation in the

beams.


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4.5. BoxgirderDecksBoxgirderisverystiffinpuretorsionandmostofthetwistinthedeckis

due to distortion unless the box is bracedwith diaphragm. The use of

transverse bracing or framing provides a very effective method for

stiffeningaboxgirderagainstdistortion.

Figuresbelowillustratethebehaviourofboxsectiontoloads:

(a)Plane, (b) section, (c) to (e) components of loads and (f) distortiondeflection

Theamountofstiffeningisequallybeneficialtoeffectiveloaddistribution

and distortional flexibilitywhich helps the structure to spread the load

betweenthesupports.

Grillagemethod can be used but the analysismay not give sufficient

detailed picture of the flexural and membrane stresses in the plate

elementandforthisadditional3dimenssionalanalysiswillbenecessary.

Foldedplateanalysiswillprobablyprovidemostaccurate information if

the deck crosssection is uniform from end to endwith few transverse

diaphragms.

For complicated variations in the deck section, finite element analysis


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methodwillbenecessary.

Belowisillustrationoftwodifferentanalysismethodsforboxgirderdeck:

(a)Finiteelementmodeland(b)to(d)framemodelTypical comparison of result for frame and grillage analysismethods is

illustratedinthefollowing:

Resultfromspaceframeanalysisofaconcreteboxgirderdeck


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Resultfromgrillageanalysisofaconcreteboxgirderdeck

Documents

Lecture 2- Bridge Superstructure (Deck Structural Forms & Behaviour)