04-aashto lrfd design provisions for prestressed concrete bridge

8/22/2019 04-AASHTO LRFD Design Provisions for Prestressed Concrete Bridge

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Dr.SamiW.Tabsh,P.E.

CivilEngineeringDepartment

AmericanUniversityofSharjah

AASHTOLRFDDesignConsiderations

forPrestresed ConcreteBridges

Prestressed ConcreteBridges

Prestressed concreteoffersmanyadvantagesoverothermaterials:

Costsavings Reducedinitialconstructioncosts

andlower

maintenance

costs.

Durability ConcreteBridgescaneasilywithstanextremetemperaturechangesandcorrosive

chemicalsinavarietyofconditions.

Competitiveness thevalueofconcreteisrepeatedlyrecognizedincompetitivebidding

situations.

Prestressed ConcreteBridges

BridgetrendintheUnitedStates

TypesofBridges

Thestructuralsystemofprestressed concretebridgesmayconsistof:

Slab

Beam/girder

Rigidframe

Cantilever

Cablestayed

TypesofBridges

CompositeIGirder

TypesofBridges

Pretensioned concrete Posttensionedconcrete


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AASHTOBackground

AASHTObeganpublishingtheStandardSpecificationsforHighway

Bridgesinthe1930s.

Upuntil

1970,

the

allowable

stress

design(ASD)wasthemainmethod

ofstructuraldesign.

Inthe1970s,AASHTObeganvaryingthefactorofsafetyforeachloadinrelationtothe

uncertaintyintheload,referredtoastheload

factordesign(LFD).

AASHTOBackground

AASHTOmadethechangefromASDtoLFDinthformofinterimrevisionstotheStandard

Specifications. AASHTOhadnevertotally

rewrittenits

Standard

Specifications.

ThisintroducedgapsandinconsistenciesintheLFDspecificationsthatmadeitdifficultto

maintainandmodernize.

Today,thebridgedesignprofessionhasmovedtLoadandResistanceFactorDesign(LRFD).

DesignPhilosophy

TheLoadandResistanceFactorDesign(LRFD)philosophyinAASHTOisbasedon:

where =factorrelatedtotheductility,redundancyandoperationalimportance,

Qi =loadeffect,

i =loadfactor(statisticallybased),

Rn =nominal

resistance,

=strengthreductionfactor(statisticallybased).

ContentsofAASHTOsLRFD

MajorChangesofAASHTOsLRFD

ThemajorchangesintheLRFDspecificationfromthestandardLFDspecificationsare:

Newlive

load

model

and

load

distribution

factors.

Calibratedloadandresistancefactors.

Adoptionofthemodifiedcompressionfieldtheory,andstrutandtiemodelingforconcretestructures.

Limitstatebasedprovisionsforfoundationdesign.

Considerationofshipcollision.

Guidanceonthedesignofcurvedsteelgirderandsegmentalconcretebridges.

SpanCapability

Foroptimizeddesign,considerthefollowingissues

Continuousspansaremoreeconomicalthanaseriesof

simplespans.

Incontinuousspans,proportionthespanlengthssuch

thattheendspansare75%82%oftheinteriorspans.

ThisresultsinM+ nearlyequalinallspans.

Formaximumeconomy,thelengthofdeckslaboverhan

is0.280.35oftheinteriorgirderspacing.

Usethesamesizeforbothinteriorandexteriorgirders.

Usingfewergirdersatwidespacing(upto5m)ismore

economicalthanusingmanygirdersatsmallspacing.


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CAST-IN-PLACE, POSTTENSIONED CONCRETE BOX GIRDER

SPAN RANGE (m) DEPTH/SPAN COST/SQ-FT

SIMPLE CONT25-75 0.045 0.040 $75-$145

Advantages Disadvantages

Commonly used in the UAELow costLow depth/span ratioHigh seismic resistance due tolarge torsional stiffness

Longer construction time thanprecast concreteLabor intensiveRequires shoring for falsework

Note: Costsareforyear2000dollarsinCalifornia

PC/PS I-GIRDER

SPAN RANGE (m) DEPTH/SPAN COST/SQ-FT

SIMPLE CONT

15-40 0.055 0.050 $90-$170

Advantages Disadvantages

Simple construction methodLow costNo false-work requiredRequire little or no maintenance

Has to be prismaticPoor seismic resistanceLow negative moment capacity

Note: Costsareforyear2000dollarsinCalifornia

TypesofLoads

Ingeneral,loadsimposedonbridgescanbeclassifiedinto:(1)Permanent,and(2)Transient.

Permanentloadsareloadsthatarealwayspresentonthebridgeanddonotchangeinmagnitudeduringitslife. Theyinclude: DC:weightofStructuralComponentsandAttachments DW:WeightofWearingSurfacesandUtilities EH:HorizontalEarthPressure ES:EarthSurchargeLoad EV:VerticalPressureofEarthFill

TypesofLoads

Transientloadsareloadsthatarenotalways

presentonthebridge,orchangeinmagnitude

duringitslife. Theyinclude:

BR:VehicleBrakingForce CE:Vehicle Centrifugal Force CT:Vehicle CollisionForce CV:Vessel CollisionForce

EQ:Earthquake LL:VehicleLiveLoad IM:DynamicLoadAllowance

IC:IceLoad PL:PedestrianLiveLoa SE:Settlement TG:TemperatureGrad

TU:UniformTemperat WL:WindonLiveLoad WS:WindonStructure

TypesofLoads

17

Earthquake PedestrianLoad

Wind VehicleLiveLoad

TypesofLoads

1

Vessel Collision Vehicle Collision

IceLoadSettlement


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LoadCombinations

Therearefiveloadcombinationsforthestrength limitstate:

LoadCombinations

Therearetwoloadcombinationsfortheextremeventlimitstateandonefatiguelimitstate:

*determinedonaprojectspecificbasis.

*

LoadCombinations

Therearefourloadcombinationsfortheserviceabilitylimitstate:

ApplicationofDeadLoad

Inadditiontoselfweight,provisionsareoftenmadeinthedesignfor:

1.0to1.5kPa forfuturewearingsurfacetothedeck.

0.75kPa foruseofstayinplacemetaldeckformsin

projectsinvolvingstructural

steelorprecastconcrete.

Theweight

of

the

integral

sacrificialwearingsurface

(thickness=10 to30mm).

ApplicationofLiveLoad

TheDesignLiveLoadisHL93consistsofadesigntruck

or

design

tandem

applied

simultaneously

withadesignlaneload,whichevergivesthe

largereffectonthebridge.

Fornegativemomentbetweeninflectionpoints,90%oftheeffectoftwodesigntrucksspacedat

aminimumof15000mmcombinedwith90%of

thedesignlaneload.


0.90x 15000mmMIN

35

kN

145

kN

145

kN

35

kN

145

kN

145

kN

ForNegativeMomentBetweenInflectionPoints

9.3N/mm

35kN 145kN 145kN 110kN 1 10kN

9.3N/mm 9.3N/

4300mm 4300to

9000mm

1200mm

ORdesigntruck+designlane designtandem+designla


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DesignTruck,DesignTandem andDesignLane loadsareappliedovera 3.0mwidth,withinthe3.6mlanewidth.

145kN

145kN

35kN1.8m

3.0m

3.0mlane

3.1kPa

DesignTruckLoad

DesignLaneLoad

1.8m

3.0m

Adpated from:Cole(PCAseminar)


Theextremeliveloadeffect

shallconsideranumberof

transverselyloadedlanes

multipliedbyamultiple

presencefactortoaccount

fortheprobabilityof

simultaneouslane

occupationbythefullHL93

designliveload.


Thedynamicloadallowance,IM,accounts

forthedynamicportion

ofthetruckloadonthe

bridge. Itisequalto:

DeckJoints:75%

Fatigue:15%

All

other

cases:

33%appliedtothetruck

portionofliveload.

AnalysisofDeckSlab

AASHTOapprovestheanalysisoftheconcrete

slabbysubdividingit

intostripsoriented

perpendiculartothe

supportingcomponents

(girder)andtreatingthe

strip

as

a

beam

on

rigid

supports.AASHTOsstripmethodforsla

AnalysisofDeckSlab

=1140+0.833X

=660+0.55S

=1220+0.25S

Definitionofthestripwidth,SW(mm)

S

AnalysisofDeckSlab

AppendixA4.1includesliveloadmomentsfortypicalconcretedecks,inlieuofmoreprecise

calculations,if

they

meet

specified

condition.


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GirderDistributionFactors

TheAASHTOSpecificationsusethedistributionfactorconcepttoapproximatethesharingofloadby

thegirdersinabridge.

The

distribution

factor

(DF)

allows

a

3

dimensional

loadtransfermechanismtobereplacedwithamuch

simpleronedimensionalsystem.

P

GirderDistributionFactors

Thegirderdistributionfactordependson: Consideredlimitstate(flexureversusshear)

Locationofgirder(exteriorversusinterior)

Type

of

bridge

superstructure Geometryofbridge(i.e.skewness,overhangwidth)

Spacingandstiffnessofgirders

Thicknessofthedeckslabandlengthofbridge

Theyareappliedasfollows:

Mdesign/girder =DF (Mper lane)LLVdesign/girder =DF (Vper lane)LL

GirderDistributionFactorsforMoment GirderDistributionFactorsforShear

TableDistributionofLiveLoadperLaneforShearinInteriorBeams.

GirderDistributionFactorsforMoment&Shear

(d)

TableDistributionofLiveLoadperLaneforMomentinInteriorBeams.

TableDistributionofLiveLoadperLaneforShearinInteriorBeams.

Prestressed Concrete

Prestressing consistsofpreloadingthestructurebeforeapplicationofdesignloadsinsuchaway

soas

to

improve

its

general

performance

by:

Reducingoreliminatingtensilestressesintheconcrete

Controllingdeflection

Allowingtheuseofhighstrengthsteelandconcrete

Permittingtheuseofshallowerstructures

Eliminatingfatigueproblems

Increasingdurability


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TypesofPrestressing

Prestressing ofconcretecanbeachievedby:

Prestensioning: requirestensioning

betweenrigidabutments,ina

fabrication

plant

and

transporting

theelementtothesite. Thus,itis

suitedformassproduction.

Posttensioning: requirestensioning

againsthardenedconcreteonsite.It

allowsforvaryingtheprestressing

eccentricity alongthememberto

suittheparticularloadeffectwithin

thestructure.

Pretensioned Concrete

Pretensioning Sequence:1. Thesteelstrandsare

stretchedthroughtheformbetweentwoendanchors.

2. Apredetermined

amount

of

stressisappliedtothesteelstrands.

3. Theconcreteisthenpoured,encasingthesteel.

4. Strandsarecutwhentheconcretereachesaspecifiedearlystrength.

Pretensioned Concrete Pretensioned Concrete

PosttensionedConcrete

Posttensioningsequence:1. Placesteelcageandpost

tensioningducts

in

formwork.

2. Castconcreteintheformwork

andcuretheconcrete.

3. Afterconcreteishardened,

thetendonsaretensionedand

anchoredagainstthemember.

4. Theductisthengroutedto

completetheposttensioning

operation.

PosttensionedConcrete


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Prestress Losses

Levelofprestress varieswithtime.Lossesduetoanchorageset,frictionandelasticshorteningareinstantaneous.Lossesduetocreep,shrinkageandrelaxationaretimedependent.

Concrete

Prestressed concretecanbenefitfromhigh

strengthconcrete(5070MPa)becauseweget:

TypicalStressStrainCurves

ConcreteinCompression

Higherallowablestresses(to

prestress atan

early

stage)

Increasedelasticmodulus

(lesslossofprestress dueto

elasticshortening)

Reducedcreepandshrinkage

(lesslossofprestress dueto

volumechanges)

Prestressing Steel

Widerangeofwires,strandsandbarsavailable

Highstrength(1860MPa forstrands&1000MPa forbars)

Diameters:

Wires


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FlexuralDesignConsiderations

BehaviorofPrestressed ConcreteMembers

FlexuralDesignConsiderations

GeneralAssumptionsforFlexuralDesignofPrestressed ConcreteMembers ServiceLoadDesign:

Concreteisuncracked

Stressin

prestressing steel

is

linearly

related

to

strain

Iteratetodeterminestrandpatternandsatisfystresslimits

CheckStrengthatCriticalSections: Concrete

inelasticincompressiveregions

tensilestrengthisneglected

Prestressing steel inelastic

StrengthReductionFactors

Theresistancefactorsforthestrengthlimitstateforprestressed concreteare:

Flexure:

Tensioncontrolledregion: 1.00

Compressioncontrolledregion: 0.75

Shearandtorsion: 0.90

Compressionin

anchorage

zone: 0.80

Tensioninanchoragezone: 1.00

ServiceLimitState

1.ComputeStressesatRelease

NonCompositeSection(BareGirder)

Loads:

Girderselfweight

Initialprestress

Topofgirder:

Bottomofgirder:

t

gdl

t

iiRt

S

M

S

eP

A

Pf

b

gdl

b

iiRb

S

M

S

eP

A

Pf

ServiceLimitState

ComputeStressesatRelease

ServiceLimitState

2.ComputeStressesatService LimitStateAfteLosseswithPermanent&TransientLoads:

LoadsonNonCompositeSection Girder,deckdeadloads

Otherdeadloadsappliedbeforeplacingdeck(e.g.diaphragms)

Finalprestress (afterlosses)

LoadsonCompositeSection Barrierandfuturewearingsurface

Otherdeadloads(utilities,etc.)

Vehicularliveloadandimpact


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ServiceLimitState

ComputeStressesatServiceLimitStateAfterLosseswithPermanentandTransientLoads

Topofdeck:

Topofgirder:

Bottomofgirder:bcg

ILLcdl

b

ncdlgdl

b

eeLPbg

S

MM

S

MM

S

eP

A

Pf

tcg

ILLcdl

t

ncdlgdl

t

eeLPtg

S

MM

S

MM

S

eP

A

Pf

tcd

ILLcdlLPtd

S

MMf

ServiceLimitState

StressesatServiceLimitStateAfterLosseswith

PermanentandTransientLoads

ServiceLimitState

StressLimitsforPrestressing Tendons(5.9.31)forPretensioned Construction:

ServiceLimitState

StressLimitsforPrestressing Tendons(5.9.31)fPosttensioned Construction:

ServiceLimitState

Compression StressLimitsforConcrete (5.9.4.1.1&5.9.4.1.2):

ForTemporary StressesBeforeLosses(FullyPrestressed,Pretensioned orPosttensionedComponents):0.60fci

ServiceLimitState

Tensile StressLimitsforConcrete forTemporaryStressesBeforeLosses(Pretensioned orPost

tensioned):


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ServiceLimitState

Compression StressLimitsforConcrete atServiceLimitState(SERVICEI)AfterLosses(Fully

Prestressed,Pretensioned orPosttensioned):

wherew =1.0formemberswithspan/thicknessratio


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StrengthLimitState

Theminimumflexuralreinforcementisbasedon(LRFD5.7.3.3.2):

Mr

=Mn1.2M

cr

NotethatthecrackingmomentMcr isobtainedfrom:

where f r =0.97 fc

rc

nc

c fSS

S

1M-)f(fSM dnccperccr

Conclusions

Therearemanybenefitsforusingprestressedconcreteinbridgestructures.

NewAASHTOLRFDcoderequirementsaremuch

differentfrom

the

old

Standard

requirements.

Prestressed concretetakesfulladvantageofhigperformanceconcreteandhighstrengthsteel.

Structuraldesignofprestressed concretemembersrequirescheckingserviceabilityand

ultimatestrengthatvariousstagesofloading.

04-aashto lrfd design provisions for prestressed concrete bridge

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