05 Pre Stressed Beam Design

8/6/2019 05 Pre Stressed Beam Design

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LRFDLRFD Prest ressedPrest ressed BeamBeamDesignDesign

LRFD Bridge Design WorkshopLRFD Bridge Design Workshop

June 12, 2007June 12, 2007

David Dahlberg, P.E.David Dahlberg, P.E.

LRFD EngineerLRFD Engineer


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Present at ion Overv iewPresent at ion Overv iew

PrestressPrestress LossesLosses

Vertical ShearVertical Shear

MethodsMethods

Interface ShearInterface Shear Variable ResistanceVariable Resistance

FactorFactor

Computer ProgramComputer Program

CapabilitiesCapabilities


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Prest ressres t ress Lossesosses

LRFD 2005 Interims included majorLRFD 2005 Interims included majorrevisions torevisions to prestressprestress losseslosses

Included changes to refined andIncluded changes to refined and

approximateapproximate prestressprestress loss methodsloss methods

New refined losses method much moreNew refined losses method much more

complicated than previous refinedcomplicated than previous refinedmethodmethod


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Instantaneous lossesInstantaneous losses Elastic shortening (same as before)Elastic shortening (same as before)

TimeTime--dependent lossesdependent losses

Computed for 2 time periods:Computed for 2 time periods:

1) from transfer to deck placement1) from transfer to deck placement

2) from deck placement to end of life2) from deck placement to end of life Includes shrinkage, creep, and strandIncludes shrinkage, creep, and strand

relaxationrelaxation


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Include both losses(+) and gains(Include both losses(+) and gains(--)) Anything that causesAnything that causes prestressprestress strands tostrands to

shorten or relax will result in a loss,shorten or relax will result in a loss,

e.g.e.g. -- shrinkage of beam concreteshrinkage of beam concrete

Anything that causesAnything that causes prestressprestress strands tostrands to

lengthen or increase in tension will result inlengthen or increase in tension will result ina gain,a gain,

e.g.e.g. -- shrinkage of deck concreteshrinkage of deck concrete


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MnDOTMnDOT continued using precontinued using pre--2005 interim2005 interimrefined loss methodrefined loss method

New loss method was studied and errorsNew loss method was studied and errors

in sign convention were foundin sign convention were found

Errors were corrected in 4Errors were corrected in 4thth Edition ofEdition of

2007 except 2007 except


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Prest ressres t ress Lossesosses Shrinkage of deckShrinkage of deck

concreteconcrete ffpSSpSS ::

defined as a gaindefined as a gain

ffcdfcdfwill have awill have a

negative valuenegative value


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Prest ressres t ress Lossesosses LRFDLRFD EqnEqn 5.9.5.4.15.9.5.4.1--1 subtracts the already1 subtracts the already

negativenegative ffpSSpSS , essentially treating it as a, essentially treating it as a

loss:loss:


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Prest ressres t ressLossesosses New approximateNew approximate

loss method:loss method:

EasilyEasily

computedcomputed For members withFor members with

V/SV/S 3.53.5


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Comparisons were made between theComparisons were made between theold and new methodsold and new methods

Significant reduction in losses overSignificant reduction in losses over

previous methodprevious method

Use of new method could create ratingUse of new method could create rating

problems unless new losses are used inproblems unless new losses are used inthe ratingthe rating


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Prestress Losses

Beam Type Span Spacing Old Refined New Refined New Approx

ft ft ksi ksi ksi

27M 62.3 9.0 62.9 44.8 47.8

MN54 116.8 9.0 66.5 46.3 51.6

72M 130.0 9.0 63.4 46.1 49.5

Comparison results:Comparison results:


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Beam Type Span Spacing Number of Strands

ft ft each

72M, Original Design 130.0 9.0 44

72M, Redesign 130.0 9.0 38

Comparison results:Comparison results:


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Prest ressres t ress Lossesosses Old method is gone, so must move onOld method is gone, so must move on

and choose either new refined method orand choose either new refined method or

new approximate methodnew approximate method


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Prest ressres t ress Lossesosses MnDOTMnDOT Bridge Office policy:Bridge Office policy:

ForForprestressedprestressed II--beam designs, usebeam designs, usethe new approximatethe new approximate prestressprestress losslossmethod given in LRFD 5.9.5.3method given in LRFD 5.9.5.3

As in the past, do not transformAs in the past, do not transformprestressprestress strands in your designstrands in your design

Designers to provideDesigners to provide prestressprestress losslossvalues to rating engineer for input intovalues to rating engineer for input intoVirtisVirtis


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Vert ic a l Shear Met hodser t ic a l Shear Met hods LRFD 2007 4LRFD 2007 4thth Edition includes 3 shearEdition includes 3 shear

methods:methods:

LRFD 5.8.3.4.1LRFD 5.8.3.4.1 -- Simplified procedure forSimplified procedure for

nonprestressednonprestressed sectionssections

LRFD 5.8.3.4.2LRFD 5.8.3.4.2 -- General procedureGeneral procedure

New LRFD 5.8.3.4.3New LRFD 5.8.3.4.3 -- Simplified procedureSimplified procedureforforprestressedprestressed && nonprestressednonprestressed sectionssections


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Vert ic a l Shear Met hodser t ic a l Shear Met hods Some background:Some background:

Since beginning of LRFD Spec in 1994,Since beginning of LRFD Spec in 1994,there were complaints about the shearthere were complaints about the shearprocedure because it requires iteration andprocedure because it requires iteration andis too complicatedis too complicated

Simplified procedure was desiredSimplified procedure was desired

NCHRP Project 12NCHRP Project 12--61 resulted in NCHRP61 resulted in NCHRPReport 549 Simplified Shear Design ofReport 549 Simplified Shear Design ofStructural Concrete MembersStructural Concrete Members


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Vert ic a l Shear Met hodser t ic a l Shear Met hods New LRFD 5.8.3.4.3 simplified procedureNew LRFD 5.8.3.4.3 simplified procedure

Utilizes aUtilizes a VVcici andand VVcwcw approach similarapproach similar

to the Standard Specificationsto the Standard Specifications

Unified to include both conventionallyUnified to include both conventionally

reinforced andreinforced and prestressedprestressed sectionssections


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Vert ic a l Shear Met hodser t ic a l Shear Met hods :New LRFD 5.8.3 .4.3ew LRFD 5.8.3.4 .3 LRFDLRFD VVcici

Std SpecsStd Specs

VVcici


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Vert ic a l Shear Met hods :er t i c a l Shear Met hods :New LRFD 5.8.3.4.3ew LRFD 5.8.3 .4.3 LRFDLRFD VVcwcw

Std SpecsStd Specs

VVcwcw


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Vert ic a l Shear Met hodser t ic a l Shear Met hods New shear limitation per LRFD 5.8.3.2New shear limitation per LRFD 5.8.3.2


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Vert ic a l Shear Met hodser t ic a l Shear Met hods LRFD Interim 2008?LRFD Interim 2008?

Similar to CSA A23.3Similar to CSA A23.3--04 with04 with andand

determined by equations for no iterationdetermined by equations for no iteration

With min VWith min Vss

Without min VWithout min Vss


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Vert ic a l Shear Met hodser t ic a l Shear Met hods LRFD Interim 2008?LRFD Interim 2008?

Strain equation revised for no iterationStrain equation revised for no iteration

Current general procedure to be placed inCurrent general procedure to be placed in

Section 5 appendixSection 5 appendix


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Vert ic a l Shear Met hodser t ic a l Shear Met hods MnDOTMnDOT Bridge Office recommendation:Bridge Office recommendation:

Continue to use LRFD 5.8.3.4.1 forContinue to use LRFD 5.8.3.4.1 for

conventionally reinforced pier caps andconventionally reinforced pier caps andfootings (footings (withwith = 2 and= 2 and = 45 degrees)= 45 degrees)

Continue to use the LRFD 5.8.3.4.2 generalContinue to use the LRFD 5.8.3.4.2 general

procedure forprocedure forprestressedprestressed sections (at leastsections (at least

for this year, anyway!)for this year, anyway!)


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In t er fac e Shearnt er fac e Shear LRFD 2007 4LRFD 2007 4thth Edition included revisions to interfaceEdition included revisions to interface

shear provisionsshear provisions Previous method required a substantial amount ofPrevious method required a substantial amount of

interface shear reinforcement if Vinterface shear reinforcement if Vuu due to all loads wasdue to all loads was

usedused

MnDOTMnDOT followed PCI Bridge Design Manual procedurefollowed PCI Bridge Design Manual procedurewhich designed only for live load and superimposedwhich designed only for live load and superimposeddead load (loading added after deck hardened)dead load (loading added after deck hardened)

PCI procedure did not require additional reinforcementPCI procedure did not require additional reinforcementto resist interface shearto resist interface shear


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In t er fac e Shearnt er fac e Shear Cohesion valuesCohesion values

increasedincreasedsignificantlysignificantly(previous value(previous value

was 0.10was 0.10 ksiksi)) Deck on concreteDeck on concrete

girder was madegirder was made

a separate casea separate case VVuu based on allbased on all

loadsloads


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In t er fac e Shearnt er fac e Shear Study results:Study results:

Beam Type Span Spacing

Old LRFD Method

Required Av f

New LRFD Method

Required Av f

ft ft in2/ft in2/ft

27M 62.3 9.0 0.74 0.30

27M 52.5 13.0 0.93 0.35

36M 66.0 13.0 0.80 0.30

81M 122.0 13.0 0.30 0.19


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Var iab le Res is t anc e Fac t orar iab le Res ist anc e Fac t or LRFD 2005 interims eliminatedLRFD 2005 interims eliminated c/dc/d maxmax

reinforcement limit and replaced with areinforcement limit and replaced with avariable resistance factorvariable resistance factor

The resistance factor drops below 1.0The resistance factor drops below 1.0forforprestressedprestressed flexural members whenflexural members whenthey becomethey become overreinforcedoverreinforced

Will rarely have an effect on beamWill rarely have an effect on beamdesigns exceptdesigns except pedped bridges where slabbridges where slab

is below top flangeis below top flange


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Com put er Programom put er ProgramCapabi l i t iesapab i l i t ies ConspanConspan

LossesLosses

Currently gives choice of old refined, newCurrently gives choice of old refined, new

refined, and new approximate lossesrefined, and new approximate losses Vertical and horizontal shear:Vertical and horizontal shear:

December 2007 (?) releaseDecember 2007 (?) release

VariableVariable factor:factor:

December 2007 (?) releaseDecember 2007 (?) release


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Com put er Programom put er ProgramCapabi l i t iesapab i l i t ies PSBeamPSBeam

LossesLosses

Currently gives choice of old and newCurrently gives choice of old and new

refined lossesrefined losses Horizontal shear and variableHorizontal shear and variable factorfactor::

June 2007 (?) releaseJune 2007 (?) release

Approximate losses and vertical shear inApproximate losses and vertical shear in

a near future releasea near future release


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Com put er Programom put er ProgramCapabi l i t iesapab i l i t ies OpisOpis

LossesLosses

Currently only new approximate loss methodCurrently only new approximate loss methodavailable and new refined loss method is onavailable and new refined loss method is on

possible enhancement listpossible enhancement list

Vertical and horizontal shear:Vertical and horizontal shear:

Fall 2007 (?) releaseFall 2007 (?) release

VariableVariable factor:factor:

Fall 2007 (?) releaseFall 2007 (?) release


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Quest ionsQuest ions

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05 Pre Stressed Beam Design