7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges http://slidepdf.com/reader/full/long-term-behaviour-of-balanced-cantilever-bridges 1/43 LONG LONG - - TERM BEHAVIOUR OF TERM BEHAVIOUR OF BALANCED CANTILEVER BRIDGES BALANCED CANTILEVER BRIDGES Milan Milan Kalný Kalný Petr Souček Petr Souček Václav Kvasnička Václav Kvasnička
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Long-Term Behaviour of Balanced Cantilever Bridges
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7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Supposed reasons for excessive deflectionSupposed reasons for excessive deflection
1.1. Actual Actual prestressingprestressing of theof the super super structure is lower than it was assumedstructure is lower than it was assumed
in the design due to the losses namely greater relaxation of in the design due to the losses namely greater relaxation of
prestressingprestressing steel.steel.
This effect might be increased by very high level of initialThis effect might be increased by very high level of initial prestressingprestressing,,
which was allowed by some previous national design codes.which was allowed by some previous national design codes.
In the Czech Republic the permissible limit of 0.935 fpIn the Czech Republic the permissible limit of 0.935 fp0.20.2 was allowedwas allowedand even its "improving" by 5and even its "improving" by 5% overstressing was possible.% overstressing was possible.
LongLong--term behaviour of balanced cantilever bridgesterm behaviour of balanced cantilever bridges ACES Workshop, ACES Workshop,
Corfu, 2010Corfu, 2010
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Modulus of elasticity according to EN 1992Modulus of elasticity according to EN 1992--11--11
( )3.0
10/*22 cmcm f E =
zz secant modulussecant modulus EEcmcm is derived from the workingis derived from the working diaghramdiaghramat the stress level of 40%at the stress level of 40% f f cmcm
zz It depends also on concrete composition:It depends also on concrete composition: – – tables are for tables are for quar quar ttz aggregatez aggregate – – limestone aggregatelimestone aggregate --10%10% – – sandstone aggregatesandstone aggregate --30%30% – – basalt aggregatebasalt aggregate +20%+20%
zz Table values should be used when more precise values areTable values should be used when more precise values are
not available or no precise analysis is not required!not available or no precise analysis is not required!zz Testing of concrete properties is required for FCM!Testing of concrete properties is required for FCM!
LongLong--term behaviour of balanced cantilever bridgesterm behaviour of balanced cantilever bridges ACES Workshop, ACES Workshop,
Corfu, 2010Corfu, 2010
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Modulus of elasticity of C30/37 as tested according to ČSN ISO 6Modulus of elasticity of C30/37 as tested according to ČSN ISO 6784:1993, Z1:2003784:1993, Z1:2003betonu C30/37 v čase 28dní
z betonárek Berger Beton a Holcim Pardubice
25000.00
30000.00
35000.00
40000.00
3 5 4 0 4 5 5 0
M o d u l p r u ž n o s t i [ M P a ]
Berger
Holcim
Lineární (Holcim)
Lineární (Berger)
25000,00
30000,00
35000,00
40000,00
45000,00
50000,00
3 0
3 5
4 0
4 5
5 0
5 5
6 0
6 5
7 0
pevnost fck [MPa]
M o d u l p r u ž n o s t i [ M P a
zz Concrete C35/45, 28 days oldConcrete C35/45, 28 days oldzz Distribution up to 25% from averageDistribution up to 25% from average
zz Samples from TBGSamples from TBG ChomutovChomutov oothersthers
Supposed reasons for excessive deflectionSupposed reasons for excessive deflection
5.5. Theoretic values of deflection are affected by the creep functioTheoretic values of deflection are affected by the creep function. In then. In the
analysis theanalysis the MoerschMoersch' function was often utilized. Application of ' function was often utilized. Application of
currently recommended function (EC2) may lead to increase of currently recommended function (EC2) may lead to increase of
calculated deflection by up to +100%.calculated deflection by up to +100%.
6.6. There may be influence of differential creep and shrinkage of coThere may be influence of differential creep and shrinkage of concretencrete
in box section (the bottom slab and the walls are at supports muin box section (the bottom slab and the walls are at supports muchch
thicker than the top slab). Delayed shrinkage should be taken althicker than the top slab). Delayed shrinkage should be taken also intoso intoaccount.account.
LongLong--term behaviour of balanced cantilever bridgesterm behaviour of balanced cantilever bridges ACES Workshop, ACES Workshop,
Corfu, 2010Corfu, 2010
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Supposed reasons for excessive deflectionSupposed reasons for excessive deflection
7.7. There may be fatigue influence of repeated live load and thermalThere may be fatigue influence of repeated live load and thermal
effects.effects.
8.8. Load of the main span was greater due to increased thickness of Load of the main span was greater due to increased thickness of thethepavement layers (blinding concrete).pavement layers (blinding concrete).
9.9. Finally theFinally the prestressingprestressing tendon's area might be reduced or even lost bytendon's area might be reduced or even lost by
corrosion.corrosion.
LongLong--term behaviour of balanced cantilever bridgesterm behaviour of balanced cantilever bridges ACES Workshop, ACES Workshop,
Corfu, 2010Corfu, 2010
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Conclusions from the testing of modulus of elasticityConclusions from the testing of modulus of elasticity
zz Values of modulus of elasticity of concrete received from compar Values of modulus of elasticity of concrete received from compar able samples areable samples arevery variable, influence of the real aggregate is a principle onvery variable, influence of the real aggregate is a principle one.e.
zz Statistic distribution of samples from one concrete plant is higStatistic distribution of samples from one concrete plant is high, however the basich, however the basictrend is clearly visible.trend is clearly visible.
zz Indicative values given in the EN 1992Indicative values given in the EN 1992--11--1 are basically correct and usable in1 are basically correct and usable instandard design practice with given limits.standard design practice with given limits.
zz In the previous Czech standards ČSN 73 6206 and ČSN 73 6207 moduIn the previous Czech standards ČSN 73 6206 and ČSN 73 6207 modulus of lus of elasticity was overestimated by up to 10% for reinforced structuelasticity was overestimated by up to 10% for reinforced structures (ČSN 73 6206)res (ČSN 73 6206)and by 5and by 5--15% for prestressed structures15% for prestressed structures (ČSN 73 6207), the difference is growing(ČSN 73 6207), the difference is growingwith the concrete class.with the concrete class.
zz In previous Czech standards no data were available on the time dIn previous Czech standards no data were available on the time development of Ec.evelopment of Ec.In the new ČSN EN 1992In the new ČSN EN 1992--11--1 the given formulas are reasonable and useful for 1 the given formulas are reasonable and useful for current analytical software.current analytical software.
zz For evaluation of the pumping effect we have not sufficient dataFor evaluation of the pumping effect we have not sufficient dataset. However theset. However thetendency to 5% reduction behind the pump is clear.tendency to 5% reduction behind the pump is clear.
LongLong--term behaviour of balanced cantilever bridgesterm behaviour of balanced cantilever bridges ACES Workshop, ACES Workshop,
Corfu, 2010Corfu, 2010
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Conclusions for the balanced cantilever bridgesConclusions for the balanced cantilever bridges
z Reliable deflection control during construction as well as back analysis of executed structures isnot possible without actual testing of modulus of elasticity of the concrete supplied to the site.This complies with the recommendation in EN 1992-1-1.
z Wide range of distribution for the samples from one concrete plant brings doubts aboutpurposefulness of advance testing for the structural analysis. For slender structures with largespan strict supervision during production, transport and casting is vital for successfulapplications.
z For demanding structures with many phases of construction and loading of young concretecomputational method using time-dependent modulus should be used. Values received fromsamples are better however standardized procedures can be also implemented.
z Road vertical alignment should be designed as curved when possible.
z If any doubts are raised, the designer should propose a sufficient initial camber and providedetails for additional superstructure strengthening by external prestressing if necessary duringits lifetime.
LongLong--term behaviour of balanced cantilever bridgesterm behaviour of balanced cantilever bridges ACES Workshop, ACES Workshop,
Corfu, 2010Corfu, 2010
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Discrepancy: different requirements for longitudinal and transvDiscrepancy: different requirements for longitudinal and transversal directionersal direction
inin spite of one exposure class and one protection level for spite of one exposure class and one protection level for prestressingprestressing
Design of partially/limited prestressed concrete structuresDesign of partially/limited prestressed concrete structures
Example 2: PartialExample 2: Partial prestressingprestressing
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Example 2: PartialExample 2: Partial prestressingprestressingfor continuity of precast girdersfor continuity of precast girders
Design of partially/limited prestressed concrete structuresDesign of partially/limited prestressed concrete structures
•• Even very low level of Even very low level of prestressingprestressingis helpful for serviceability behaviour is helpful for serviceability behaviour and robustnessand robustness
•• ProgressiveProgressive prestressingprestressing of of composite structure is possiblecomposite structure is possible
=> cost effectiveness=> cost effectiveness
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Remarks:Remarks: PrestressingPrestressing protection level should be more emphasized, one load combinatioprotection level should be more emphasized, one load combination only wouldn only wouldbe better for design process, partial and limitedbe better for design process, partial and limited prestressingprestressing should not beshould not be handicapedhandicaped
Design of partially/limited prestressed concrete structuresDesign of partially/limited prestressed concrete structures
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Remark: DraftRemark: Draft,, not yet approved by the Czech Standards Committee for bridges.not yet approved by the Czech Standards Committee for bridges.
Design of partially/limited prestressed concrete structuresDesign of partially/limited prestressed concrete structures
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges
Limitation of concrete tensile stressesLimitation of concrete tensile stresses
No direct requirementsNo direct requirements in EN 1992in EN 1992--1 or MC20101 or MC2010 are available, however the design value of are available, however the design value of
the concrete tensile strength is given in the material chapter bthe concrete tensile strength is given in the material chapter by the formula:y the formula:f f ctdctd == ααctct f f ctk0,05ctk0,05 / / γcc , (i.e. for C30/37 ~ 2,0, (i.e. for C30/37 ~ 2,0 MPaMPa))
where the recommended values in SLS are:where the recommended values in SLS are:αα
ctct = 1,0= 1,0
coefficient taking account of longcoefficient taking account of long
--term effects on the tensile strengthterm effects on the tensile strength
coefficientcoefficient
and of and of unfavourableunfavourable effects, resulting from the way the load is applied,effects, resulting from the way the load is applied,γcc = 1,0 partial safety factor for concrete in SLS.= 1,0 partial safety factor for concrete in SLS.
For the combination of compression and bending this value can beFor the combination of compression and bending this value can be increased by up to ~1,6,increased by up to ~1,6,which depends on the depth, way of loading and sensitivity of thwhich depends on the depth, way of loading and sensitivity of the steel to corrosion.e steel to corrosion.
For theFor the prestressedprestressed structures we can assume that if the concrete tensile stress dostructures we can assume that if the concrete tensile stress does notes notexceed the value of exceed the value of f f ctdctd == f f ctctmm for frequent andfor frequent and / /or or f f ctdctd == f f ctk0,05ctk0,05 for for quasiquasi--permanentpermanent
combination of load, no cracking is developcombination of load, no cracking is developeded =>=> structurestructure comply with SLS conditions.comply with SLS conditions.
Design of partially/limited prestressed concrete structuresDesign of partially/limited prestressed concrete structures
7/27/2019 Long-Term Behaviour of Balanced Cantilever Bridges