Research introductionResearch introductionMay, 2010May, 2010

TANTIPIDOK TANTIPIDOK PatarapolPatarapolM2 studentM2 student

NiwaNiwa LaboratoryLaboratory

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BEHAVIOR OF DIAGONAL COMPRESSION BEHAVIOR OF DIAGONAL COMPRESSION FAILURE WITH DIFFERENT STIRRUP RATIO IN FAILURE WITH DIFFERENT STIRRUP RATIO IN

HIGHHIGH--STRENGTH CONCRETE BEAMSSTRENGTH CONCRETE BEAMS

2BackgroundBackground

More economically design by Increasing span length. Reducing cross sectional area (I-shaped).

AdvantageAdvantage in bridge designin bridge design

Advance in new construction materials Advance in new construction materials High-strength concrete (f’c≧ 60 MPa)High-strength steel bar (fy ≧ 700 MPa)

The diagonal compression failurewill occur.

Very thin web Excessive stirrupsDrawbackDrawback

3What is Diagonal Compression Failure?What is Diagonal Compression Failure?

The diagonal compression failure is a failure due to the crushing of concrete at the web in the diagonal direction prior to the yielding of stirrups, resulting in a brittle failure

CL

CL Compression chord

Tension chordCompression Diagonal

Truss Analogy steelconcrete

Research significanceResearch significance

VVJSCEJSCE == 1.251.25f'f'cc1/21/2bbwwddQuite conservative for safety reasons.Covers only for concrete with f’c < 50 MPa.

JSCE Standard (2007)JSCE Standard (2007)

Does not include the effect of stirrup ratio (rw).

Lack of data in high-strength concrete beam with rw<2%

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5

0 1 2 3 4 5

Kobayashi et al.(f’c = 100 MPa)v/f’c1/2

Stirrup ratio, rw

Previous studyPrevious study

In rw<2% region, the turning point of failure mode may exist.

Diagonal tension capacity, V=Vc+Vs (JSCE standard)

ObjectiveObjective

To investigate the effect of stirrup ratio to the diagonal compressive capacity in high-strength concrete beams with stirrup ratio lower than 2%.

To examine the change of failure mode when reducing stirrup ratio.

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0 1 2 3 4 5

Kobayashi et al.(f’c = 100 MPa)

v/f’c1/2

Stirrup ratio, rw

Diagonal tension capacity, V=Vc+Vs (JSCE standard)

Outline of specimensOutline of specimensThree point bending tests

d d =

220

= 22

0aa=660=660CL

bw=40

Unit: mmStirrup(fwy≥953MPa) Tensile bar, D22 (fy≥1198MPa)

250250

A’ B’

BB--BB’’BA

146146

AA--AA’’250250

146146

5050

7070s

Constant variablesConstant variablesCompressive strength,f’c ~ 100 MPaLongitudinal reinforcement ratio, pw = 8.8%

Nominal diameter of stirrups, d = 10 mm. 1.8

1.5

1.2

Stirrup ratio, rw (%)

100

120

150

Stirrup spacing, s (mm)

Specimen name

UH1.8

UH1.5

UH1.2

Experimental casesExperimental cases

hh =26

0=2

60

Shear span to effective depth ratio, a/d = 3.0

Strain gauge

7ExperimentExperiment

CastingCasting Loading test Loading test

Specimen after loadingSpecimen after loading

DSLR cameraDSLR camera

SpecimenSpecimen

Load Load –– deflection relationship and deflection relationship and failure modefailure mode

Discussion of results 1Discussion of results 1

LoadLoad--deflection relationshipdeflection relationship

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50

100

150

200

250

300

0 2 4 6 8 10 12 14Deflection (mm)

Load (kN)rw =1.5%rw =1.8%

rw =1.2%

Flexure crack

Crushing at web.

Diagonal crack

No yielding of stirrups.Diagonal compression failure occurred.

Effect of stirrup ratio on diagonal Effect of stirrup ratio on diagonal compressive capacity compressive capacity

and failure mode criteriaand failure mode criteria

Discussion of results 2Discussion of results 2

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0 1 2 3 4 5stirrup ratio, r w (%)

Authors

Kobayashi et al.

Shear capacity normalized by f’c1/2 was used in order to neglect the effect of f’c. (JSCE standard)

Effect of stirrup ratioEffect of stirrup ratio

v : shear strength (=V/(bwd)) (MPa)

Capacity increases as rw increases.Linear relationship agrees well with the experimental results.

Fitting curve

Normalized shear capacity, v/f’c1/2

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0 1 2 3 4 5stirrup ratio, r w (%)

Authors

Kobayashi et al.

Criteria of failure modeCriteria of failure mode

Fitting curve(Diagonal compression)

Failure mode may change around rw = 1%.

Diagonal tension capacity, V=Vc+Vs(JSCE standard)

Diagonal tension failure

Diagonal compression failure

Blue line is used to predict criteria between diagonal tension failure and diagonal compression failure.

Diagonal tension failure will occur if the blue line is below the green line.

Normalized shear capacity, v/f’c1/2

Effect of stirrup ratio on capacityEffect of stirrup ratio on capacityAn increase in rw results in more numbers of wide diagonal cracks at the peak load.

Diagonal compressive stress concentrates in more specific portions.

Diagonal compressive

capacity decreases.

Lower stirrup ratio

rrww = 1.2= 1.2 rrww = 1.8= 1.8

ConclusionsConclusions

By considering crushing of concrete at web without yielding of stirrups, all specimens failed in diagonal compression.

As stirrup ratio increases, diagonal compressive capacity increases with a linear relationship and diagonal cracks are closely distributed.

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Thank you very much for Thank you very much for your kind attention !!your kind attention !!