anchorage of high-strength reinforcement with standard ......~100 splice, side and shallow blowout,...

Anchorage of High-Strength Reinforcement with Standard Hooks

and Heads

David Darwin

62nd Annual Structural Engineering ConferenceMarch 2, 2017

High-strength reinforcing steel – Studies at the University of Kansas Anchorage of High-Strength Reinforcing Bars with Standard HooksAnchorage of High-Strength Reinforcing Bars with HeadsUse of High-Strength Shear Reinforcement Anchored with HeadsHigh-Strength Reinforcement in Concrete Walls and Coupling Beams

OutlineBasis for ACI Provisions Scope of StudyWhat We’ve LearnedCracking Patterns and Failure ModesComparison of Test Data with ACIDescriptive EquationsDesign ApproachConclusions

Why high-strength reinforcing steel?

Rautenberg (2011)

Photo Courtesy of Remy Lequesne, University of Kansas

Photo Courtesy of Andres Lepage, University of Kansas

Sponsors

ACI Equations

Hooks:

Heads:

ψ ψ ψ50λy e c r

dh bc

fd

f

= ′

0.016 ψy edt b

c

fd

f

= ′

Modification factorsEpoxy-coated reinforcement:

Cover*:

Confining reinforcement*:

Lightweight concrete*:

Excess reinforcement*:

* Hooks only

( )( )

required provided

s

s

AA

ψ 0.7c =

ψ 0.8r =

λ 0.75=

ψ 1.2e =

Limitations on and Head Size

Hooks:

Heads:

Abrg ≥ 4Ab

and c yf f′

10,000 psi; 80,000 psic yf f′ ≤ ≤

6,000 psi; 60,000 psic yf f′ ≤ ≤

ACI 318-14

Limitations on bar spacing

Headed bars:Cover = as required for straight bars,Clear spacing – horizontal layerClear spacing – vertical layers

2 bd≥ 4 bd≥ 4 bd≥

Proposed Design Equations

Hooks:

Heads:

1.50.25

ψ ψ ψ0.003

λy e cs o

dh bc

fd

f

= ′

1.50.25

ψ ψ ψ0.0024 y e cs odt b

c

fd

f

= ′

Basis for ACI Provisions for Hooked Bars

38 tests of standard hooked bars that failed in bond

Concrete strength = 3.8 – 5.1 ksi

Yield strengths = 64 and 68 ksi

Basis for ACI Provisions for Headed Bars ~100 splice, side and shallow blowout, and CCT node testsConcrete strength ≤ 3.5 – 5.5 ksi Yield strength ≤ 69 ksiNormalweight concrete

Scope of StudyTest SpecimensRange of Variables

Test Specimens

Beam-Column Joints

Test Apparatus

Hooked and Headed Bar Specimens

Headed BarsHooked Bars

Multiple Hook and Headed Bar Test Specimens

Slab tests – Headed Bars

CCT Node Tests – Headed Bars

eh eh

Splices – Headed Bars

Range in VariablesParameters Range

Bar Size No. 5, 6, 8, 11

Hook Bend Angle 90°, 180°

Head Size, Abrg 3.8Ab to 14.9Ab

Concrete Compressive Strength, fcm (ksi) 4.3 to 16.5

Placement of Hooks: Inside or Outside Column Core i/o

Amount of Confining Reinforcement (Number and Bar Size)

0, 1 No. 3, 2 No. 3, 4 No. 3, 5 No. 3, 6 No. 3, 1 No. 4, 2 No.

4, 4 No. 4, and 5 No. 4

Range in Variables con’dParameters Range

Nominal Side Cover, cso (in.) 1.5, 2.5, 3, 3.5, 4

Nominal Tail Cover, cth (in.) 2 to 18

Nominal Embedment Length, eh (in.) 4 to 26

Number of Bars 2 to 6

Center-to-Center Spacing of Bars 2db to 11db (< 1.3db for splices)

Stress at Failure, fsu (ksi) 23 to 142

Headed Bars

Cold-Swaged Threaded Coupling Sleeve

Friction-Forged

Taper-Threaded

Cold-Swaged

What we’ve learned – 1 Hooked and headed bars behave a lot alikeFor the same embedment length, headed bars provide a higher anchorage force than hooked barsClosely spaced hooked and headed bars are weaker, individually, than widely spaced hooked and headed bars

What we’ve learned – 2 Hooked bars with 90° and 180°degree bends have similar anchorage strengthsConfining reinforcement parallel to the bar increases anchorage strength of hooked or headed bars Confining reinforcement perpendicular to the bar increases anchorage strength of hooked but not headed bars

What we’ve learned – 3 The ACI Code does not accurately represent the anchorage capacity of hooked or headed bars in terms of the effect of bar size and the contribution of concrete compressive strengthMore!

Crack Progression – Hooked bars

Failure Modes

Front Failure Side Failure Tail Kickout

Comparison of Crack Patterns

Hooked bars Headed Bars

Standard Hooks

ACI Equation

ψ ψ ψ50λy e c r

dh bc

fd

f=

′

,

50λψ ψ ψ

eh cms ACI

b e c r

ff

d=

Comparison to ACI for No Confining Reinforcement – Two-hook specimens

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 5 10 15 20

f su/f

s,AC

I

Concrete Compressive Strength, fcm (ksi)

No. 5 Hook

No. 6 Hook

No. 7 Hook

No. 8 Hook

No. 9 Hook

No. 11 Hook

Comparison to ACI for 2 No. 3 Ties– Two-hook specimens

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 5 10 15 20

f su/f

s,AC

I

Concrete Compressive Strength, 'fcm (ksi)

No. 5 Hook

No. 8 Hook

No. 11 Hook

Comparison to ACI for No. 3 Ties Spaced at 3db – Two-hook specimens

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 5 10 15 20

f su/f

s,AC

I


No. 5 Hook

No. 6 Hook

No. 8 Hook

No. 7 Hook

No. 11 Hook

Confining Reinforcement Orientation

After ACI 318-14

Confining Reinforcement Orientation

Hooked Bar

Parallel Hoop

Hooked Bar

Perpendicular Hoop

Descriptive Equation

1.020.295 1.085 0.47 0.73294 55,000 thh b s cm eh b b

AT A f f d dn

= = +

Two widely-spaced hooked bars: confining reinforcement parallel to the bar

Ath = total area of confining steel parallel to straight portion of hooked bars within 8db of top of hook for No. 3 - 8 or 10db for No. 9 - 11 hooked barsn = number of hooked bars

Region for confining reinforcement Ath parallel to straight portion of hooked bar


Ath = total area of confining reinforcement of confining reinforcement perpendicular to straight portion of hooked bar within embedment (development) length eh

Two widely-spaced hooked bars: confining reinforcement perpendicular to the bar

1.020.295 1.085 0.47 0.73294 13,750 thh b s cm eh b b

AT A f f d dn

= = +

Region for confining reinforcement Athperpendicular to straight portion of hooked bar

Test vs Calculated w/o confining reinforcement

0

50

100

150

200

250

0 50 100 150 200 250

Mea

sure

d Fa

ilure

Loa

d,T

Calculated Failure Load, Tc

No.11

No.8

No.7

No.6

No.5

Test vs Calculated w/ confining reinforcement

0

50

100

150

200

250

0 50 100 150 200 250

Mea

sure

d Fa

ilure

Loa

d,T

Calculated Failure Load, Th

No. 11

No. 8

No. 5

Closely spaced and staggered hooked bars

Hooked bars without confining reinforcement

y = 0.0907x + 0.4175

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 2 4 6 8 10 12 14

Test

/Cal

cula

ted,

T/T

c

Spacing/db

Hooks ≥ 6 db

No. 5 ≤ 6 db

No. 7 ≤ 6 db

No. 8 ≤ 6 db

No. 11 ≤ 6 db

Hooked bars with confining reinforcement

y = 0.0357x + 0.7402

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 2 4 6 8 10 12 14

Test

/Cal

cula

ted,

T/T

h

Spacing/db

Hooks ≥ 6 db

No. 5 ≤ 6 db

No. 8 ≤ 6 db

No. 11 ≤ 6 db

Hook placement within member

Insidecolumn core

Outsidecolumn core

Ratio Toutside to Tinside

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15

T out

side

/Tin

side

fcm (ksi)

No. 8, 0TR

No. 8, 5#3

No. 11, 0TR

No. 11, 6#3

Design ApproachConvert descriptive equation to one for development length dhModify equation for hooked bar spacing of 2db, and then account for

1. wider hooked bar spacing2. confining reinforcement3. hooked bar location within the member

Incorporate a reliability-based φ-factor

Design Equation After Modification

ψ confinement and spacing factorψ location factor

cs

o

==

1.50.25

ψ ψ ψ0.003

λy e cs o

dh bc

fd

f

= ′

Confinement and Spacing Factor ψcs

Confinement level

Yield Strength

(ksi)

c-c Spacing

2db ≥ 6db

None - 1.0 0.6

Ath/Ahs > 0.2*Ath/Ahs > 0.8**

60 0.6 0.5

120 0.66 0.55

* Confining reinf. parallel to dh** Confining reinf. perpendicular to dh

Bar location factor ψo

Hooks within a column core with side cover ≥ 2.5 in. or in a wall with side cover ≥ 6db: ψo = 1.0

Otherwise: ψo = 1.25

Toutside ~ 0.80 Tinside

Drop Excess Reinforcement Factor for Hooks

( ) ( ), ,s required s providedA Abecause T is proportional to 1.085eh

Design EquationFor hooked bars without confining reinforcement

0

50

100

150

200

250

0 50 100 150 200 250

Mea

sure

d Fa

ilure

Loa

d,T

Calculated Failure Load, Tc

No. 11 ≥ 6 db

No. 8 ≥ 6 db

No. 7 ≥ 6 db

No. 6 ≥ 6 db

No. 5 ≥ 6 db

No. 11 ≤ 6 db

No. 8 ≤ 6 db

No. 5 ≤ 6 db

Design EquationFor hooked bars with confining reinforcement

0

50

100

150

200

250

0 50 100 150 200 250

Mea

sure

d Fa

ilure

Loa

d,T

Calculated Failure Load, Th

No. 11 ≥ 6 db

No. 8 ≥ 6 db

No. 5 ≥ 6 db

No. 11 ≤ 6 db

No. 8 ≤ 6 db

No. 5 ≤ 6 db

Reinforcing Bars with Heads

Comparison to ACI 318-14

0.016 ψy edt b

c

fd

f

= ′

,

62.5ψ

eh cms ACI

e b

ff

d=

Comparison to ACI for Two-Head Specimens with No Confining Reinforcement

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20

f su/f

s,AC

I


No. 5, 13AbNo. 5, 4AbNo. 8, 15AbNo. 8, 4AbNo. 8, 9AbNo. 8, 6AbNo. 11, 6AbNo. 11, 4AbNo. 11, 9AbNo. 5, 13AbNo. 5, 4AbNo. 8, 15AbNo. 8, 4AbNo. 8, 9AbNo. 8, 6AbNo. 11, 6AbNo. 11, 4AbNo. 11, 9Ab

Comparison to ACI for Two-Head Specimens with No. 3 Ties Spaced at 3db

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20

f su/f

s,AC

I


No. 5, 13AbNo. 5, 4AbNo. 8, 9AbNo. 8, 15AbNo. 8, 4AbNo. 8, 6AbNo. 11, 6AbNo. 11, 4AbNo. 11, 9AbNo. 5, 13AbNo. 5, 4AbNo. 8, 9AbNo. 8, 15AbNo. 8, 4AbNo. 8, 6AbNo. 11, 6AbNo. 11, 4AbNo. 11, 9Ab


0.24 1.03 0.35 0.88781 48,800 tth b s cm eh b bAT A f f d dn

= = +

For two widely spaced headed bars with confining reinforcement oriented parallel to the bar:

Region for confining reinforcement Attparallel to headed bar

Headed Bar Specimens with more than two headed bars

Headed Bars with No Confining Reinforcement

y = 0.0835x + 0.3433

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2 4 6 8 10 12 14

T/T

c

Center-to-center Spacing/db

No. 5No. 8No. 11widely spaced

Headed Bars with Confining Reinforcement

y = 0.0622x + 0.5428

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2 4 6 8 10 12 14

T/T

h

Center-to-center Spacing /db

No. 5No. 8No. 11widely spaced

Design Approach – same as for hooked barsConvert descriptive equation to one for development length dtModify equation for headed bar spacing of 2db, and then account for

1. wider headed bar spacing2. confining reinforcement3. headed bar location within the member

Incorporate a reliability-based φ-factor

Design Equation After Modification

ψ confinement and spacing factorψ location factor

cs

o

==

1.50.25

ψ ψ ψ0.0024 y e cs odt b

c

fd

f

= ′

Confinement and Spacing Factor ψcs

Confinement level

Yield Strength

(ksi)

c-c Spacing

2db ≥ 8db

None - 1.0 0.5

Att/Ahs > 0.3*60 0.6 0.4

120 0.7 0.45

* Confining reinf. parallel to dt

Design EquationFor headed bars without confining reinforcement

Design EquationFor headed bars with confining reinforcement

Proposed design provisions for hooked and headed bars cover:

Compressive strengths to 16 ksi Yield strengths to 120 ksi

ConclusionsACI provisions for hooked and headed bar development length overestimateAnchorage strength of larger barsEffect of concrete compressive strengthContribution of confining reinforcement (hooks)

ACI provisions do not account for reduced strength of closely-spaced hooked or headed bars

Conclusions con’dEffect of concrete compressive strength is adequately represented by

For a given development length, anchorage force with and without confining reinforcement increases as size of hooked or headed bar increases

0.25cf ′

Conclusions con’dContribution of confining reinforcement oriented parallel to and located within 8 to 10 db of straight portion of bar is proportional to ratio of area of confining reinforcement to total area of hooked/headed bars

Conclusions con’dContribution of confining reinforcement perpendicular to straight portion of hooked bar differs from that of reinforcement oriented parallel to bar, with more legs contributing but with each leg making a smaller contribution

The University of Kansas

David Darwin, Ph.D., P.E.

Deane E. Ackers Distinguished Professor and ChairDept. of Civil, Environmental & Architectural Engineering2150 Learned HallLawrence, Kansas, 66045-7609785 864-3827 Fax: 785 864-5631

[email protected]

Anchorage of High-Strength Reinforcement with Standard Hooks and Heads��David Darwin��High-strength reinforcing steel – Studies at the University of Kansas OutlineWhy high-strength reinforcing steel?Slide Number 5Slide Number 6SponsorsACI EquationsModification factorsLimitations on and Head Size Limitations on bar spacingProposed Design EquationsBasis for ACI Provisions for Hooked BarsBasis for ACI Provisions for Headed BarsScope of StudyTest SpecimensBeam-Column JointsTest ApparatusHooked and Headed Bar SpecimensMultiple Hook and Headed Bar Test SpecimensSlab tests – Headed BarsCCT Node Tests – Headed BarsSplices – Headed BarsRange in VariablesRange in Variables con’dHeaded BarsWhat we’ve learned – 1 What we’ve learned – 2 What we’ve learned – 3 Slide Number 30Failure ModesComparison of Crack PatternsStandard HooksACI EquationComparison to ACI for No Confining Reinforcement – Two-hook specimensComparison to ACI for 2 No. 3 Ties� – Two-hook specimensComparison to ACI for No. 3 Ties Spaced at 3db – Two-hook specimensConfining Reinforcement OrientationSlide Number 39Slide Number 40Confining Reinforcement OrientationDescriptive EquationRegion for confining reinforcement Ath parallel to straight portion of hooked barDescriptive EquationRegion for confining reinforcement Ath perpendicular to straight portion of hooked barTest vs Calculated w/o confining reinforcementTest vs Calculated w/ confining reinforcement Closely spaced and staggered hooked barsHooked bars without confining reinforcementHooked bars with confining reinforcementHook placement within memberRatio Toutside to TinsideDesign ApproachDesign Equation After ModificationConfinement and Spacing Factor ycsBar location factor yoDrop Excess Reinforcement Factor for HooksDesign EquationDesign EquationReinforcing Bars with HeadsComparison to ACI 318-14�Comparison to ACI for Two-Head Specimens with No Confining ReinforcementComparison to ACI for Two-Head Specimens with No. 3 Ties Spaced at 3dbDescriptive EquationRegion for confining reinforcement Att parallel to headed barHeaded Bar Specimens with more than two headed barsHeaded Bars with No Confining ReinforcementHeaded Bars with Confining ReinforcementDesign Approach – same as for hooked barsDesign Equation After ModificationConfinement and Spacing Factor ycsDesign EquationDesign EquationProposed design provisions for hooked and headed bars cover:ConclusionsConclusions con’dConclusions con’dConclusions con’dSlide Number 79

anchorage of high-strength reinforcement with standard ......~100 splice, side and shallow blowout,...

Documents