New ACI Provisions on Hooked and Headed Reinforcing Bars
David Darwin
69th Annual Concrete Conference December 5, 2019 Outline .ACI 318-14 Provisions and Basis for Provisions .Scope of Studies .What We’ve Learned .Comparison of Test Data with ACI 318- 14 .Design Approach in ACI 318-19 .Things We Need to Work On Emphasis on high-strength reinforcing steel but full range of bar stresses studied .Anchorage of High-Strength Reinforcing Bars with Standard Hooks .Anchorage of High-Strength Reinforcing Bars with Heads Sponsors ACI 318-14 Equations f yecrψψψ .Hooks: = d dh ′ b 50λ fc 0.016 f yeψ .Heads: = d dt ′ b fc Modification factors
.Epoxy-coated reinforcement: ψe = 1.2
.Cover*: ψc = 0.7
.Confining reinforcement*: ψr = 0.8
.Lightweight concrete*: λ= 0.75 A required .Excess reinforcement*: ( s ) ( As provided) * Hooks only ACI 318-14 Limitations on ff cy′ and and Head Size
. Hooks: ffcy′ ≤≤10,000 psi; 80,000 psi
.Heads: ffcy′ ≤≤ 6,000 psi; 60,000 psi
Abrg ≥ 4Ab
ACI 318-14 Limitations on bar spacing
.Headed bars: . Cover = as required for straight bars ≥ 2db . Clear spacing – horizontal layer ≥ 4db
.Clear spacing – vertical layers ≥ 4db
ACI 318-14 Design Equations in ACI 318-19 f yerocψψψψ 1.5 .Hooks: = d dh ′ b 55λ fc f yepocψψψψ 1.5 .Heads: = d dt ′ b 75 fc Basis for ACI 318-14 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 tests .Concrete strength ≤ 3.5 – 5.5 ksi .Yield strength ≤ 69 ksi .Normalweight concrete Scope of Studies .Test Specimens .Range of Variables Test Specimens Beam-Column Joints Test Apparatus Hooked and Headed Bar Specimens
Hooked Bars Headed Bars Multiple Hook and Headed Bar Test Specimens Slab tests – Headed Bars CCT Node Tests – Headed Bars
eh eh Splices – Headed Bars Range in Variables Parameters 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 i/o Column Core
Amount of Confining Reinforcement 0 to 5 hoops: No. 3 and No. 4 (Number and Bar Size) Range in Variables con’d Parameters 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 153 Headed Bars
Cold-Swaged Threaded Friction- Taper- Cold- Coupling Sleeve Forged Threaded Swaged What we’ve learned – 1 .Hooked and headed bars behave a lot alike .For the same embedment length, headed bars provide a higher anchorage force than hooked bars .Closely 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 strengths .Confining reinforcement parallel to the bar increases anchorage strength of hooked and headed bars .Confining reinforcement perpendicular to the bar increases anchorage strength of hooked but not headed bars What we’ve learned – 3 .Confining reinforcement makes a bigger contribution for closely-spaced bars than for widely-spaced bars .ACI 318-14 did not accurately represent anchorage strength of hooked or headed bars in terms of the effect of bar size and the contributions of confining reinforcement and concrete compressive strength Crack Progression – Hooked bars Failure Modes
Breakout Side Splitting Tail Kickout Comparison of Crack Patterns
Hooked bars Headed Bars Standard Hooks ACI 318-14 Equation (in.-lb)
f yecrψψψ dh = db 50λ fc′
50λ ehf cm fs, ACI = dbecrψψψ Comparison to ACI 318-14 for No Confining Reinforcement – Two-hooks 2.0
1.8
1.6
1.4 No. 5 Hook
1.2 No. 6 Hook
s,ACI No. 7 Hook f 1.0 / No. 8 Hook su f 0.8 No. 9 Hook
0.6 No. 11 Hook
0.4
0.2
0.0 0 5 10 15 20
Concrete Compressive Strength, fcm (ksi) Comparison to ACI 318-14 for No. 3
(No. 10) Ties Spaced at 3db – Two-hooks 2.0
1.8
1.6
1.4 No. 5 Hook 1.2 No. 6 Hook s,ACI f 1.0 No. 8 Hook /
su No. 7 Hook f 0.8 No. 11 Hook 0.6
0.4
0.2
0.0 0 5 10 15 20
Concrete Compressive Strength, fcm (ksi) Confining Reinforcement Orientation
After ACI 318-14
Confining Reinforcement Orientation
Parallel Hooked Hoop Bar
Perpendicular Hooked Hoop Bar Descriptive Equation Two widely-spaced hooked bars with confining
1.02 0.295 1.085 0.47 Ath 0.73 Th= Af b s = 294 fcm eh d b + 55,000db n
Ath = total area of confining steel n = number of hooked bars Closely spaced and staggered hooked bars Hooked bars without confining reinforcement 1.6
/ 1.2 T Hooks ≥ 6 db 1.0 No. 5 ≤ 6 db 0.8 No. 7 ≤ 6 db No. 8 ≤ 6 db 0.6 y = 0.0907x + 0.4175 Test/Calculated, Test/Calculated, No. 11 ≤ 6 db 0.4
0.2
0.0 0 2 4 6 8 10 12 14 Spacing/db Hooked bars with confining reinforcement 1.6
1.4 h T
/ 1.2 T
1.0 Hooks ≥ 6 db No. 5 ≤ 6 db 0.8 No. 8 ≤ 6 db 0.6 No. 11 ≤ 6 db
Test/Calculated, Test/Calculated, y = 0.0357x + 0.7402 0.4
0.2
0.0 0 2 4 6 8 10 12 14
Spacing/db Hook placement within member
Inside column core
Outside column core Ratio Toutside to Tinside 1.2
No. 8, 0TR 1
0.8 No. 8, 5#3 inside T / 0.6
outside No. 11, 0TR
T 0.4
0.2 No. 11, 6#3 0 0 5 10 15
fcm (ksi) Reinforcing Bars with Heads
largest obstruction permitted
obstruction not considered to detract from the net bearing area of the head. Comparison to ACI 318-14
0.016 f ψ = ye d dt ′ b fc
62.5 ehf cm fs, ACI = ψebd Comparison to ACI for Two-Head Specimens with No Confining Reinforcement 4
3.5 No. 5, 13Ab No. 5, 4Ab 3 No. 8, 15Ab No. 8, 4Ab No. 8, 9Ab 2.5 No. 8, 6Ab
s,ACI No. 11, 6Ab f 2 / No. 11, 4Ab su
f No. 11, 9Ab 1.5 No. 5, 13Ab No. 5, 4Ab 1 No. 8, 15Ab No. 8, 4Ab 0.5 No. 8, 9Ab No. 8, 6Ab 0 No. 11, 6Ab No. 11, 4Ab 0 5 10 15 20 No. 11, 9Ab Concrete Compressive Strength, fcm (ksi) Comparison to ACI for Two-Head Specimens with No. 3 Ties Spaced at 3db 4
3.5 No. 5, 13Ab No. 5, 4Ab 3 No. 8, 9Ab No. 8, 15Ab No. 8, 4Ab 2.5 No. 8, 6Ab
s,ACI No. 11, 6Ab f 2 / No. 11, 4Ab su
f No. 11, 9Ab 1.5 No. 5, 13Ab No. 5, 4Ab 1 No. 8, 9Ab No. 8, 15Ab 0.5 No. 8, 4Ab No. 8, 6Ab 0 No. 11, 6Ab No. 11, 4Ab 0 5 10 15 20 No. 11, 9Ab Concrete Compressive Strength, fcm (ksi) Descriptive Equation For two widely spaced headed bars with confining reinforcement oriented parallel to the bar:
0.24 1.03 0.35 Att 0.88 Th= Af b s = 781 fcm eh d b + 48,800db n Headed Bar Specimens with more than two headed bars Headed Bars with No Confining Reinforcement 1.4
1.2 y = 0.0835x + 0.3433
1
0.8 c
T No. 5 /
T No. 8 0.6 No. 11 widely spaced 0.4
0.2
0 0 2 4 6 8 10 12 14
Center-to-center Spacing/db Headed Bars with Confining Reinforcement 1.4
1.2 y = 0.0622x + 0.5428
1
h 0.8 T
/ No. 5 T No. 8 0.6 No. 11 widely spaced 0.4
0.2
0 0 2 4 6 8 10 12 14
Center-to-center Spacing /db Design Approach – Initial Proposal .Convert descriptive equations (without confining reinforcement, hooked bar spacing > 6db, headed bar spacing > 8db) to equations for development length dh, dt .Then account for 1. closer bar spacing 2. confining reinforcement 3. bar location within the member . Incorporate a reliability-based φ-factor Design Equations – Initial Proposal (in.-lb)
f yψψ e cs ψ o = d 1.5 dh ′0.25 b 500λfc
f yψψ e cs ψ o = d 1.5 dt ′0.25 b 800 fc
Applicable for f c ′ up to 16 ksi Design Equations – Initial Proposal
ψe = coating factor
ψcs = confinement and spacing factor
ψo = location factor λ = concrete density (hooks only) Confinement based on:
.Ath/Ahs, Att/Ahs
where
.Ath, Att = total cross-sectional area of ties or stirrups confining hooked or headed bars
. Ahs = total cross-sectional area of hooked or headed bars developed at critical section Region for confining reinforcement Ath parallel to straight portion of hooked bar Region for confining reinforcement Ath perpendicular to straight portion of hooked bar Region for confining reinforcement Att parallel to headed bar Bar location factor ψo
Toutside ~ 0.80 Tinside
Bars 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 Comparison with test results For hooked bars without confining reinforcement
2
1.8 No. 11
1.6 No. 7 s,calc
f No. 8
/ 1.4 su
f No. 5 1.2 No. 6 1 No. 11 0.8 No. 7 0.6 No. 8
Test/Calculated, 0.4 No. 5
0.2 No. 6 0 0 5 10 15 20 widely spaced Concrete Compressive Strength, fcm (ksi) Comparison with test results For hooked bars with confining reinforcement
2
1.8 No. 11 1.6 calc ,
s No. 8 f
/ 1.4 su f 1.2 No. 5 1
0.8 No. 11
0.6 No. 8
Test/Calculated, 0.4
0.2 No. 5
0 0 5 10 15 20 widely spaced Concrete Compressive Strength, fcm (ksi) ACI 318-19
.Stay with f c′ but modify equation to represent the effect of compressive 0.25 strength based on f c ′ for compressive strengths < 6000 psi
.Limit maximum f c ′ to 10,000 psi for use in calculating dh and dt .Simplify representation of confining reinforcement and bar spacing ACI 318-19 (in.-lb) Hooked Bars f ψψψψ = yeroc d 1.5 dh ′ b 55λ fc
ψe = coating factor
ψr = confining reinforcement factor
ψo = location factor
ψc = concrete strength factor λ = concrete density ACI 318-19 (in.-lb) Headed Bars f ψψψψ = yepoc d 1.5 dt ′ b 75 fc
ψe = coating factor
ψp = parallel tie reinforcement factor
ψo = location factor
ψc = concrete strength factor ACI 318-19 Confinement and Spacing No. 11 (No. 36) and smaller bars
Modification Condition Value Factor Confining Hooked A ≥ 0.4A or s ≥ 6d 1.0 Reinforcement th hs b bars ψr Other 1.6 Parallel tie Headed Att ≥ 0.3Ahs or s ≥ 6db 1.0 reinforcement bars Other 1.6 ψp ACI 318-19 Location Factor No. 11 (No. 36) and smaller bars
Modification Condition Value Factor Hooked Location (1) Terminating inside column ψ core with side cover normal to bars o plane of hook ≥ 2.5 in. (65 mm), 1.0 or (2) With side cover normal to plane of hook ≥ 6db Other 1.25 Headed Location (1) Terminating inside column core with side cover ≥ 2.5 in. (65 ψ 1.0 bars o mm), or (2) With side cover ≥ 6db Other 1.25 ACI 318-19 Concrete Strength Factor
Modification Condition Value Factor ′ Hooked & Concrete fc < 6000 psi fc′/15,000 + 0.6 Headed strength Bars ′ ψc fc > 6000 psi 1.0 Drop Excess Reinforcement Factor for Hooks
( AAs,, required) ( s provided )
1.085 because T is proportional to eh Things we need to work on .Hooked and headed bars larger than No. 11 .Seismic provisions for hooked bars .Add more detailed expressions to allow designers to take advantage of confining reinforcement Ath < 0.4Ahs or Att < 0.3Ahs for bar spacing s less than 6db
The University of Kansas
David Darwin, Ph.D., P.E.
Deane E. Ackers Distinguished Professor and Chair Dept. of Civil, Environmental & Architectural Engineering 2150 Learned Hall Lawrence, Kansas, 66045-7609 785 864-3827 Fax: 785 864-5631 [email protected]