splice and development length of high relative rib area reinforcing bars in tension

Splice and Development Length of High Relative Rib Area Reinforcing Bars in Tension 408.3-01 and Commentary 408.3R-01 ACI 408.3-01/ 408.3R-01 Reported by ACI Committee 408 Originally p

ACI Committee 408 adopted ACI T2-01 (unpublished) as ACI 408.3-01 on April

23, 2001 ACI T2-01 superseded provisional standard ACI ITG-2-98 and became effective March 9, 2001.

Copyright  2001, American Concrete Institute.

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de-sired by the Architect/Engineer to be a part of the contract

documents, they shall be restated in mandatory language

for incorporation by the Architect/Engineer

408.3-1

This standard was created to help designers take advantage of the

improved bond characteristics of high relative rib area deformed

reinforce-ment This type of reinforcement can be produced by increasing rib height,

decreasing rib spacing, or employing a combination of the two.

This standard is intended to be a more efficient means of providing a

development and splice length expression for the high relative rib area bars

than altering the current ACI 318-99 Chapter 12 provisions to

accommo-date bars that are not yet in commercial production.

Splice and Development Length of High Relative

Rib Area Reinforcing Bars in Tension (408.3-01)

and Commentary (408.3R-01)

ACI 408.3-01/ 408.3R-01

Reported by ACI Committee 408

Originally prepared by TTTC Subcommittee ITG-2

Atorod Azizinamini Anthony L Felder John F McDermott Gyorgy L Balazs Robert J Frosch Denis Mitchell Joann P Browning Bilal S Hamad Stavroula J Pantazopoulou

Richard A Devries Roberto T Leon Julio A Ramirez Rolf Eligehausen Leroy A Lutz Telvin Rezansoff

Richard N White Chairman David P Gustafson Leroy A Lutz Roberto T Leon Jack P Moehle

Non-ITG-2 voting members:

Jacob S Grossman S Ali Mirza John C McDermott

Keywords: bar ribs; bond; development length; high relative rib area;

reinforcing bars; splice length.

CONTENTS

1.0—Notation, p 408.3-2 2.0—Definition, p 408.3-2 3.0—Scope, p 408.3-2 4.0—Development of high relative rib area reinforcing bars in tension, p 408.3-2 5.0—Splices of high relative rib area reinforcing bars in tension, p 408.3-3

David Darwin Chairman

Adolfo B Matamoros Secretary

High Relative Rib Area Bars—Deformed reinforcing bars

with a relative rib area R r equal to 0.10 or larger

3.0—Scope

3.1—Evaluation of splice and development lengths of

coated and uncoated reinforcing bars in tension having a

high relative rib area, provided that:

3.1.1 The relative rib area is at least 0.10, but no larger

than 0.14;

3.1.2 The ribs are at an angle of 45 to 65 degrees

inclu-sive with respect to the axis of the bar Ribs shall not cross

4.0—Development of high relative rib area reinforcing bars in tension

4.1—Development length l d , in terms of diameter d b for

bars in tension shall be determined from 4.2, but l d shall not

be less than 12 in

4.2—The development length of high relative rib area

re-inforcing bars in tension l d divided by the bar diameter d b

shall be taken as

(4.1)

in which the term (cω + K tr )/d b shall not be taken greater than 4

The value of f′c1/4 shall not exceed 11.0

The value of f yshall not exceed 80 ksi

The variable ω shall be taken as 1.0 or evaluated as

(4.2)

The variable K tr shall be evaluated as

(4.3)

where

(4.4) with

Alternatively, it shall be permitted to take K tr = 0

4.3—The factors used in the expressions for development

of high relative rib area bars in tension are as follows:

α = reinforcement location factor

Horizontal reinforcement so placed that more than 12 in of fresh concrete is cast in the member below the development length

or splice 1.3 Other reinforcement 1.0

β = coating factor

All epoxy-coated bars 1.2

tr

forcement that is within the spacing s and crosses

the potential plane of splitting through the

rein-forcement being developed, in.2

c = c min + 0.5 d b, in

c b = cover of reinforcement being developed,

mea-sured to tension face of member, in

c max = maximum value of c s or c b, in

c min = minimum value of c s or c b, in

c s = minimum value of c si + 0.25 in or c so, in

c si = one-half of average spacing between bars or splices

in a single layer, in

c so = side cover of reinforcing bars, in

C R = relative rib factor as defined by Eq (4.4)

d b = nominal bar diameter, in

f′c = specified compressive strength of concrete, psi

f′c1/4 = fourth root of f′c, expressed in psi units

f ct = average splitting tensile strength of lightweight

aggregate concrete, psi

f y = yield strength of reinforcement being spliced or

developed, psi

K tr = transverse reinforcement index for high relative

rib area bars as defined by Eq (4.3)

l d = development length, in

l s = splice length, in

n = number of bars being developed or spliced along

plane of splitting

R r = relative rib area, ratio of projected rib area normal

to bar axis to product of nominal bar perimeter

and average center-to-center rib spacing

s = maximum center-to-center spacing of transverse

reinforcement within l d or l s, in

α = reinforcement location factor; see 4.3

β = coating factor; see 4.3

λ = lightweight aggregate concrete factor; see 4.3

ω = factor reflecting benefit of large cover/spacing

perpendicular to controlling cover/spacing as

de-fined by Eq (4.2)

l d

d b

- f y f′1 4 ⁄

c

72 cω+K tr

d b

-=

ω 0.1c max

c min

-+0.9≤1.25

=

K tr C R(0.72d b+0.28)A tr

sn

-=

C R = 44+330 R( r–0.10)

0.10≤R r≤0.14

Uncoated reinforcement 1.0

λ = lightweight aggregate concrete factor

When lightweight aggregate concrete is used 1.3

However, when f ct is specified, λ shall be

permitted to be taken as but not

less than 1.0

When normalweight concrete is used 1.0

4.4 Excess reinforcement—Reduction in development

length shall be permitted where reinforcement in a flexural

member is in excess of that required by analysis except

where anchorage or development for f y is specifically

re-quired or the reinforcement is designed under provisions of

21.2.1.4 of ACI 318-99(A s required)/(A s provided)

5.0—Splices of high relative rib area reinforcing

bars in tension

5.1—Minimum length of lap for tension lap splices shall

be as required for Class A or B splices, but not less than 12

in., where:

Class A splice 1.0 l d

Class B splice 1.3 l d

where l d is the tensile development length for the specified

yield strength f y in accordance with 4.2 without the

modifi-cation factor of 4.4

5.2—Lap splices of high relative rib deformed bars in

ten-sion shall be Class B splices except that Class A splices shall

be allowed when the criteria of 5.2.1 or 5.2.2 are met:

5.2.1 When the splice length is confined with transverse

reinforcement at two or more locations with a spacing not

greater than 10 in., providing a K tr /d b of at least 0.5;

5.2.2 With no transverse reinforcement or with

trans-verse reinforcement less than that of 5.2.1, when: a) the area

of reinforcement provided is at least twice that required by

analysis over the entire length of the splice; and b) no more

than one-half of the total reinforcement is spliced within the

required lap length

5.3—Tension lap splices shall not be used for high relative

rib area bars larger than No 11

6.7 f′c⁄f ct

COMMENTARY

R2.0—This standard is provided to help designers take

ad-vantage of high relative rib area on the tension splice and de-velopment length of reinforcing bars It includes an expression for development and splice length applicable only for high relative rib area bars Inasmuch as high relative rib area bars have only been evaluated as straight bars in tension, the integrity of high relative rib area bars in compression or

as hooked bars in tension should be evaluated using appropri-ate Chapter 12 provisions of ACI 318-99

The relative rib area is expressed as

where

A r = projected rib area normal to reinforcing bar axis, in.2

s r = average center-to-center rib spacing, in.

The variables A r and s r are illustrated in Fig R2.0 The figure

includes expressions for the approximate values of A r and R r

To use this standard, the ASTM A 615/A 615M specification for billet-steel reinforcing bars should have the supplementary

πd b s r

-=

Fig R2.0—Definition of Rr.

Fig R3.1—Definition of average rib width.

requirements imposed by the Recommended Supplement to

ASTM A 615/A 615M for High Relative Area Bars that is

ap-pended to this document With modifications to the section

reference numbers, this supplement can be adapted for use

with the ASTM A 706/A 706M specification for low-alloy

steel reinforcing bars

R3.1—A high relative rib area bar is defined as a

rein-forcing bar with R r greater than or equal to 0.10, as

conven-tionally deformed reinforcement has relative rib areas of

0.06 to 0.085 Based on available experimental results, the

use of these provisions is limited to reinforcing bars with a

maximum R r = 0.14 Furthermore, consistent with the

small-est spacing used in tsmall-ests, the rib spacing s r shall not be less

than 44% of the nominal bar diameter, as indicated in 3.1.3

A lower limit on width of the concrete between ribs is

indi-rectly prescribed in 3.1.4 to avoid having a reduction in bond

capacity due to a local shear failure of the concrete between

the ribs The variables in 3.1.4 are illustrated in Fig R3.1

For calculating the average rib width, the width at 0.75 of the

rib height, as illustrated in Fig R3.1, was chosen in the

rec-ommended supplement to ASTM A 615 due to possible presence of rounded corners on the ribs The reinforcing bars used in the experimental studies (Darwin and Graham 1993; Darwin et al 1996a; Zuo and Darwin 1998) leading up to this standard were either machined or special rolled; both bar

types had ribs with flat upper faces Therefore, the

support-ing research results are based on the actual width of the upper face

Reinforcing bars with X and diamond deformation patterns are excluded from this standard because their bond properties are markedly lower than bars with parallel ribs Earlier bond strength tests on X pattern No 6 and 11 epoxy-coated bars (Treece and Jirsa 1989) gave the lowest bond strengths re-ported in the literature, even though the bars had relative rib areas of 0.099 and 0.110, respectively These bond values were significantly lower than values measured on epoxy-coated bars with parallel ribs and lower relative rib areas Also, X pattern bars are not allowed in the Canadian Code (CSA 1992) because Canadian bond pullout tests on X pat-tern bars gave significantly lower strengths than did parallel rib bars In addition to the bond strength issue, the NCHRP study (Helgason et al 1976) indicated that X pattern bars have lower fatigue life than bars with other types of deforma-tion patterns; three unpublished studies done in the 1970s by John McDermott corroborate this finding

The bamboo pattern for ribs (ribs oriented at 90 degrees to the bar axis) are also excluded by the angle restrictions adopted in

Fig R4.2.1—Histogram of test/prediction ratio for all uncoated high Rr bars (No 5, 8, and 11 bars).

Table R4.2.1—Database size for bottom-cast

uncoated bars

Bar pattern

No of specimens

f′c < 6000 psi f′c ≥ 6000 psi

Fig R4.2.2—Histogram of test/prediction ratio for all coated high Rr bars (No 5, 8, and 11 bars).

this standard because of problems associated with the bending

of conventionally deformed bars with this rib orientation

No 11 bars were the largest high relative rib area bars used

in the experimental program forming the basis for these new

provisions Thus, these provisions are not intended to be used

for No 14 and No 18 reinforcing bars

R4.2—Equation (4.1) represents the beneficial effect of the

high relative rib reinforcing bars as well as the influence of

oth-er poth-ertinent variables (Darwin and Graham 1993; Darwin et al

1996a,b) Equation (4.1) was derived by statistical analysis of

experimental data and does not represent a mechanistic model

for bond behavior Thus, it should not be extended to cases

oth-er than those explicitly covoth-ered in these provisions The

rela-tive rib area R r would be fixed at a specified value for the

reinforcement being used

Table R4.2.1 indicates the size of the database used in

de-veloping the provisions in 4.2 for high relative rib area bars

in normal- and high-strength concrete, as well as the current

size of the database for conventionally deformed bars

Fig R4.2.1 summarizes the development length test

re-sults for the uncoated high R r bars using a histogram of test/

prediction ratio A similar histogram of the test results for

coated high R r bars is given in Fig R4.2.2

Use of the fourth root of the concrete strength is limited to

11.0 because testing at strengths in excess of 14,000 psi is

very limited The yield strength is limited to 80 ksi inasmuch

as the maximum bar stress in tests was 81 ksi

The upper limit on the confinement parameter (cω + K tr )/d b

of 4 is specified because higher values of the parameter

corre-spond to pullout failures, which occur at lengths correcorre-spond-

correspond-ing to Eq (4.1) with the confinement parameter at a value of

4 No specific limit is placed on the concrete or the

trans-verse reinforcement terms in the parameter

The K tr parameter includes the influence of the high relative

rib properties as well as the amount of transverse

reinforce-ment confining the developing bar The yield strength of the

transverse reinforcement is not present in the K tr parameter

because it has been found that the transverse reinforcement

seldom reaches the yield value when confining the

develop-ing bar

The parameter ω typically reflects the benefit of wide spacing

when the cover to the tension face c b dictates the value of c It

can, however, also reflect the benefit of large cover when

close spacing dictates the value of c.

Evaluations of crack width and crack spacing outside the

splice region have indicated no measurable difference

be-tween conventional and high relative rib area bars As

com-pared with conventionally deformed bars, coated high

relative rib area bars typically produce fewer cracks with

larger crack width than uncoated bars

Studies at PCA (Helgason et al 1976) established that the

principal geometric variable in the fatigue life of reinforcing

bars is the ratio of the radius at the base of a deformation r to

its height h The absolute values of rib height and rib spacing

were not found to be critical parameters The results of the

work by Helgason et al (1976) are incorporated in the

AASHTO Bridge Specifications (1996) in an expression that

establishes the maximum allowable stress range as a function of

r/h and the minimum stress As with conventional bars, that

ex-pression should be applied when fatigue is of concern

High relative rib area bars have thus far exhibited no prob-lems when subjected to standard bend tests at the producing mills or in fabrication tests in the research used to develop this standard

No modifier factors are included in this standard for bundled bars in tension Although no testing has been conducted for high relative rib area bars in bundles, there is no reason to believe that the length modifiers for bundled bars in 12.4 of ACI 318-99 are not just as appropriate for use with high relative rib area bars as with the conventionally deformed reinforcing bars

R4.3—The presence of the higher ribs, and specifically ribs

with a larger relative rib area, and the elimination of rib patterns with poor bond properties produces a beneficial effect on the bond of epoxy-coated bars (Choi et al 1991) The epoxy coat-ing thickness has less impact on the bearcoat-ing area with high rib bars The resulting reduced bearing stress decreases the differ-ence between the behavior of uncoated and coated reinforcing bars, which leads to use of a 1.2 factor for all situations The use of high ribs has little effect on the ratio of the embed-ment length for top-cast bars to the embedembed-ment length for bot-tom-cast bars With this information, it was felt there was no basis for changing the 1.3 top bar factor

R5.0—Analyses of test data (Orangun, Jirsa, and Breen

1977; Darwin et al 1996b) have concluded that the splice length and the development length can be predicted by the same ex-pression when conditions are the same Therefore, there is tech-nically no need to have Class B splices However, Class B splices have been retained for consistency with current practice

In this document, Class A splices can be used in all situations except those with high stress [that is, not meeting 5.2.2(a)], little

or no staggering of splices and little or no confinement from transverse reinforcement where Class B splices are indicated With adequate transverse reinforcement, there is a more predict-able and a more ductile failure mode that permits the use of Class A splice lengths

R5.3—This restriction is identical to Section 12.14.2.1 in

ACI 318-99 for tension lap splices of conventionally deformed reinforcing bars

COMMENTARY REFERENCES

AASHTO Subcommittee on Bridges and Structures, 1996, Standard Specifi-cations for Highway Bridges, 16th Edition, American Association of State

Highway and Transportation Officials, Washington, D.C., 676 pp.

ASTM A 615/A 615M-94, Standard Specification for Deformed and Plain Billet-Steel Bars for Concrete Reinforcement, American Society for Testing

Materials, West Conshohocken, Pa.

ASTM A 706/A 706M-92b, Standard Specification for Low-Alloy Steel Deformed Bars for Concrete Reinforcement, American Society for Testing

Materials, West Conshohocken, Pa.

Choi, O C.; Hadje-Ghaffari, H.; Darwin, D.; and McCabe, S., 1991, “Bond

of Epoxy-Coated Reinforcement: Bar Parameters,” ACI Materials Journal, V.

88, No 2, Mar.-Apr., pp 207-217.

CSA, 1992, Billet-Steel Bars for Concrete Reinforcement,

(CAN/CSA-G30.18-M92), Canadian Standards Association, Rexdale (Toronto), Ontario,

18 pp.

Darwin, D., and Graham, E K., 1993, “Effect of Deformation Height

and Spacing on Bond Strength of Reinforcing Bars,” ACI Structural Journal, V 90, No 6, Nov.-Dec., pp 646-657.

D.C., 90 pp.

Orangun, C O.; Jirsa, J O.; and Breen, J E., 1977, “Re-Evaluation of Test

Data on Development Length and Splices,” ACI J OURNAL, Proceedings V 74,

No 3, Mar., pp 114-122.

Treece, R A., and Jirsa, J O., 1989, “Bond Strength of Epoxy-Coated

Rein-forcing Bars,” ACI Materials Journal, V 86, No 2 Mar.-Apr., pp 167-184.

Zuo, J., and Darwin, D., 1998, “Bond Strength of High Relative Rib Area

Reinforcing Bars,” SM Report No 46, University of Kansas Center for

Research, Inc., Lawrence, Kans., Jan., 350 pp

CODES CITED IN STANDARD

ACI Committee 318, 1999 “Building Code Requirements for Structural

Concrete (ACI 318-99) and Commentary (318R-99),” American Concrete

Institute, Farmington Hills, Mich., 391 pp.

APPENDIX A—RECOMMENDED SUPPLEMENT

TO ASTM A 615/A 615M FOR HIGH RELATIVE RIB

AREA BARS

The following supplementary requirements shall apply

only when specified in the purchase order or contract

S.1—Requirements for deformations

S.1.1—The deformations on high relative rib area bars

shall meet all requirements in Section 7

S.1.2—In addition, the relative rib area (as defined in

S.2.1) shall meet the requirements and limitations of 3.1 of

the standard.1

S.2—Relative rib area

S.2.1—The relative rib area R r is defined in 2.0 and R2.0

of the provisional standard

1 The value of R r should be specified by the purchaser

S.2.2—For bars that meet the requirements of S.2.1, it

shall be permitted to calculate R r using Eq (S-1).

(S-1)

where

S.2.3—The average height of deformations shall be

deter-mined from measurements made on not less than two typical deformations on each side of the bar Determinations shall be based on five measurements per deformation, one at the cen-ter of the overall length, two at the ends of the overall length, and two located halfway between the center and the ends The measurements at the ends of the overall length shall be aver-aged to obtain a single value and that value shall be combined with the other three measurements to obtain the average rib

height h r Deformation height shall be measured using a depth gage with a knife edge support that spans not more than two adjacent ribs Alternatively, it shall be permitted to use a knife edge that spans more than two adjacent ribs, in which case the average rib height shall be multiplied by 0.95 prior

to use in Eq (S-1)

S.2.4—The average rib width shall be determined from

measurements made on not less than two typical deformations

on each side of the bar Determinations shall be based on three measurements per deformation, one at the center and one at each end The measurements shall be taken at three-quarters of the rib height at each location The average of the ments at the ends shall be averaged with the center measure-ment to obtain a value for the one side of the deformation

Note S.2—A knife edge is required to allow measurements

to be made at the ends of the overall length of deformations, usually adjacent to a longitudinal rib The calculation of hr

is based on a knife edge that spans only two ribs because measurements made with a longer knife edge result in unre-alistically high average rib heights and an overestimate of the relative rib area for some bars When a longer knife edge

is used, hr shall be reduced by 5%.

S.3—Type of steel

S.3.1—All bars produced to these supplementary

require-ments shall be identified by the letter H, in place of the letter S

specified in 20.3.3, indicating that the bar was produced to meet both the specification and these supplementary requirements

S.4—References

1 ACI Committee 408, 2001, “Splice and Development Length of High Relative Rib Area Reinforcing Bars in Tension (ACI 408.3-01) and Com-mentary (408.3R-01),” American Concrete Institute, Farmington Hills, Mich., 6 pp.

h r = average height of deformations (measured

according to S.2.3), in or mm

s r = average spacing of deformations, in or mm

R r h r

s r

1 ∑gaps

p

-–

=