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

ACI Committee 408 adopted ACI T2-01 (unpublished) as ACI 408.3-01 on April 23, 2001. ACI T2-01 superseded provisional standard ACI ITG-2-98 and became effective March 9, 2001.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. All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral, or recording for sound or visual reproduc- tion or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors. ACI Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in planning, de- signing, executing, and inspecting construction. The Com- mentary is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it con- tains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom. Reference to the Commentary shall not be made in con- tract documents. If items found in this document are 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 John H. Allen Steven L. McCabe 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 James V. Cox Neil M. Hawkins Max L. Porter Richard A. Devries Roberto T. Leon Julio A. Ramirez Rolf Eligehausen Leroy A. Lutz Telvin Rezansoff Fernando E. Fagundo Jun Zuo 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 ACI STANDARD AND COMMENTARY 408.3-2 Commentary, p. 408.3-3 Appendix A—Recommended supplement to ASTM A 615/A 615M for high relative rib area bars, p. 408.3-6 1.0—Notation 2.0—Definition 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. Use of X-patterns and diamond patterns for ribs is not per- mitted; 3.1.3 The rib spacing is at least 0.44 of the nominal di- ameter d b of the reinforcing bar; 3.1.4 The average rib width is less than or equal to one- third of the average rib spacing. 3.1.5 The bar size does not exceed No. 11. 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 (c ω + K tr )/d b shall not be taken greater than 4. The value of f ′ c 1/4 shall not exceed 11.0. The value of f y shall 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 A s = area of nonprestressed tension reinforcement, in. 2 A tr = total cross-sectional area of all transverse rein- 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 ′ c 1/4 = 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′ 14 ⁄ c ⁄ 1900ω–()αβλ 72 c ω K tr + d b   = ω 0.1 c max c min 0.91.25≤+= K tr C R 0.72d b 0.28+ () A tr sn = C R 44330R r 0.10 – () += 0.10 R r 0.14 ≤≤ SPLICE AND DEVELOPMENT LENGTH OF REINFORCING BARS IN TENSION 408.3-3 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 R r A r πd b s r = Fig. R2.0—Definition of R r . Fig. R3.1—Definition of average rib width. ACI STANDARD AND COMMENTARY 408.3-4 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 tests, 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 R r 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 Conventional 207 54 High R r 68 25 Fig. R4.2.2—Histogram of test/prediction ratio for all coated high R r bars (No. 5, 8, and 11 bars). SPLICE AND DEVELOPMENT LENGTH OF REINFORCING BARS IN TENSION 408.3-5 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 pertinent 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 covered 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 (c ω + K tr )/d b of 4 is specified because higher values of the parameter corre- spond to pullout failures, which occur at lengths 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 bearing 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 embedment 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. ACI STANDARD AND COMMENTARY 408.3-6 Darwin, D.; Tholen, M. L.; Idun, E. K.; and Zuo, J., 1996a, “Splice Strength of High Relative Rib Area Reinforcing Bars,” ACI Structural Journal, V. 93, No. 1, Jan Feb., pp. 95-107. Darwin, D.; Zuo, J.; Tholen, M. L.; and Idun, E. K., 1996b, “Develop- ment Length Criteria for Conventional and High Relative Area Reinforc- ing Bars,” ACI Structural Journal, V. 93, No. 3, May-June, pp. 347-359. Helgason, T.; Hanson, J. M.; Somes, N. F.; Corely, W. G.; and Hognes- tad, E., 1976, “Fatigue Strength of High Yield Reinforcing Bars,” NCHRP Report No. 164, Transportation Research Board, Washington, 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 measure- 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 h r 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, h r 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 accordingto S.2.3), in. or mm s r = average spacing of deformations, in. or mm R r h r s r 1 gaps ∑ p –    = ∑gaps = sum of the gaps between ends of deformations asdefined in Section 7.4, plus the width of any continuous longitudinal lines used to represent the grade of the bar, multiplied by the ratio of the height of the line to h r , in. or mm p = nominal perimeter of the bar, in. [Table 1(a)] or mm [Table 1(b)] . 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 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. 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