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Design of Concrete Structure I University of Palestine بسم الله الرحمن الرحيم Chapter 7 Instructor: Eng. Mazen Alshorafa 2

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 1 University of Palestine Bond, Development Lengths, and Splices Concept of Bond Stress Bond stresses are existent whenever the stress or force in a reinforcing bar changes from point to point along the length of the bar in order to maintain equilibrium. C M Concrete T Reinforcing bar T2=fs2Ab db µ=Bond stress T1=fs1Ab l T=Asfy Bond stress fs2=fs1+∆fs Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 2 University of Palestine Bond, Development Lengths, and Splices Concept of Bond Stress Equilibrium Condition for Rebar µavg= bond stress (coefficient of friction) C T2=fs2Ab db µ=Bond stress T1=fs1Ab l M T L fs2=fs1+∆fs T Bond stress Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 3 University of Palestine Bond, Development Lengths, and Splices Mechanism of Bond Transfer A smooth bar embedded in concrete develops bond by Adhesion between concrete & reinforcement, and a small amount Friction. Note: These bonds are quickly lost when the bar is loaded in tension On the other hand, a deformed bar generates bond by friction and by bearing on the deformations of the bar against the concrete longitudinal Radial (a) Forces on bar (c) Components of forces on concrete (d) Splitting stresses (b) Forces on concrete Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 4 University of Palestine Bond, Development Lengths, and Splices Mechanism of Bond Transfer Splitting cracks result in loss of bond transfer. Reinforcement can be used to restrain these cracks. The load at which splitting failure develops is a function of : The minimum distance from the bar to the surface of the concrete or to the next bar. The smaller the distance, the smaller is the splitting load. The tensile strength of the concrete. The average bond stress. The higher the average bond stress, the higher is the splitting resistance. Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 5 University of Palestine Bond, Development Lengths, and Splices Mechanism of Bond Transfer If the concrete cover and bar spacing are large compared to the bar diameter, a pullout failure can occur. Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 6 University of Palestine Bond, Development Lengths, and Splices Development Length The development length ld is shortest length of bar in which the bar stress can increase from zero to the yield strength, fy The development Lengths are different in tension and compression, because a bar loaded in tension is subject to in-and-out bond stresses and hence requires a considerably longer development length Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 7 University of Palestine Bond, Development Lengths, and Splices Development Length of Deformed Bars in Tension According to ACI Code, the development length for deformed bars in tension is given by to safeguard against pullout type failure where, ld = development length, cm db = nominal diameter of bar, cm fy = specified yield strength of reinforcement, kg/cm2 = square root of specified compressive strength of concrete, kg/cm2 C = spacing or cover dimension, cm Ktr = transverse reinforcement index Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 8 University of Palestine Bond, Development Lengths, and Splices Development Length of Deformed Bars in Tension [contd.] C is the smaller of (a) the smallest distance measured from the center of the bar to the nearest concrete surface (b) one-half the center-to-center spacing of bars being developed. α is a bar location factor (a) Horizontal reinforcement so placed that more than 30 cm of fresh concrete is cast in the member below the development length or splice……………………………………………………………………. 1.3 (b) Other reinforcement………………………………………………………….. 1.0 β is a coating factor that reflects the adverse effects of epoxy coating (a) Epoxy-coated bars or wires with cover less than 3db or clear spacing less than 6db ………………………………………….. 1.5 (b) All other epoxy-coated bars or wires………………………………... 1.2 (c) Uncoated reinforcement……………………………………………………… 1.0 Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 9 University of Palestine Bond, Development Lengths, and Splices Development Length of Deformed Bars in Tension [contd.] However, the product αβ is not to be greater than 1.7 γ is a reinforcement size factor that reflects better performance of the smaller diameter reinforcement (a) Φ20mm and smaller bars.……………………………………………….. 0.8 (b) Φ22mm and larger bars.…………..…….………………………………. 1.0 λ is a lightweight aggregate concrete factor that reflects lower tensile strength of lightweight concrete, & resulting reduction in splitting resistance. (a) When lightweight aggregate concrete is used…….……..……. 0.8 (b) When normal weight concrete is used…………………..…………. 1.0 Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 10 University of Palestine Bond, Development Lengths, and Splices Development Length of Deformed Bars in Tension [contd.] Ktr is a transverse reinforcement index that represents the contribution of confining reinforcement Where Atr = total cross sectional area of all transverse reinforcement within the spacing s, which crosses the potential plane of splitting along the reinforcement being developed with in the development length cm2 fyt = specified yield strength of transverse reinforcement, kg/cm2 s = maximum center-to-center spacing of transverse reinforcement within development length ld , cm. n = number of bars being developed along the plane of splitting. Note: It is permitted to use Ktr= 0.0 as design simplification even if transverse reinforcement is present. Atr Potential plane of splitting Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 11 University of Palestine Bond, Development Lengths, and Splices Development Length of Deformed Bars in Tension [contd.] Excessive Reinforcement According to ACI Code, reduction in development length is allowed Where As provided > As required. the reduction is given by -Except as required for seismic design -Good practice to ignore this factor, since use of structure may change over time. Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 12 University of Palestine Bond, Development Lengths, and Splices Example # 1 Determine the development length required for the uncoated bottom bars as shown in figure. Use fc’ = 250 kg/cm2 normal weight concrete and fy = 420 kg/cm2 Solution: α=1.0 for bottom bars, β=1.0 for uncoated bars α β =1.0 <1.7 OK γ=0.8 for Φ20mm, λ=1.0 for normal weight concrete C the smallest of 4.0+1.0+1.0=6 cm [40-2(4.0)-2(1.0)-2.0]/(3)(2)=4.67 cm i.e., C is taken as 4.67 cm 4Φ20 40 cm 60 cm Φ10@20 Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 13 University of Palestine Bond, Development Lengths, and Splices Example # 1 [contd.] 4Φ20 40 cm 60 cm Φ10@20 Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 14 University of Palestine Bond, Development Lengths, and Splices Development Length of Deformed Bars in Compression Shorter development lengths are required for compression than for tension since flexural tension cracks are not present for bars in compression. According to ACI Code, the development length ld , for deformed bars in compression is computed as the product of the basic development length ldc and applicable modification factors, but ld is not to be less than 20 cm. ld = ldc x applicable modification factors ≥ 20 cm. The basic development length ldb for deformed bars in compression is given as Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 15 University of Palestine Bond, Development Lengths, and Splices Development Length of Deformed Bars in Compression [contd.] Applicable Modification Factors Excessive reinforcement factor =As required / As provided Spirals or Ties: the modification factor for reinforcement, enclosed with spiral reinforcement ≥ 6mm in diameter and ≤ 10 cm pitch or within Φ12mm ties spaced at ≤ 10 cm on center is given as 0.75 Development Lengths for Bundled Bars Based on ACI Code, development length of individual bars within a bundle, in tension or compression, is taken as that for individual bar, increased 20% for three-bar bundle, and 33% for four-bar bundle. For determining the appropriate modification factors, a unit of bundled bars is treated as a single bar of a diameter derived from the equivalent total area of bars. Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 16 University of Palestine Bond, Development Lengths, and Splices Development of Standard Hooks in Tension Hooks are used to provide additional anchorage when there is insufficient length available to develop a bar. According to ACI Code development length ldh , for deformed bars in tension terminating in a standard hook is computed as the product of the basic development length lhb and applicable modification factors, but ldh is not to be less than 8db, nor less than 15 cm. ldh = lhb x applicable modification factors ≥ 15 cm or 8db. The basic development length lhb for hooked bars is given as Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 17 University of Palestine Bond, Development Lengths, and Splices Development of Standard Hooks in Tension [contd.] Applicable Modification Factors 1. Concrete cover: for db ≤ Φ36mm, side cover (normal to plane of hook) ≥ 6.35 cm, and for 90 degree hook, cover on bar extension beyond hook ≥ 5.0 cm, the modification factor is taken as 0.7. 2. Excessive reinforcement factor =As required / As provided 3. Spirals or Ties: for db ≤ Φ36mm, hooks enclosed vertically or horizontally within ties or stirrups spaced along the full development length ldh not greater than 3 db , where db is diameter of hooked bar, is taken as 0.8. 4. Lightweight aggregate concrete: the modification factor is 1.3. 5. Epoxy-coated reinforcement: the modification factor for hooked bars with epoxy coating is taken as 1.2. Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 18 University of Palestine Bond, Development Lengths, and Splices Development of Standard Hooks in Tension [contd.] Development length ldh is measured from the critical section of the bar to the out-side end or edge of the hooks. Either a 90 or a 180-degree hook, shown in Figure, may be used Development of reinforcement- General * ACI code notes that hooks are not considered effective in compression and may not be used as anchorage. * The values of used in this Lecture Shall not exceed 26.5 kg/cm2. ldh Part (a) 4db or 64mm Φ10 through Φ25 Φ28 through Φ36 Φ44 through Φ56 ldh Part (b) Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 19 University of Palestine Bond, Development Lengths, and Splices Splices of Reinforcement Splicing of reinforcement bars is necessary, either because the available bars are not long enough, or to ease construction, in order to guarantee continuity of the reinforcement according to design requirements. Types of Splices (a) Welding (b) Mechanical connectors (c) Lap splices (simplest and most economical method) In a lapped splice, the force in one bar is transferred to the concrete, which transfers it to the adjacent bar. Splice length is the distance the two bars are overlapped. Forces on bar at splice Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 20 University of Palestine Bond, Development Lengths, and Splices Splices of Reinforcement Important note: Lap splices have a number of disadvantages, including congestion of reinforcement at the lap splice and development of transverse cracks due to stress concentrations. It is recommended to locate splices at sections where stresses are low. Types of lap Splices Direct Contact Splice as figure a Non-Contact Splice (spaced) the distance between two bars cannot be greater than 1/5 of the splice length nor 15 cm T T ls Direct contact T s T Bars are spaced ls Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 21 University of Palestine Bond, Development Lengths, and Splices Splices of Deformed Bars in Tension ACI code divides tension lap splices into two classes, A and B. the class of splice used is dependent on the level of stress in the reinforcing and on the percentage of steel that is spliced at particular location. The splice lengths for each class of splice are as follow Class A splice: 1.0 ld Class B splice: 1.3 ld Tension Lap Splices Maximum Percent of As spliced within required lap length As provided As required 50 100 Equal to or greater than 2 Less than 2 Class A Class B Instructor: Eng. Mazen Alshorafa

Design of Concrete Structure I Page 22 University of Palestine Bond, Development Lengths, and Splices Splices of Deformed Bars in Compression Bond behavior of compression bars is not complicated by the problem of transverse tension cracking and thus compression splices do not require provisions as strict as those specified for tension Based on the ACI code the Compression lap splice length shall be 0.007 fy db ≥ 30.0 cm for fy ≤ 4200 kg/cm2 (0.013 fy-24) db ≥ 30.0 cm for fy > 4200 kg/cm2 The computed splice length should be increase by 33% if fc’<210kg/cm2 According to ACI code when bars of different size are lap spliced in compression, splice length shall be the larger of either development length of larger bar, or splice length of smaller bar. Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 23 University of Palestine Bond, Development Lengths, and Splices Example # 2 Determine the development or embedment length required for the epoxy-coated top bars of the beam as shown in figure. If the bars are straight If a 180 hook is used If a 90 hook is used Use fc’ = 280 kg/cm2 and fy = 4200 kg/cm2 Solution: (a) Straight Bars α=1.3 for top bars, β=1.5 for coated bars α β =1.3x1.5 = 1.95 > 1.7 use 1.7 γ=1.0 for Φ32mm, λ=1.0 for normal weight concrete C the smallest of 4.0+1.2+1.6=6.8 cm [40-2(4.0)-2(1.2)-3.2]/(3)(2)=4.4 cm i.e., C is taken as 4.4 cm 4Φ32 40 cm 50 cm Φ12@15 4Φ32 Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 24 University of Palestine Bond, Development Lengths, and Splices Example # 2 [contd.] (b) Using 180 hook ldh = lhb x applicable modification factors ≥ 150 mm or 8db. applicable modification factors =1.2 for epoxy-coated hooks 4Φ32 40 cm 50 cm Φ12@15 Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 25 University of Palestine Bond, Development Lengths, and Splices Example # 2 [contd.] (c) Using 90 hook ldh=70 cm Critical section 5db =16 4db =12.8 180o hook Ldh=70 cm Critical section 12db=38.4 90o hook Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 26 University of Palestine Bond, Development Lengths, and Splices Example # 3 To facilitate construction of a cantilever retaining wall, the vertical reinforcement shown in Figure, is to be spliced to dowels extending from the foundation. Determine the required splice length when all reinforcement bars are spliced at the same location. Use fc’ = 300 kg/cm2 and fy = 4200 kg/cm2 Solution: Class B splice is required where ls = 1.3 ld α=1.0, β=1.0 → α β =1.0 < 1.7 OK γ=0.8, λ=1.0 for normal weight concrete C the smallest of 7.5+0.8=8.3 cm 25/2=12.5 cm i.e., C is taken as 8.3 cm Ktr =0.0, since no stirrups are used ls Φ16 @ 250 Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 27 University of Palestine Bond, Development Lengths, and Splices Example # 3 [contd.] Φ16 @ 25 Ls=50 cm Φ16 @ 25 Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 28 University of Palestine Bond, Development Lengths, and Splices Example # 4 Design a compression lap splice for a tied column whose cross section is shown in Figure when: (a) Φ16 mm bars are used on both sides of the splice. (b) Φ 16 mm bars are lap spliced with φ 18 mm bars. Use fc’ = 300 kg/cm2 and fy = 4200 kg/cm2 Solution: (a) For bars of similar diameter splice length in compression and for fy =4200 kg/cm2 is equal to 0.07 fy db = 0.07 (4200)(1.6) = 47 cm >30 cm taken as 47 cm Instructor: Eng. Mazen Alshorafa

Bond, Development Lengths, and Splices Design of Concrete Structure I Page 29 University of Palestine Bond, Development Lengths, and Splices Example # 4 [contd.] (b) For bars of different diameters splice length in compression shall be the larger of either development length of larger bar, or splice length of smaller bar. The development length of larger bar ld = ldb x applicable modification factors applicable modification factors =1.0 Splice length of smaller diameter bar is evaluated in part (a) as 470mm. Thus, the splice length is taken as 470 mm. Instructor: Eng. Mazen Alshorafa