Development Length Hook Splice Of Reinforcements
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Development Length Hook Splice Of Reinforcements

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Development Length Hook Splice Of Reinforcements Development Length Hook Splice Of Reinforcements Document Transcript

  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com CHAPTER DEVELOPMENT LENGTH, 23 HOOK REINFORCEMENTS & SPLICE OF23.1 INTRODUCTION The failure of the reinforced concrete structure commonly caused by incorrect reinforcements detail. Reinforcement detail includes the development length, hook (anchorage) and splice between reinforcements. The strength of reinforcing bar is based on the bond strength between steel reinforcement and concrete material. Due to external load the bond stress between steel reinforcement and concrete can be exceeded and cause crushing and splitting of the surrounding concrete. The followings are the major factors of the bond strength, as follows : Adhesion between concrete and steel reinforcement. Gripping effect from drying shrinkage of the surrounding concrete. Shear interlock of bar deformation and surrounding concrete. Concrete quality. Diameter of the steel reinforcement. This chapter describes the analysis of development length, standard hook, development of flexural reinforcement, bar cut off and splice of reinforcements.23.2 DEVELOPMENT OF BOND STRESS 23.2.1 GENERAL Bond stress is the primary result of the shear interlock between the steel reinforcement and surrounding concrete. Bond stress can be defined as local shearing stress per unit area of the bar surface. Three types of test can be used to determine the bond quality which is pull-out test, embedded rod test and beam test. 23.2.2 PULL OUT BOND The pull out bond is determined based on the pull out force applied to the embedded steel reinforcement with prescribed embedded length. The pull out bond strength can be calculated based on the average bond stress μ, as follows : Tnb = μ(πdb )ld [23.1] 23 - 1
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com where : Tnb = bond strength of embedded reinforcement μ = average bond stress per unit area of bar surface db = diameter of reinforcement ld = embedded length (development length) The tensile force at the bar cross section is : 1 2 T= πdb fs [23.2] 4 where : T = tensile force at bar cross section db = diameter of reinforcement fs = stress of bar The two variables above must be in static horizontal equilibrium, as follows : μ(πdb )ld = 1 2 πdb fs [23.3] 4 So the development length is derived as : db fs ld = [23.4] 4μ23.3 DEVELOPMENT LENGTH 23.3.1 GENERAL Development length is defined as minimum length of bar in which the bar stress can increase from zero to the yield strength. If the distance is less than the development length the bar will pull out the concrete. The development length is a function of yield stress, bar diameter and average bond stress at surrounding concrete. 23.3.2 BASIC DEVELOPMENT LENGTH ACI code uses the concept of development length rather than average bond stress. The average bond stress is determined based on the test result and function of the concrete compressive strength. Empirically the average bond stress is calculated, as follows : 9.5 f c lb μ= ≤ 800 [23.5] db in2 where :23 - 2
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com μ = average bond stress per unit area of bar surface f’c = concrete compressive strength db = diameter of reinforcement or can be simply written as : μ = k f c [23.6] Rewritten the above condition we can obtain the basic development length, as follows : μ(πdb )ld = A b fy k f c (πdb )ld = A b fy [23.7] ⎛ A b fy ⎞ ldb = k1⎜ ⎟ ⎜ f ⎟ ⎝ c ⎠ 23.3.3 DEVELOPMENT LENGTH OF TENSION BAR A. Original Development Length The basic development length of tension bar is : TABLE 23.1 DEVELOPMENT LENGTH OF TENSION BAR SI psi ld 15fy αβγλ ld 3fy αβγλ = = db ⎛ c + K tr ⎞ db ⎛ c + K tr ⎞ 16 f c ⎜ ⎜ d ⎟ ⎟ 40 f c ⎜ ⎜ d ⎟ ⎟ ⎝ b ⎠ ⎝ b ⎠ ⎛ c + K tr ⎞ ≤ 1.5⎜ ⎜ d ⎟ ≤ 2. 5 ⎟ ⎝ b ⎠ The transverse reinforcement index is defined as : TABLE 23.2 KTR SI psi A tr fyt A tr fyt K tr = K tr = 260sn 1500sn where : Ktr = transverse reinforcement index Atr = area of transverse reinforcement through the longitudinal bar being developed fyt = yield strength of transverse reinforcement Ktr can be used conservatively = 0. 23 - 3
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com B. α, β, γ, λ & c Factor The α factor is bar location factor determined, as follows : TABLE 23.3 BAR LOCATION FACTOR α LOCATION α Horizontal reinforcement placed more than 12” (300 mm) fresh concrete 1.3 Other Reinforcement 1.0 The β factor is coating factor determined, as follows : TABLE 23.4 COATING FACTOR β COATING β Epoxy coated bar with cover less than 3db / clear spacing less than 6db 1.5 All epoxy coated bar 1.2 Uncoated reinforcement 1.0 The product of αβ must not exceed 1.7. The γ factor is bar size factor determined, as follows : TABLE 23.5 BAR SIZE FACTOR γ BAR SIZE γ < 20 mm 0.8 > 25 mm 1.0 The c factor is spacing / cover dimension factor determined as the smaller of : Distance from center of bar to the nearest concrete surface. 0.5 of center to center spacing of the bar being developed. C. Simplified Development Length For the design purpose the simplified development length formula is often used, as follows : TABLE 23.6 SIMPLIFIED DEVELOPMENT LENGTH OF TENSION BAR – PSI UNIT ≤ NO. 6 CASE > NO. 7 (DEFORMED BAR) Clear spacing of developed bar > db, stirrup not less than the ld fy αβλ ld fy αβλ code minimum requirement = = db 25 f c db 20 f c Clear spacing of developed bar > 2db, clear cover > db ld 3fy αβλ ld 3fy αβλ Other = = db 50 f c db 40 f c The development length ld must be greater than 12 inch.23 - 4
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com TABLE 23.7 SIMPLIFIED DEVELOPMENT LENGTH OF TENSION BAR – SI UNIT ≤ NO. 6 CASE > NO. 7 (DEFORMED BAR) Clear spacing of developed bar > db, stirrup not less than the ld 12fy αβλ ld 12fy αβλ code minimum requirement = = db 25 f c db 20 f c Clear spacing of developed bar > 2db, clear cover > db ld 18 fy αβλ ld 18 fy αβλ Other = = db 25 f c db 20 f c The development length ld must be greater than 300 mm. 23.3.4 DEVELOPMENT LENGTH OF COMPRESSION BAR The development length for compression bar is shorter than in the tension bar, because there is no concrete cracking occurs. The development length of compression bar is : TABLE 23.8 DEVELOPMENT LENGTH OF COMPRESSION BAR psi SI 0.02fy db fy db ld = ≥ 0.0003 fy db ld = ≥ 0.044 fy db f c 4 f c The development length of compression bar ld must be greater than 8 inch / 200 mm. 23.3.5 DEVELOPMENT LENGTH OF BUNDLED BAR The development length of bundled bar either in tension or compression is greater than development length of single bar, because the bundled bar reduce the surface area surrounding concrete. TABLE 23.9 DEVELOPMENT LENGTH OF BUNDLED BAR 3 BUNDLED 4 BUNDLED 1.2ld 1.33ld ld is calculated based on the equivalent single bar area having the same area of bundled bar. 23.3.6 DEVELOPMENT LENGTH OF WELDED WIRE FABRIC The development length of plain welded wire fabric in tension is : ⎛ A w fy λ ⎞ ld = 0.27⎜ ⎟ ⎜ s f ⎟ [23.8] ⎝ w c ⎠ where : Aw = cross section area of wire 23 - 5
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com sw = spacing of wire fy = yield strength of wire (psi) f’c = concrete compressive strength (psi) The development length must be greater than 6 inch or (sw + 2 inch). 23.3.7 DEVELOPMENT LENGTH OF WEB REINFORCEMENT The following figure shows the development length of double U stirrup, as follows : FIGURE 23.1 DOUBLE U STIRRUP If the development length above can not fit the depth of the member, the development length can be extended to full depth of member.23.4 STANDARD HOOK 23.4.1 GENERAL When the insufficient length can not be provided to develop a bar then the bar needed to be anchorage. Two type of standard hooks can be used which is 90o hook and 180o hook. 23.4.2 EMBEDMENT LENGTH OF HOOK The hook development length is obtained from the basic development length for standard hook lhb multiplied with factor. The basic development length for standard hook is : TABLE 23.10 BASIC DEVELOPMENT LENGTH OF STANDARD HOOK psi SI 1200 db 100db lhb = lhb = f c f c23 - 6
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com The hook development length then calculated as follows : ldh = λlhb [23.9] where : ldh = hook development length λ = multiplier factor lhb = basic development length of standard hook The following is the multiplier factor λ, as follows : TABLE 23.11 MULTIPLIER FACTOR OF HOOK DEVELOPMENT LENGTH CONDITION λ fy λ= 400 fy different from 400 MPa / 60000 psi fy λ= 60000 o For 90 hook cover not less than 2” λ = 0 .7 No. 11 bar and smaller cover not less than 2.5” No. 11 bar and smaller stirrup spacing less than 3d b λ = 0 .8 Light weight concrete λ = 1 .3 Epoxy coating λ = 1 .2 23.4.3 90O HOOK AND 180O HOOK The figure below is the standard hook for 90o hook and 180o hook. FIGURE 23.2 STANDARD HOOK The diameter of the bend of hook is : TABLE 23.12 BEND DIAMETER OF HOOK NO. 3 – 8 NO. 9, 10, 11 NO. 14 & 18 D = 6db D = 8db D = 10db 23 - 7
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com The figure below is the hook for No. 3 bar stirrup. FIGURE 23.3 HOOK FOR STIRRUP NO. 3 The diameter of the bend of stirrup is : TABLE 23.13 BEND DIAMETER OF STIRRUP NO. 3 – 5 NO. 6 – 8 D = 4db D = 6db23.5 DEVELOPMENT OF FLEXURAL REINFORCEMENT & CUT OFF POINT 23.5.1 GENERAL Flexural reinforcement has different treatment of development length. The flexural reinforcement in one span may designed due to different value of bending moment so the reinforcement is different. We have to determine the location where the bar can be cut and the development length from the point of maximum moment. 23.5.2 DEVELOPMENT LENGTH OF FLEXURAL REINFORCEMENT A. General The flexural reinforcements are designed using the maximum bending moment value such as at mid span (positive moment) and at support (negative moment). To ensure the full development the flexural reinforcement must be extended at least development length ld from the point of maximum bending moment. B. Rules of Positive Moment Reinforcement The followings are the rules of the development length of flexural reinforcement for positive moment, as follows : The reinforcement must be extended at least development length ld from the point of maximum bending moment.23 - 8
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com In simple beam structure, at least 1/3 of positive moment reinforcement must be extended at least 6 inch into support without bending. In continuous beam, at least ¼ of positive moment reinforcement must be extended at least 6 inch into support without bending. Interior continuous beam without closed stirrup, at least ¼ of positive moment reinforcement shall be spliced with spliced class A. C. Rules of Negative Moment Reinforcement The followings are the rules of the development length of flexural reinforcement for negative moment, as follows : The reinforcement must be extended at least development length ld from the point of maximum bending moment. Negative moment reinforcement must be anchored to the supporting column or member. At least 1/3 of total reinforcement for negative moment must be extended beyond the inflection point > d or 12 db or 1/16 of clear span the larger value is taken. 23 - 9
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com 23.5.3 BAR CUT OFF POINT A. General The critical location of the flexural reinforcement is where there is rapid drop in the bending moment such as inflection point (zero moment). To ensure the full development length the flexural reinforcement must be extended beyond the inflection point with a distance 12db or d which is greater. B. Rules for All Reinforcements The followings are the rules of the bar cut off for all reinforcements, as follows : Bars must be extended d or 12 db beyond the theoretical flexural cut off points except at support / end of cantilever. Bars must be extended ld from the theoretical flexural cut off point of adjacent bar. 23.5.4 SKETCH OF FLEXURAL DEVELOPMENT LENGTH A. General This section shows the flexural development sketch of positive moment reinforcement and negative moment reinforcement based on the all rules at previous section. B. Positive Moment Reinforcement The figure below shows the flexural development length of positive moment reinforcement. FIGURE 23.4 FLEXURAL DEVELOPMENT LENGTH – POSITIVE MOMENT REINFORCEMENT23 - 10
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com C. Negative Moment Reinforcement The figure below shows the flexural development length of negative moment reinforcement. FIGURE 23.5 FLEXURAL DEVELOPMENT LENGTH – NEGATIVE MOMENT REINFORCEMENT23.6 SPLICE OF REINFORCEMENTS 23.6.1 GENERAL The bars are produced in standard length so sometime it is needed to be spliced. The splice of the reinforcement must ensure that it can develop yield stress along the splice length. There are three types of splice, as follows : Lap Splice, lapping of two bars with determined splice length (< bar No. 11). Mechanical Connecting, splice of reinforcement using the connector / coupler. Welding, splice by weld the two reinforcements (> bar No. 11). 23.6.2 LAP SPLICE OF TENSION BAR There are two types of lap splice of tension bar according to ACI code, as follows : Class A. Class B. The splice length of splice class A is : ls = 1.0ld ≥ 12" [23.10] where : ls = splice length ld = development length 23 - 11
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com The splice length of splice class B is : ls = 1.3ld ≥ 12" [23.11] where : ls = splice length ld = development length The following table shows the conditions of tension lap splice, as follows : TABLE 23.14 TENSION LAP SPLICE MAXIMUM % OF SPLICED BAR As PROVIDED / As REQUIRED 50% 100% ≥2 Class A Class B <2 Class A Class B 23.6.3 LAP SPLICE OF COMPRESSION BAR The lap splice of compression bar is : TABLE 23.15 COMPRESSION LAP SPLICE fy psi SI ≤ 60000 psi / 400 MPa ls ≥ 0.0005 fy db ls ≥ 0.07 fy db > 60000 psi / 400 MPa ( ls ≥ 0.0009fy − 24 db ) ( ls ≥ 0.13fy − 24 db )23.7 DETAIL OF REINFORCEMENTS 23.7.1 GENERAL The most important thing in the reinforced concrete structure is the reinforcement detail. After the reinforced concrete member is analyzed and designed a structural engineer must make a reinforcement detail, splice of reinforcement, bar bending schedule because the engineer is the only person who knows the location of critical section of the member, these information then used by the contractor when they build the structure. 23.7.2 SPACING LIMITS A. General For ensure the workability of the concrete the spacing of the reinforcement must be limited so the spacing is not o small compared to the size of the coarse aggregate. B. Minimum Spacing Minimum clear spacing of between bars is : db ≥ 1" [23.12]23 - 12
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com where : db = diameter of bar Minimum clear spacing of bar more than one layers is : 1" [23.13] Minimum clear spacing of longitudinal reinforcement in compression member with tied and spiral transverse reinforcement is : (1 − 1.5)db ≥ 1"−1.5" [23.13] where : db = diameter of bar C. Maximum Spacing Maximum spacing between bars must not spaced greater than : 3hf ≤ 18" [23.14] where : hf = slab thickness 23.7.3 END SPAN OF CONTINUOUS BEAM The figure below shows the typical detail of reinforcement for end span in continuous reinforced concrete structure. FIGURE 23.6 END SPAN OF CONTINUOUS BEAM 23 - 13
  • http://syaifulsipil96.blogspot.com/ syaiful_ashari@yahoo.com 23.7.4 INTERIOR SPAN OF CONTINUOUS BEAM The figure below shows the typical detail of reinforcement for interior span in continuous reinforced concrete structure. FIGURE 23.7 INTERIOR SPAN OF CONTINUOUS BEAM 23.7.5 COLUMN The figure below shows the typical detail of reinforcement for column. FIGURE 23.8 COLUMN23 - 14