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RC Lecture 6 - TOPIC 7 BOND ANCHORAGE AND CURTAILMENT OF...

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CIVL2007 Theory and Design of Structures II RC Component (20 April 2007) page 110 TOPIC 7: BOND, ANCHORAGE AND CURTAILMENT OF REINFORCEMENT - LECTURE NOTES - General Unless otherwise noted, all clauses cited in this lecture have been taken from Chapters 8 and 9 of the Hong Kong Code of Practice for Structural Use of Concrete (2004). BOND A reinforcing bar must be adequately anchored otherwise it will withdraw from the concrete before it has reached its full tensile strength. Efficiency of anchorage depends on the length of the bar, the shape of the bar at the end, roughness of the bar and concrete strength, to name the more important factors. Bond stress is the shear stress acting parallel to the reinforcement bar on the interface between the bar and the concrete as a result of the change of stress in the reinforcement bar. Bond is due to the combined effects of adhesion, friction and mechanical bearing. In deformed reinforcement bars, bearing stresses against the ribs contribute to most of the bond. With reference to Figure 7.1, if bond shear stresses are assumed to be uniformly distributed over the effective anchorage (bond) length, then from the equilibrium of forces at the ultimate load: F s = π φ e l f bu (7.1) where: F s = force in the reinforcing bar φ e = nominal bar diameter l = anchorage length f bu = design ultimate anchorage bond stress Figure 7.1. Anchorage of a Straight Bar
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CIVL2007 Theory and Design of Structures II RC Component (20 April 2007) page 111 Values for the design ultimate anchorage bond stress may be obtained from the following equation: cu bu f f β = (7.2) where: f cu = characteristic strength of concrete β = a coefficient dependent of the bar type (see Figures 7.1 and 7.2) barb2right can be obtained from Table 8.3 of the HK RC Code (2004). A partial safety factor of 1.4 for bond strength, obtained from Table 2.2 (HK RC Code 2004), has been included in Table 8.3. Table 7.1. Values of Bond Coefficient β (Table 8.3, HK RC Code 2004) Figure 7.2. Type 2 Deformed (Ribbed) Bar Figure 7.3. Type 1 Reinforcement Bar
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CIVL2007 Theory and Design of Structures II RC Component (20 April 2007) page 112 for full development of tensile strength: F s = 0.87 f y ¼ π φ e 2 cu e y cu e 2 e 4 1 y bu e s f 4 f 87 . 0 f f 87 . 0 f F l β φ = β φ π φ π = φ π = (7.3) Hence a bar must extend a distance l beyond any section at which it is required to develop its full tensile strength. Table 7.2 (Table 8.4, HK RC Code 2004) lists values of anchorage length for various reinforcement bars embedded in concrete of various strengths. Table 7.2. Ultimate Anchorage Bond Lengths (l b ) as Multiples of Bar Diameter (Table 8.4, HK RC Code 2004)
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CIVL2007 Theory and Design of Structures II RC Component (20 April 2007) page 113 Tension and Compression Lap Lengths (Clause 8.7.3) The tension lap lengths are listed in Table 7.3 (Table 8.5, HK RC Code 2004). The length of a tension lap should be at least equal to the design tension anchorage length.
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