Lecture_16 - 3.3 Bond Laps and Bearing Stresses in Bends...

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130 3.3 Bond, Laps, and Bearing Stresses in Bends 3.3.1 Anchorage bond The reinforcing bar subject to direct tension (Fig. 3.3-1) must be firmly anchored when it is not to be pulled out of the concrete. Fig. 3.3-1 Anchorage bond Anchorage is the embedment of a bar in concrete so that it can carry load through bond between the steel and the concrete. o Bond is the grip due to adhesion or mechanical interlock and bearing in deformed bars between the steel and the concrete. o The anchorage depends on the bond between the bar and the concrete, and the area of contact. The force in the bar should be developed by an appropriate embedment length in the concrete (Clause 3.12.8.1). If the anchorage bond length is sufficient, the full strength of the bar can be developed by bond. In Fig. 3.3-1, o Tensile pull-out force ( ) 2 / 4 s e s F f πφ = (3.20) where f s – the direct tensile (or compressive) stress in the bar F s
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131 φ e – the effective bar size which, for a single bar is equal to bar size and, for a group of bars is equal to the nominal diameter (the diameter of a bar of equal total area). o Anchorage force = ( ) e b l f πφ (3.21) where l – the anchorage bond length to prevent pull out f b – the design anchorage bond stress, which is assumed to be constant over the anchorage length l , given by ( ) / b s e f F l πφ = (3.22) o By equating Eq. (3.20) and Eq. (3.21), ( ) 2 / 4 ( ) e s e b f l f πφ πφ = Thus the anchorage bond length (often simply called the anchorage length ), which is the length of the reinforcing bar required to develop the stress f s , is given by 4 s e b f l f φ = (3.23) The ultimate anchorage bond length (often refereed to as the ultimate anchorage length or the full anchorage length) is the length of the reinforcing bar required to develop the full design strength. Hence, writing f s = 0.95 f y in Eq. (3.23) gives 0.95 4 y e bu f l f φ = (3.24)
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