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Structural Concrete - Hassoun

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258 Chapter 7 Development Length of Reinforcing Bars<br />

7.2 DEVELOPMENT OF BOND STRESSES<br />

7.2.1 Flexural Bond<br />

Consider a length dx of a beam subjected to uniform loading. Let the moment produced on one<br />

side be M 1 and on the other side be M 2 with M 1 being greater than M 2 . The moments will produce<br />

internal compression and tension forces, as shown in Fig. 7.1. Because M 1 is greater than M 2 , T 1<br />

is greater than T 2 ; consequently, C 1 is greater than C 2 .<br />

At any section, T = M/jd, wherejd is the moment arm:<br />

T 1 − T 2 = dT = dM<br />

jd<br />

but<br />

T 1 = T 2 + u ∑ Odx<br />

where u is the average bond stress and ∑ O is the sum of perimeters of bars in the section at the<br />

tension side. Therefore,<br />

T 1 − T 2 = u ∑ Odx = dM<br />

jd<br />

u = dM<br />

dx × 1<br />

jd ∑ O<br />

The rate of change of the moment with respect to x is the shear, or dM/dx = V. Therefore,<br />

V<br />

u =<br />

jd ∑ O<br />

The value u is the average bond stress; for practical calculations, j can be taken to be approximately<br />

equal to 0.87:<br />

V<br />

u =<br />

0.87d ∑ O<br />

In the strength design method, the nominal bond strength is reduced by the capacity reduction<br />

factor, φ = 0.75. Thus,<br />

V<br />

U u = u<br />

φ(0.87)d ∑ (7.2)<br />

O<br />

Based on the preceding analysis, the bond stress is developed along the surface of the reinforcing<br />

bar due to shear stresses and shear interlock.<br />

(7.1)<br />

Figure 7.1<br />

Flexural bond.

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