Structural Concrete - Hassoun

264 Chapter 7 Development Length of Reinforcing Bars
Equations 7.10 and 7.11 can only be applied if one of the following two different sets of
conditions is satisfied:
Set 1: The following three conditions must simultaneously be satisfied:
1. The clear spacing of bars being developed or spliced should not be less than the diameter of
reinforcement being developed (S ≥ d b ).
2. The clear cover for reinforcement being developed should not be less than d b .
3. Minimum amount of stirrups or ties throughout l d should not be less than the minimum value
specified in ACI Code, Section 9.7.6.2, for beams or ACI Code, Section 9.7.6.4, for columns.
Set 2: The following two conditions must simultaneously be satisfied:
1. The clear spacing of reinforcement being developed or spliced should not be less than 2d b .
2. The clear cover should not be less than d b .
If all the conditions of set 1 and set 2 cannot be satisfied then Eqs. 7.12 and Eqs. 7.13 must
be used.
It is quite common to use f c ′ = 4ksi and f y = 60 ksi in the design and construction of reinforced
concrete buildings. If these values are substituted in the preceding equations, and assuming
normal-weight concrete (λ = 1.0), then
Equation 7.10 becomes l d = 38d b (≤ no. 6 bars) (7.10a)
Equation 7.11 becomes l d = 47.5d b (≥ no. 7 bars) (7.11a)
Equation 7.12 becomes l d = 57d b (≤ no. 6 bars) (7.12a)
Equation 7.13 becomes l d = 71.2d b (≥ no. 7 bars) (7.13a)
For design simplicity other values of l d /d b ratios are shown in Table 7.1. Table 7.2 gives the development
length, l d , for different reinforcing bars (when f y = 40 or 60 ksi and f ′ c = 3and4ksi).
7.4 DEVELOPMENT LENGTH IN COMPRESSION
The development length of deformed bars in compression is generally smaller than that required
for tension bars, due to the fact that compression bars do not have the cracks that develop in tension
concrete members that cause a reduction in the bond between bars and the surrounding concrete.
Table 7.1 Values of l d
/d b
for Various Values of f
c ′ and f y
(Tension Bars), (λ = 1.0)
f ′ c (ksi)
f y
= 40 ksi
f y
= 60 ksi
No. 6 Bars ≥ No. 7 Bars No. 6 Bars ≥ No.7Bars
Conditions
Met
Other
Cases
Conditions
Met
Other
Cases
Conditions
Met
Other
Cases
Conditions
Met
Other
Cases
3 29.3 43.9 36.6 54.8 43.9 65.8 54.8 82.2
4 25.3 38.0 31.7 47.5 38.0 57.0 47.5 71.2
5 22.7 34.0 28.3 42.5 34.0 51.0 42.5 63.7
6 20.7 31.0 25.9 38.8 31.0 46.5 38.8 58.1