Structural Concrete - Hassoun

5.7 Design of Vertical Stirrups 203
Figure 5.9
Effective length and spacing of bent bars.
The stirrups that are commonly used in concrete sections are made of two-leg no. 3 or no. 4
U-stirrups with f yt = 60 ksi. If no. 3 stirrups are used, then Eq. 5.16 becomes
S
d = A v f y
= 0.22(60) = 13.2
(5.24)
V s V s V s
If no. 4 stirrups are used, then
S
d = A v f y
= 0.4(60) = 24
(5.25)
V s V s V s
The ratio of stirrups spacing relative to the effective depth of the beam, d, depends on V s .The
values of S/d for different values of V s when f y = 60 ksi are given in Tables 5.2 and 5.3 for no. 3 and
no. 4 U-stirrups, respectively. The same values are plotted in Figs. 5.10 and 5.11. The following
observations can be made:
1. If no. 3 stirrups are used, S = d∕2 when V s ≤ 26.4. When V s increases, S/d decreases in a
nonlinear curve to reach 0.132 at V s = 100 K. If the minimum spacing is limited to 3 in., then
Table 5.2
S/d Ratio for Different Values of V s
(f yt
= 60 ksi, S/d = 13.2/V s
), No. 3 Stirrups
V s (K) 26.4 30 40 50 52.8 60 70 80 90 100 125
S/d 0.5 0.44 0.33 0.264 0.25 0.22 0.19 0.165 0.15 0.132 0.106
Table 5.3
S/d Ratio for Different Values of V s
(f yt
= 60 ksi, S/d = 24/V s
), No. 4 Stirrups
V s (K) 48 50 60 70 80 90 96 100 110 120 150 175
S/d 0.50 0.48 0.40 0.34 0.3 0.27 0.25 0.24 0.22 0.20 0.16 0.137