Structural Concrete - Hassoun

256 Chapter 6 Deflection and Control of Cracking
Figure 6.16
Skin reinforcement.
include the self-weight of the beams. Use f c ′ = 4ksi, f y = 60 ksi, and a time limit of more than 5 years.
Refer to Fig. 6.13.
No.
b
(in.)
d
(in.)
h
(in.)
A s
(in. 2 )
A ′ s
(in. 2 )
W D
(K/ft)
W L
(K/ft)
a 15 20.5 24 8 no. 9 2 no. 9 3.5 2.0 — —
b 18 22.5 26 6 no. 10 — 2.0 1.5 7.4 5.0
c 12 19.5 23 8 no. 8 2 no. 8 2.4 1.6 — —
d 14 20.5 24 8 no. 9 2 no. 9 3.0 1.1 5.5 4.0
Note h–d = 2.5 in. indicates one row of bars, whereas h–d = 3.5 in. indicates two rows of bars. Concentrated
loads are placed as shown.
6.3 A 28-ft simply supported beam carries a uniform dead load of 2 K/ft (including self-weight) and a live
load of 1.33 K/ft. Design the critical section at midspan using the maximum steel ratio allowed by the
ACI Code and then calculate the instantaneous deflection. Use f c ′ = 4ksi,f y = 60 ksi, and b = 12 in. See
Fig. 6.14.
6.4 Design the beam in Problem 6.3 as doubly reinforced, considering that compression steel resists 20% of
the maximum bending moment. Then calculate the maximum instantaneous deflection.
6.5 The four cross sections shown in Fig. 6.15 belong to four different beams with f c ′ = 4ksiandf y = 60 ksi.
Check the spacing of the bars in each section according to the ACI Code requirement using f s = 0.6f y .
Then calculate the tolerable crack width, W.
6.6 Determine the necessary skin reinforcement for the beam section shown in Fig. 6.16. Then choose
adequate bars and spacings. Use f c ′ = 4ksiandf y = 60 ksi.
P D
(K)
P L
(K)