Simplified Design for Beams and One-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 90

75"

Joist top bar

Slab bars

1 1 / 2 "

Stirrups

12

Bottom bars

9"

1 1 / 2 "

Figure 3-19 Joist Section

Effective flange width: (30 12)/10 = 36 in. (governs

6.25 12 = 75 in.

9 + 2(8 4.5) = 81 in.

A s = 0.70/3 = 0.23 in. 2 /ft

maximum bar spacing for No.4 bars,

s max = 3h = 3(4.5) = 13.5 in. < 18 in.

From Table 3-6: Use No. 4 @ 9 in. (A s = 0.27 in. 2 /ft)

(b) Bottom bars in end spans:

A s = 86.4/(4 18.0) = 1.2 in. 2

a = A s f y /0.85 › b e = (1.2

60)/(0.85 4 36) = 0.59 in. < 4.5 in. O.K.

From Table 3-5: Use 2-No. 7 (A s = 1.20 in. 2 )

Check ρ = A s /b w d = 1.2/(9 18.0) = 0.0074 > ρ min = 0.0033 O.K.

The 2-No.7 bars satisfy all requirements for minimum and maximum number of bars
in a single layer.

(c) Top bars at first interior beams:

A s = [118.8/(4 18.0) = 1.65 in. 2 ]/3 = 0.55 in. 2 /ft

From Table 3-6, assuming No.7 bars (s max = 10.8 in. from Table 3-6),

Use No. 7 @ 10 in. (A s = 0.72 in. 2 /ft)