Simplified Design for Beams and One-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 86

36"

Joist top bars

Slab bars

12

Bottom bars

30" clear

30" clear 6"

Figure 3-17 Joist Section (Example 3.8.1)

Maximum bar spacing for an interior exposure and 3 / 4 -in. cover will be controlled by
provisions of ACI 7.6.5

s max = 3h = 3(3.5) = 10.5 in. < 18 in.

From Table 3-6: Use No.3 @ 10 in. (A s = 0.13 in. 2 /ft)

(b) Bottom bars in end spans:

A s = 41.1/(4 18.25) = 0.56 in. 2

Check rectangular section behavior:

a = A s f y /0.85 ›b e = (0.56 60)/(0.85 4 36) = 0.27 in. < 3.5 in. O.K.

c

From Table 3-5: Use 2-No.5 (A s = 0.62 in. 2 )

Check ρ = A s /b w d = 0.56/(6 18.25) = 0.005 > ρ min = 0.0033 O.K.

(c) Top bars at first interior beams:

A s = [56.3/ (4 18.25) = 0.77 in. 2 ]/3 = 0.26 in. 2 /ft

From Table 3-6, with s max = 10.5 in.:

Use No.5 @ 10 in. (A s = 0.37 in. 2 /ft)