Simplified Design for Beams and One-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 87

(d) Bottom bars in interior spans:

A s = 34.6/(4 18.25) = 0.47 in. 2

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

(e) Top bars at interior beams:

A s = [50.4/(4 18.25) = 0.69 in. 2 ]/3 = 0.23 in. 2 /ft

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

Use No.4 @ 10 in. (A s = 0.24 in. 2 /ft)

(f) Slab reinforcement normal to ribs (ACI 8.13.6.2):

Slab reinforcement is often located at mid-depth of slab to resist both positive and negative moments.

Use M u = w u ˜ n 2 /12 (see Fig. 2-5)

= 0.19(2.5) 2 /12 = 0.10 ft-kips

where w u = 1.2(44 + 30) + 1.6(60) = 185 psf = 0.19 kips/ft
˜ n = 30 in. = 2.5 ft (ignore rib taper)

With bars on slab centerline, d = 3.5/2 = 1.75 in.

A s = 0.10/4(1.75) = 0.015 in. 2 /ft (but not less than required temperature and shrinkage
reinforcement)

From Table 3-4, for a 3 1 / 4- in. slab: Use No.3 @ 16 in.

Note: For slab reinforcement normal to ribs, space bars per ACI 7.12.2.2 at 5h or 18 in.
Check for fire resistance: From Table 10-3, required cover for fire resistance rating of
1 hour = 3 / 4 -in. O.K.

Sym about L

9'-9" 9'-9"

7'-11"
#3 @ 10"

#3 @ 16" (typ)

#4 @ 10"

3.5"

1-#5

1-#5

16"

6" 1-#5 1-#5

0"

3'-5" 3'-5"

˜ n = 27.5'
30'-0"

3'-0" ˜ n = 27.0'

1'- 8 "
8"

30'-0"

End Span

Interior Span (Typ)

Figure 3-18 Reinforcement Details for 30 in. Standard Joist Floor System [Building #1—Alternate (1)]