Simplified Design for Beams and One-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 85

Try 16 in. pan forms * + 3 1 / 2 -in. slab

h = 19.5 in. > 17.8 in. O.K.
slab thickness = 3.5 in. > 3.2 in. O.K.

(4) Determine width of joist rib

(a) Code minimum (ACI 8.13.2):

b w > 16/3.5 = 4.6 in. > 4.0 in.

(b) For flexural strength:

20M u
d 2

20(57.5)
18.25 2

b w = = = 3.45 in.

where d = 19.5 - 1.25 = 18.25 in. = 1.52 ft

Check for fire resistance: from Table 10-4 for restrained members, the required cover for a fire
resistance rating of 1 hr = 3 / 4 -in. for joists.

(c) For shear strength

V u @ distance d from support face = 12.0 - 0.76(1.52) = 10.84 kips

φ V c = 1.1(0.095b w d) = 0.11b w d**

∴ b w

= 10.84/(0.11 x 18.25) = 5.40 in.

Use 6 in. wide joists (see Table 9-3 and Fig. 3-17).

(5) Determine Flexural Reinforcement

(a) Top bars at spandrel beams:

M u 24.0

4(18.25) =

A s =

4d =

0.33 in. 2

Distribute bars uniformly in top slab:

A s = 0.33/3 = 0.11 in. 2 /ft

* See Table 9-3 for standard form dimensions for one-way joists.

** For standard joist ribs conforming to ACI 8.13 a 10% greater shear strength fVc is allowed. Also, minimum shear
reinforcement is not required (see ACI 11.4.6).