Simplified Design for Beams and One-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 89

(3) Preliminary size of joists (beams) and slab thickness

From Table 3-1: depth of joist h = ˜ n /18.5 = (27.5 12)/18.5 = 17.8 in.

where ˜ n (end span) = 30 - 1.0 - 1.5 = 27.5 ft (governs)
˜ n (interior span) = 30 - 3 = 27.0 ft

Check for fire resistance: from Table 10-1, required slab thickness = 4.6 in. for 2-hour rating.

Try 16 in. pan forms + 4 1 / 2 -in. slab

h = 20.5 in. > 17.8 in. O.K.

(4) Determine width of joist rib

(a) For moment strength:

20M u
d 2

20(118.8)
18.0 2

b w = = = 7.33 in.

where d = 20.5 - 2.5 = 18.0 in. = 1.50 ft

Check for fire resistance: for joists designed as beams, specified cover per ACI 7.7 = 1.5 in.
From Table 10-4, the required cover for restrained beams for a fire resistance rating of
2 hr = 1 / 4 -in. < 1.5 in. O.K.

(b) For shear strength

V u @ distance d from support face = 25.3 - 1.6(1.50) = 22.9 kips

φ V c /2 = 0.048b w d*

b w = 22.9/(0.048 18.0) = 26.5 in.

Use 9 in.-wide joists (standard width) and provide stirrups where required. A typical
cross-section of the joist is shown in Fig. 3-19.

(5) Determine Flexural Reinforcement

(a) Top bars at spandrel beams:

M u 50.4

4(18.0) =

A s =

4d =

0.70 in. 2

Distribute bars uniformly in top slab according to ACI 10.6.6.

* For joists designed as beams, the 10% increase in φ V c is not permitted. Also, minimum shear reinforcement is required when

V u > φ V c /2.