Simplified Design for Beams and One-Way Slabs - Simplified Design of Reinforced Concrete Buildings - page 84

3.8.1 Example: Design of Standard Pan Joists for Alternate (1) Floor System
(Building #1)

(1) Data: › = 4000 psi (normal weight concrete, carbonate aggregate)

f y = 60,000 psi

Floors: LL = 60 psf
DL = 130 psf (assumed total for joists and beams + partitions + ceiling & misc.)
Required fire resistance rating = 1 hour

Floor system - Alternate (1): 30-in. wide standard pan joists

width of spandrel beams = 20 in.
width of interior beams = 36 in.

Note: The special provisions for standard joist construction in ACI 8.13 apply to the pan joist floor
system of Alternate (1).

(2) Determine the factored shears and moments using the approximate coefficients of ACI 8.3.3.

Factored shears and moments for the joists of Alternate (1) are determined in Chapter 2, Section 2.3.2
(Figure 2-8).
w u = [1.2(130) + 1.6(60) = 252 psf] 3 ft = 756 plf

Note: All shear and negative moment values are at face of supporting beams.

V u @ spandrel beams = 10.5 kips

V u @ first interior beams = 12.0 kips

-M u @ spandrel beams = 24.0 ft-kips

+M u @ end spans = 41.1 ft-kips

-M u @ first interior beams = 56.3 ft-kips

+M u @ interior spans = 34.6 ft-kips

-M u @ interior beams = 50.4 ft-kips

(3) Preliminary size of joist rib 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

From Table 10-1, required slab thickness = 3.2 in. for 1-hour fire resistance rating. Also, from
ACI 8.13.6.1:

Minimum slab thickness > 30/12 = 2.5 in. > 2.0 in.