Building Design and Construction

Analysis and Design of One-Way Slabs

Structural strength, fire resistance, crack control, and deflections of one-way slabs must be satisfactory under service loads.
Strength and Deflections. Approximate methods of frame analysis can be used with uniform loads and spans that conform to ACI 318 Building Code requirements (see Art. 9.41). Deflections can be computed as indicated in Art. 9.51, or in lieu of calculations the minimum slab thicknesses listed in Table 9.15 may be used. In Fig. 9.22 is a plot of ratios of moments of inertia of cracked to gross concrete section for one-way slabs. These curves can be used to simplify deflection calculations.
Strength depends on slab thickness and reinforcement and properties of materials used. Slab thickness required for strength can be computed by treating a 1-ft width of slab as a beam (Arts. 9.45 and 9.46).
Fire Resistance. One-way reinforced concrete slabs, if not protected by a fireresistant ceiling, must have a thickness that conforms to the fire-resistant rating required by the statutory building code. Table 9.17 gives minimum slab thickness for various fire-resistance ratings for normal-weight and structural-lightweightconcrete construction. Providing a minimum 3⁄4-in. concrete cover for reinforcement in restrained construction is adequate under the Uniform Building Code and Standard Building Code for fire-resistance ratings up to 4 hours.
Reinforcement. Requirements for minimum reinforcement for crack control are summarized in Art. 9.50. Table 9.18 lists minimum reinforcement when Grade 60 bars are used. Reinforcement required for flexural strength can be computed by treating a 1-ft width of slab as a beam (Arts. 9.44 to 9.46).
Rebar weights, lb / ft2 of slab area, can be estimated From Fig. 9.24a for oneway, continuous, interior spans of floor or roof slabs made of normal-weight concrete.
One-way reinforced concrete slabs with spans less than 10 ft long can be reinforced with a single layer of draped welded-wire fabric for both positive and negative factored moments. These factored moments can be taken equal to wuL^2 / 12, where wu is the total factored uniform load and L is the span, defined in Art. 9.41,

if the slab meets ACI 318 Building Code requirements for approximate frame analysis with uniform loads.
For development (bond) of reinforcement, see Art. 9.49.
Shear. Shear strength is usually not critical in one-way slabs carrying uniform loads, but the ACI 318 Building Code requires that it be investigated (see Art. 9.47).

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