Building Design and Construction Handbook - Merritt - Ventech!

ONE-WAY REINFORCED-CONCRETE SLABS
CONCRETE CONSTRUCTION 9.75
A one-way reinforced-concrete slab is a flexural member that spans in one direction
between supports and is reinforced for flexure only in one direction (Art. 9.52). If
a slab is supported by beams or walls on four sides, but the span in the long
direction is more than twice that in the short direction, most of the load will be
carried in the short direction; hence, the slab can be designed as a one-way slab.
One-way slabs may be solid, ribbed, or hollow. (For one-way ribbed slabs, see
Arts. 9.54 to 9.58.) Hollow one-way slabs are usually precast (Art. 9.100). Castin-place,
hollow one-way slabs can be constructed with fiber or cardboard-cylinder
forms, inflatable forms that can be reused, or precast hollow boxes or blocks. Oneway
slabs can be haunched at the supports for flexure or for shear strength.
9.52 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/ft 2 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 w uL 2 /12,
where w u is the total factored uniform load and L is the span, defined in Art. 9.41,