Structural Concrete - Hassoun

328 Chapter 9 One-Way Slabs
9.3 DESIGN LIMITATIONS ACCORDING TO ACI CODE
The following limitations are specified by the ACI Code:
1. A typical imaginary strip 1 ft (or 1 m) wide is assumed.
2. The minimum thickness of one-way slabs using grade 60 steel according to the ACI Code,
Sections 7.3.1.1, for solid slabs and for beams or ribbed one-way slabs should be equal to the
following:
• For simply supported spans: solid slabs, h = L∕20 (ribbed slabs, h = L∕16).
• For one-end continuous spans: solid slabs, h = L∕24 (ribbed slabs, h = L∕18.5).
• For both-end continuous spans: solid slabs, h = L∕28 (ribbed slabs, h = L∕21).
• For cantilever spans: solid slabs, h = L∕10 (ribbed slabs, h = L∕8).
• For f y other than 60 ksi, these values shall be multiplied by (0.4 + 0.01f y ),wheref y is in
ksi. This minimum thickness should be used unless computation of deflection indicates a
lesser thickness can be used without adverse effects.
3. Deflection is to be checked when the slab supports are attached to construction likely to be
damaged by large deflections. Deflection limits are set by the ACI Code, Table 24.2.2.
4. It is preferable to choose slab depth to the nearest 1 in. (or 10 mm).
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5. Shear should be checked, although it does not usually control.
6. Concrete cover in slabs shall not be less than 3 in. (20 mm) at surfaces not exposed to weather
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or ground. In this case, d = h − 3 in. −(half − bar diameter). Refer to Fig. 9.1d.
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7. In structural one way slabs of uniform thickness, the minimum amount of reinforcement in
the direction of the span shall not be less than that required for shrinkage and temperature
reinforcement (ACI Code, Sections 7.6.1 and 24.4.3).
8. The main reinforcement maximum spacing shall be the lesser of three times the slab thickness
and 18 in. (ACI Code, Section 7.7.2.3).
9. Straight-bar systems may be used in both tops and bottoms of continuous slabs. An alternative
bar system of straight and bent (trussed) bars placed alternately may also be used. Bars may
also be placed at the center of the slab using a lesser effective depth but accounting for the
highest moments.
10. In addition to main reinforcement, steel bars at right angles to the main must be provided. This
additional steel is called secondary, distribution, shrinkage, ortemperature reinforcement.
9.4 TEMPERATURE AND SHRINKAGE REINFORCEMENT
Concrete shrinks as the cement paste hardens, and a certain amount of shrinkage is usually anticipated.
If a slab is left to move freely on its supports, it can contract to accommodate the shrinkage.
However, slabs and other members are joined rigidly to other parts of the structure, causing a certain
degree of restraint at the ends. This results in tension stresses known as shrinkage stresses. A
decrease in temperature and shrinkage stresses is likely to cause hairline cracks. Reinforcement is
placed in the slab to counteract contraction and distribute the cracks uniformly. As the concrete
shrinks, the steel bars are subjected to compression.
Reinforcement for shrinkage and temperature stresses normal to the principal reinforcement
should be provided in a structural slab in which the principal reinforcement extends in one direction