Because of the effectiveness of reinforcement in limiting crack widths, the ACI 318 Building Code requires minimum areas of steel and limits reinforcement spacing, to control cracking.

Beams and One-Way Slabs. If, in a structural floor or roof slab, principal reinforcement extends in one direction only, shrinkage and temperature reinforcement should be provided normal to the principal reinforcement, to prevent excessive cracking. The additional reinforcement should provide at least the ratios of reinforcement area to gross concrete area of slab given in Table 9.13, but not less than 0.0014.

To control flexural cracking, tension reinforcement in beams and one-way slabs should be well distributed in zones of maximum concrete tension when the design yield strength of the steel Æ’y is greater than 40,000 psi. Spacing of principal reinforcement in slabs should not exceed 18 in or 3 times the slab thickness, except in concrete-joist construction.

Where slab flanges of beams are in tension, a part of the main reinforcement of the beam should be distributed over the effective flange width or a width equal to one-tenth the span, whichever is smaller. When the effective flange width exceeds one-tenth the span, some longitudinal reinforcement should be provided in the outer portions of the flange. Also, reinforcement for one-way joist construction should be uniformly distributed throughout the flange.

To control concrete cracking in beams and one-way slabs, the spacing, s, of flexural reinforcement adjacent to a concrete surface in tension should not be greater than

where the calculated service load stress, Æ’s, can be taken as 60% of specified yield strength and cc is the clear concrete cover. This change in ACI 318-99 replaces the z factor of ACI 318-95 and previous code editions and directly specifies the maximum bar spacing for crack control without reference to interior or exterior exposure.

For beams with Grade 60 reinforcement and tension bars with 2 in clear concrete cover, the maximum bar spacing s = 540/36 - 2.5(2) = 15 - 5 = 10 in.

Two-Way Slabs. Flexural cracking in two-way slabs is significantly different from that in one-way slabs. For control of flexural cracking in two-way slabs, such as solid flat plates and flat slabs with drop panels, the ACI 318 Building Code restricts the maximum spacing of tension bars to twice the overall thickness h of the slab but not more than 18 in. In waffle slabs or over cellular spaces, however, reinforcement should be the same as that for shrinkage and temperature in one-way slabs (see Table 9.13).

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