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Structural Concrete - Hassoun

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622 Chapter 17 Design of Two-Way Slabs<br />

Other ACI Code limitations are summarized as follows:<br />

1. For panels with discontinuous edges, end beams with a minimum α equal to 0.8 must be<br />

used; otherwise, the minimum slab thickness calculated by Eqs. 17.1 and Eqs. 17.2 must be<br />

increased by at least 10% (ACI Code, Section 8.3.1.2.1).<br />

2. The drop panels should extend in each direction from the centerline of support a distance not<br />

less than one-sixth of the span length in that direction between center to center of supports<br />

and also project below the slab at least h/4.<br />

3. Regardless of the values obtained by Eqs. 17.1 and 17.2, the thickness of two-way slabs<br />

shall not be less than the following (a) for slabs without interior beams or drop panels,<br />

5 in. (125 mm) (ACI Code, Section 8.3.1.1); (b) for slabs without interior beams but with<br />

drop panels, 4 in. (100 mm) (ACI Code, Section 8.3.1.1); and (c) for slabs with beams on<br />

all four sides with α fm > 2.0, 3 1 2 in. (90 mm), and for α fm ≤ 2.0, 5 in. (125 mm) (ACI Code,<br />

Section 8.3.1.2).<br />

Example 17.1<br />

A flat-plate floor system with panels 4 × 20 ft is supported on 20-in. square columns. Using the ACI Code<br />

equations, determine the minimum slab thickness required for the interior and corner panels shown in<br />

Fig. 17.6. Edge beams are not used. Use f c ′ = 4ksiandf y ′ = 60 ksi.<br />

Figure 17.6 Example 17.1.

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