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Structural Concrete - Hassoun

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

SUMMARY<br />

Sections 17.1–17.5<br />

1. A two-way slab is one that has a ratio of length to width less than 2. Two-way slabs may be<br />

classified as flat slabs, flat plates, waffle slabs, or slabs on beams.<br />

2. The ACI Code specifies two methods for the design of two-way slabs: the direct design<br />

method and the equivalent frame method. In the direct design method, the slab panel is divided<br />

(in each direction) into three strips, one in the middle (referred to as the middle strip) and one<br />

on each side (referred to as column strips).<br />

Section 17.6<br />

To control deflection, the minimum slab thickness, h, is limited to the values computed by<br />

Table 17.1 or Eqs. 17.1 and 17.2 and as explained in Examples 17.1 and 17.2.<br />

Section 17.7<br />

For two-way slabs without beams, the shear capacity of the concrete section in one-way shear is<br />

V c = 2λ √ f ′ cbd (17.33)<br />

The shear capacity of the concrete section in two-way shear is<br />

V c =<br />

(2 + 4 )<br />

λ √ f<br />

β<br />

cb ′ 0 d ≤ 4 √ f cb ′ 0 d (17.34)<br />

c<br />

When shear reinforcement is provided, V n ≤ 6 √ f ′ cb 0 d.<br />

Section 17.8<br />

In the direct design method, approximate coefficients are used to compute the moments in the<br />

column and middle strips of two-way slabs. The total factored moment is<br />

M 0 =(q u l 2 ) l2 1<br />

(Eq. 17.11)<br />

8<br />

The distribution of M 0 into negative and positive span moments is given in Fig. 17.14. A summary<br />

of the direct design method is given in Section 17.8.8. The modified stiffness method is explained<br />

in Section 17.8.7.<br />

Sections 17.9–17.11<br />

1. Unbalanced loads on adjacent panels cause a moment in columns that can be computed by<br />

Eq. 17.22<br />

2. Approximately 60% of the moment transferred to both ends of a column at a joint is transferred<br />

by flexure, M f , and 40% is transferred by eccentric shear, M v . The fraction of the<br />

unbalanced moment transferred by flexure, M f ,isγ f M u ,whereγ f is computed from Eq. 17.25.<br />

The shear stresses produced by M v must be combined with shear stresses produced by the<br />

shearing force V u .<br />

3. Waffle slabs are covered in Section 17.11.

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