1. Preliminary Slab Thickness Sizing

In lieu of detailed calculation for deflections, ACI 318 Code gives minimum thickness for two-way slab with beams spanning between supports on all sides in Table 8.3.1.2.

Beam-to-slab flexural stiffness (relative stiffness) ratio (α_f) is computed as follows:

The moment of inertia for the effective beam and slab sections can be calculated as follows:

Then,

For Edge Beams:

The effective beam and slab sections for the computation of stiffness ratio for edge beam is shown in Figure 2.

For North-South Edge Beams:

f = 1.47 using Figure 3.

For East-West Edge Beam:

f = 1.47 using Figure 3.

Figure 2 – Effective Beam and Slab Sections (Edge Beam)

Figure 3 – Beam Stiffness (Edge Beam)

For Interior Beams:

The effective beam and slab sections for the computation of stiffness ratio for interior beam is shown in Figure 4.

For North-South Interior Beams:

f = 1.61 using Figure 5.

For East-West Interior Beam:

f = 1.61 using Figure 5.

Figure 4 – Effective Beam and Slab Sections (Interior Beam)

Figure 5 – Beam Stiffness (Interior Beam)

Since α_f > 2.0 for all beams, the minimum slab thickness is given by:

Where:

Use 6 in. slab thickness.