# Two-Way Slab Design Limits per ACI 318-19 with ideCAD

How does ideCAD control two-way slab design limits according to ACI 318-19?

• The minimum slab thickness is controlled and reported automatically according to Table 8.3.1.1 for flat slabs.

• The minimum slab thickness is controlled and reported automatically according to Table 8.3.1.2 for beam-to-slabs.

• For nonprestressed slabs not satisfying 8.3.1, immediate and time-dependent deflection is calculated according to 24.2 and is not exceed the limits in 24.2.

### Symbols

h = overall thickness, height, or depth of member, in.
ln = length of clear span measured face-to-face of supports, in.
fy = specified yield strength for nonprestressed reinforcement, psi
αfm = average value of αf for all beams on edges of a panel

Minimum slab thickness

There are two different slab thickness limits according to flat slab and beam-to-slab.

Flat slab thickness limit

For nonprestressed flat slabs having a maximum ratio of long-to-short span of 2, overall slab thickness h is not less than the limits in Table 8.3.1.1 and is at least the value in a and b if the calculated deflection limits of 8.3.2 are not provided:

a) Slabs without drop panels = 5 in.

b) Slabs with drop panels = 4 in.

Note: Deflection is determined with a reduced modulus of rupture fr= 5√fc for fy exceeding 80 000 psi.

fy, psi

Without drop panels

With drop panels

40,000

ln/33

ln/36

60,000

ln/30

ln/33

80,000

ln/27

ln/30

Beam-to-slab thickness limit

For nonprestressed beam-to-slabs, overall slab thickness h provides the limits in Table 8.3.1.2 if the calculated deflection limits of 8.3.2 are not provided.

αfm

Minimum h, in

5.0

Calculated deflection limits

According to 8.3.2, immediate and time-dependent deflections are determined in accordance with 24.2. Deflections should not exceed the limits in 24.2.2 for nonprestressed slabs not provided 8.3.1, nonprestressed slabs without interior beams spanning between the supports on all sides and having a ratio of long-to-short span exceeding 2.0, and prestressed slabs.

Reinforcement strain limit in nonprestressed slabs

It’s automatically designed according to Strength Reduction Factors per ACI 318-19 with ideCAD.

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