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722.2.2.1 Reinforced and Prestressed Floors and Roofs

Building Code 2018 of Illinois > 7 Fire and Smoke Protection Features > 722 Calculated Fire Resistance > 722.2 Concrete Assemblies > 722.2.2 Concrete Floor and Roof Slabs > 722.2.2.1 Reinforced and Prestressed Floors and Roofs

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The minimum thicknesses of reinforced and prestressed concrete floor or roof slabs for fire-resistance ratings of 1 hour to 4 hours are shown in Table 722.2.2.1.

Exception: Minimum thickness shall not be required for floors and ramps within parking garages constructed in accordance with Sections 406.5 and 406.6.

TABLE 722.2.2.1
MINIMUM SLAB THICKNESS (inches)
CONCRETE TYPE	FIRE-RESISTANCE RATING (hours)
CONCRETE TYPE	1	1¹/₂	2	3	4
Siliceous	3.5	4.3	5	6.2	7
Carbonate	3.2	4	4.6	5.7	6.6
Sand-lightweight	2.7	3.3	3.8	4.6	5.4
Lightweight	2.5	3.1	3.6	4.4	5.1

For SI: 1 inch = 25.4 mm.

722.2.2.1.1 Hollow-Core Prestressed Slabs

For hollow-core prestressed concrete slabs in which the cores are of constant cross section throughout the length, the equivalent thickness shall be permitted to be obtained by dividing the net cross-sectional area of the slab including grout in the joints, by its width.

722.2.2.1.2 Slabs With Sloping Soffits

The thickness of slabs with sloping soffits (see Figure 722.2.2.1.2) shall be determined at a distance 2t or 6 inches (152 mm), whichever is less, from the point of minimum thickness, where t is the minimum thickness.

FIGURE 722.2.2.1.2
DETERMINATION OF SLAB THICKNESS FOR SLOPING SOFFITS

722.2.2.1.3 Slabs With Ribbed Soffits

The thickness of slabs with ribbed or undulating soffits (see Figure 722.2.2.1.3) shall be determined by one of the following expressions, whichever is applicable:

For s > 4t, the thickness to be used shall be t

For s ≤ 2t, the thickness to be used shall be t_e

For 4t > s > 2t, the thickness to be used shall be

(Equation 7-5)

where:

s = Spacing of ribs or undulations.

t = Minimum thickness.

t_e = Equivalent thickness of the slab calculated as the net area of the slab divided by the width, in which the maximum thickness used in the calculation shall not exceed 2t.

FIGURE 722.2.2.1.3
SLABS WITH RIBBED OR UNDULATING SOFFITS

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