722.2.2.1 Reinforced and Prestressed Floors and Roofs
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.
TABLE 722.2.2.1
MINIMUM SLAB THICKNESS (inches)
| CONCRETE TYPE | FIRE-RESISTANCE RATING (hours) | ||||
| 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. |
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.
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.
| For SI: 1 inch = 25.4 mm. |
FIGURE 722.2.2.1.2
DETERMINATION OF SLAB THICKNESS
FOR SLOPING 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:
 | (Equation 7-5) |
| s | = | Spacing of ribs or undulations. |
| t | = | Minimum thickness. |
| te | = | 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. |
| For SI: 1 inch = 25.4 mm. |
FIGURE 722.2.2.1.3
SLABS WITH RIBBED OR UNDULATING SOFFITS