Concrete fire resistance
The prescriptive fire-resistance requirements of a concrete wall, floor or roof slab, beam or column: the minimum thickness, size and cover for a required rating of 1 to 4 hours, by the aggregate of the concrete. The method is ACI 216.1 / TMS 216.1 as the IBC adopts it in Section 722.2; the masonry counterpart is the masonry fire resistance type. US and metric units.
Edition. IBC 2021 Section 722.2 (pp. 7-65 to 7-72) is on hand and is the basis: it is the code's own copy of the ACI 216.1 tables. ACI 216.1 / TMS 216.1 itself is not on hand; differences in its current edition are to be confirmed against the code text.
Inputs of the concrete fire resistance #
- Element: wall, floor or roof slab, beam or column.
- Aggregate: siliceous, carbonate, sand-lightweight or lightweight concrete. Siliceous aggregate needs the most thickness and cover.
- Required rating: 1, 1-1/2, 2, 3 or 4 hours, from IBC Tables 601 and 602 and Chapter 7 for the element.
- Restrained (slabs and beams): off by default, the conservative case. Whether an assembly is restrained against thermal expansion is your judgment by ASTM E119 Appendix X3.
- Prestressed (slabs and beams): selects Table 722.2.3(2) or 722.2.3(4).
- Equivalent thickness (walls and slabs): the actual thickness of a solid member with flat faces. Hollow-core, ribbed, undulating or tapered members: enter the equivalent thickness by 722.2.1.1.1 to 722.2.1.1.4 or 722.2.2.1.1 to 722.2.2.1.3.
- Interpolate between ratings: off by default, so the rating is the highest tabulated one met. IBC 722.2 does not state interpolation for concrete; turn it on if ACI 216.1 permits it for your case.
- Cover: to the positive moment reinforcement (slabs, beams) or the longitudinal bars (columns). For several bars in a beam, enter the average cover with the corner bars at half (Table 722.2.3(3) note c, 722.2.3.3.1).
- Width and depth: the beam width (the depth is drawn only) or the column's two dimensions; 36 in. or more on two parallel sides permits notes a and b of Table 722.2.4.
- Restrained beams spaced 4 ft or less: note a of Tables 722.2.3(3) and (4), 3/4 in. cover for 4 hours or less.
- f′c (columns): above 12,000 psi the least dimension is 24 in.
- Wall finishes: gypsum wallboard or plaster on either side, the fire side (both, or side 1 only for an exterior wall more than 5 ft from the lot line), and whether the wall is load-bearing.
Auto-design varies the wall or slab thickness, the cover and the beam width or column size. Objective: the thinnest member, then the least cover. The aggregate, restraint, prestressing, finishes and the required rating are kept.
Checks performed #
| Check | Basis |
|---|---|
| Wall rating | Table 722.2.1.1; finishes by 722.2.1.4.1 (Table 722.2.1.4(1) factor × thickness away from the fire) and 722.2.1.4.2 (Table 722.2.1.4(2) time on the fire side), both sides (722.2.1.4.3) |
| Concrete alone at least half the rating; finish away from the fire at most half the concrete's contribution (load-bearing walls) | 722.2.1.4.4 |
| Slab rating | Table 722.2.2.1 |
| Slab cover | Table 722.2.3(1) (reinforced) or 722.2.3(2) (prestressed) |
| Beam cover | Table 722.2.3(3) (reinforced, interpolated by width, note b) or 722.2.3(4) (prestressed, 8 to 12 in. interpolated); note a for restrained beams spaced 4 ft or less |
| Column least dimension | Table 722.2.4 with notes a and b; 24 in. above f′c = 12,000 psi (722.2.4.1.2) |
| Column cover | min(1 in. per hour, 2 in.) (722.2.4.2) |
A table entry printed as a dash (an unrestrained 5 in. beam at 3 or 4 hours, an 8 in. unrestrained prestressed beam at 4 hours) is reported as not available for that rating. The drawing is a section through the element with its thickness or size and its cover; the bars are schematic.
Limitations #
- Multiwythe and foam-plastic sandwich walls (Figure 722.2.1.2, Eq. 7-4), joints between precast panels (722.2.1.3), multicourse floors and roofs (Figures 722.2.2.2, 722.2.2.3), Table 722.2.3(5) (prestressed beams by area), columns built into walls (722.2.4.4) and the ties of high-strength columns (722.2.4.3) are not covered.
- The analytical methods of ACI 216.1 (heat transfer, moment redistribution in continuous members) are not covered: the prescriptive tables only.
- A tested and listed assembly (IBC 703.2) may be used instead of this calculation.
Verification examples #
Verification status: 15 values compared in 11 examples (8 published, 3 hand calculations from the cited equations): 15 within tolerance, 0 recorded deviations. Code basis: not yet recorded for this module.
Published values are rows of IBC 2021 Tables 722.2.1.1, 722.2.2.1, 722.2.3(1) to (4) and 722.2.4: a member at a tabulated thickness or size gives the tabulated rating or requirement. The interpolation by width (notes b), the wall finishes and the 722.2.1.4.4 limits are hand calculations from the same tables; no published worked example is on hand.