Masonry fire resistance
The calculated fire-resistance rating of a concrete masonry or clay masonry wall from its equivalent thickness, with finishes, plaster and a second wythe, by IBC 2021 Section 722 (722.3 concrete masonry, 722.4 clay brick and tile). Section 722 is the building code's copy of the calculation method of TMS 216 / ACI 216.1. US and metric units.
Edition. IBC 2021 Section 722 was read (pp. 7-65 to 7-75). TMS 216 / ACI 216.1 itself is not on hand; any difference in its current edition is to be confirmed. The matching of the aggregate types of Table 722.3.2 to the columns of the finish factor table, Table 722.2.1.4(1), was made from the column headings and is marked to confirm.
Inputs of the fire resistance calculation #
- Masonry: concrete masonry units, solid clay brick (at least 75 percent solid), or hollow clay brick or tile.
- Aggregate (concrete masonry): the four rows of Table 722.3.2. Units made with a blend of aggregates (note d) need the supplier's value; enter the row that is conservative.
- Actual thickness and percent solid: the equivalent thickness is the percent solid times the actual thickness (net volume by ASTM C140 or C67, from the supplier).
- Cells: hollow or partially grouted (counts as hollow, 722.3.1.2), solidly grouted, or every cell filled with an approved loose fill: these two count at the actual thickness.
- Required rating: from the IBC for the wall's use (Tables 601 and 602, Chapter 7).
- Finishes (concrete masonry): gypsum wallboard, Type X wallboard, Portland cement-sand plaster or gypsum sand plaster on each side. Away from the fire a finish adds its thickness times the factor of Table 722.2.1.4(1); on the fire side it adds the time of Table 722.2.1.4(2). Portland cement-sand plaster 5/8 in. or less applied directly counts in the equivalent thickness.
- Fire from: either side (the wall is rated from both and takes the lower), or side 1 only, which 722.3.2.3 allows for exterior walls more than 5 ft from the lot line. Side 1 is then the inside.
- Sanded gypsum plaster (clay masonry): 1/2, 5/8 or 3/4 in. on one or two sides, Eq. 7-9 with Table 722.4.1(4).
- Second wythe: concrete masonry of the same aggregate, or solid brick, by its equivalent thickness, and whether a continuous airspace of 1/2 to 3-1/2 in. separates the wythes.
- Combustible members framed into the wall: the solid material behind them must be at least 93 percent of the tabulated thickness.
Auto-design (editor toolbar) varies the unit thickness and the filling of the cells. Objective: the thinnest wall, then no fill. The aggregate, percent solid, finishes and the required rating are kept.
Checks performed #
| Check | Basis |
|---|---|
| Rating at least the required rating | IBC 2021 722.3.2 (Table 722.3.2) or 722.4.1 (Table 722.4.1(1)), interpolated |
| Finishes away from and on the fire side, both directions | 722.3.2.1 to 722.3.2.3, Tables 722.2.1.4(1) and (2) |
| Masonry alone at least half the required rating where finishes contribute | 722.3.2.4 |
| Plaster on clay masonry | 722.4.1.2, Eq. 7-9 |
| Two wythes | 722.3.3 Eq. 7-7, 722.4.1.3 Eq. 7-10, 722.4.2 Eq. 7-11 |
| Solid material behind framed combustible members | Table 722.3.2 note b, Table 722.4.1(1) note c |
The drawing is a plan section through the wall with its finishes or second wythe.
Limitations #
- Walls only: lintels and columns (Tables 722.3.4, 722.3.5, 722.4.1(5), (6)) and steel-framed brick veneer (Table 722.4.1(2)) are not covered.
- A wall thinner than the first row of its table rates as zero; above the last row it rates at the last row's hours (4 hours).
- With two wythes, finishes are not added.
- Joints, penetrations and the attachment of finishes (722.3.2.5) are not checked. A tested and listed assembly may replace this calculation.
Verification examples #
Verification status: 17 values compared in 12 examples (8 published, 4 hand calculations from the cited equations): 17 within tolerance, 0 recorded deviations. Code basis: not yet recorded for this module.
Published: rows of IBC 2021 Table 722.3.2 (all four aggregates) and Table 722.4.1(1) (all three clay rows), each a solidly grouted or solid wall of the tabulated thickness giving the tabulated rating. Hand calculations: interpolation, finishes from both sides, plaster on brick by Eq. 7-9, and two wythes by Eq. 7-7. No published worked example of the calculation is on hand.