Code of Massachusetts Regulations
780 CMR - STATE BOARD OF BUILDING REGULATIONS AND STANDARDS
Chapter 7 - Fire and Smoke Protection Features
Section 721 - CALCULATED FIRE RESISTANCE
Subsection 721.3 - Concrete masonry

Universal Citation: 780 CMR MA Code of Regs 721.3

Current through Register 1531, September 27, 2024

The provisions of this section contain procedures by which the fire-resistance ratings of concrete masonry are established by calculations.

721.3.1 Equivalent thickness. The equivalent thickness of concrete masonry construction shall be determined in accordance with the provisions of this section.

721.3.1.1 Concrete masonry unit plus finishes. The equivalent thickness of concrete masonry assemblies, Tea, shall be computed as the sum of the equivalent thickness of the concrete masonry unit, Te, as determined by Section 721.3.1.2, 721.3.1.3 or 721.3.1.4, plus the equivalent thickness of finishes, Tef, determined in accordance with Section 721.3.2:

Tea = Te + Tef (Equation 7-6)

721.3.1.2 Ungrouted or partially grouted construction. Te shall be the value obtained for the concrete masonry unit determined in accordance with ASTM C 140.

721.3.1.3 Solid grouted construction. The equivalent thickness, Te, of solid grouted concrete masonry units is the actual thickness of the unit.

721.3.1.4 Airspaces and cells filled with loose-fill material. The equivalent thickness of completely filled hollow concrete masonry is the actual thickness of the unit when loose-fill materials are: sand, pea gravel, crushed stone, or slag that meet ASTM C 33 requirements; pumice, scoria, expanded shale, expanded clay, expanded slate, expanded slag, expanded fly ash, or cinders that comply with ASTM C 331; or perlite or vermiculite meeting the requirements of ASTM C 549 and ASTM C 516, respectively.

721.3.2 Concrete masonry walls. The fire-resistance rating of walls and partitions constructed of concrete masonry units shall be determined from Table 721.3.2. The rating shall be based on the equivalent thickness of the masonry and type of aggregate used.

TABLE 721.3.2 MINIMUM EQUIVALENT THICKNESS (inches) OF BEARING OR NONBEARING CONCRETE MASONRY WALLSa,b,c,d

TYPE OF AGGREGATE FIRE-RESISTANCE RATING (hours)
1/2 3/4 1 11/4 11/2 13/4 2 21/4 21/2 23/4 3 31/4 31/2 33/4 4
Pumice or expanded slag 1.5 1.9 2.1 2.5 2.7 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.5 4.7
Expanded shale, clay or slate 1.8 2.2 2.6 2.9 3.3 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 4.9 5.1
Limestone, cinders or unexpanded slag 1.9 2.3 2.7 3.1 3.4 3.7 4.0 4.3 4.5 4.8 5.0 5.2 5.5 5.7 5.9
Calcareous or siliceous gravel 2.0 2.4 2.8 3.2 3.6 3.9 4.2 4.5 4.8 5.0 5.3 5.5 5.8 6.0 6.2
For SI: 1 inch = 25.4 mm.
a. Values between those shown in the table can be determined by direct interpolation.
b. Where combustible members are framed into the wall, the thickness of solid material between the end of each member and the opposite face of the wall, or between members set in from opposite sides, shall not be less than 93 percent of the thickness shown in the table.
c. Requirements of ASTM C 55, ASTM C 73, ASTM C 90 or ASTM C 744 shall apply.
d. Minimum required equivalent thickness corresponding to the hourly fire-resistance rating for units with a combination of aggregate shall be determined by linear interpolation based on the percent by volume of each aggregate used in manufacture.
721.3.2.1 Finish on nonfire-exposed side. Where plaster or gypsum wallboard is applied to the side of the wall not exposed to fire, the contribution of the finish to the total fire-resistance rating shall be determined as follows: The thickness of gypsum wallboard or plaster shall be corrected by multiplying the actual thickness of the finish by applicable factor determined from Table 721.2.1.4(1). This corrected thickness of finish shall be added to the equivalent thickness of masonry and the fire-resistance rating of the masonry and finish determined from Table 721.3.2.

721.3.2.2 Finish on fire-exposed side. Where plaster or gypsum wallboard is applied to the fire-exposed side of the wall, the contribution of the finish to the total fire-resistance rating shall be determined as follows: The time assigned to the finish as established by Table 721.2.1.4(2) shall be added to the fire-resistance rating determined in Section 721.3.2 for the masonry alone, or in Section 721.3.2.1 for the masonry and finish on the nonfire-exposed side.

721.3.2.3 Nonsymmetrical assemblies. For a wall having no finish on one side or having different types or thicknesses of finish on each side, the calculation procedures of this section shall be performed twice, assuming either side of the wall to be the fire-exposed side. The fire-resistance rating of the wall shall not exceed the lower of the two values calculated.

Exception: For exterior walls with a fire separation distance greater than 5 feet (1524 mm) the fire shall be assumed to occur on the interior side only.

721.3.2.4 Minimum concrete masonry fire-resistance rating. Where the finish applied to a concrete masonry wall contributes to its fire-resistance rating, the masonry alone shall provide not less than one-half the total required fire-resistance rating.

721.3.2.5 Attachment of finishes. Installation of finishes shall be as follows:
1. Gypsum wallboard and gypsum lath applied to concrete masonry or concrete walls shall be secured to wood or steel furring members spaced not more than 16 inches (406 mm) on center (o.c.).

2. Gypsum wallboard shall be installed with the long dimension parallel to the furring members and shall have all joints finished.

3. Other aspects of the installation of finishes shall comply with the applicable provisions of Chapters 7 and 25.

721.3.3 Multiwythe masonry walls. The fire-resistance rating of wall assemblies constructed of multiple wythes of masonry materials shall be permitted to be based on the fire-resistance rating period of each wythe and the continuous airspace between each wythe in accordance with the following formula:

RA = (R10.59 + R20.59 + ... + Rn0.59 + A1 + A2 + ... + An)1.7 (Equation 7-7)

where:

RA = Fire-resistance rating of the assembly (hours).

R1, R2, ..., Rn = Fire-resistance rating of wythes for 1, 2, n (hours), respectively.

A1, A2, ...., An = 0.30, factor for each continuous airspace for 1, 2, ...n, respectively, having a depth of 1/2 inch (12.7 mm) or more between wythes.

721.3.4 Concrete masonry lintels. Fire-resistance ratings for concrete masonry lintels shall be determined based upon the nominal thickness of the lintel and the minimum thickness of concrete masonry or concrete, or any combination thereof, covering the main reinforcing bars, as determined according to Table 721.3.4, or by approved alternate methods.

TABLE 721.3.4 MINIMUM COVER OF LONGITUDINAL REINFORCEMENT IN FIRE-RESISTANCE-RATED REINFORCED CONCRETE MASONRY LINTELS (inches)

NOMINAL WIDTH OF LINTEL (inches) FIRE-RESISTANCE RATING (hours)
1 2 3 4
6 11/2 2 - -
8 11/2 11/2 13/4 3
10 or greater 11/2 11/2 11/2 13/4

For SI: 1 inch = 25.4 mm.

721.3.5 Concrete masonry columns. The fire-resistance rating of concrete masonry columns shall be determined based upon the least plan dimension of the column in accordance with Table 721.3.5 or by approved alternate methods.

TABLE 721.3.5 MINIMUM DIMENSION OF CONCRETE MASONRY COLUMNS (inches)

FIRE-RESISTANCE RATING (hours)
1 2 3 4
8 inches 10 inches 12 inches 14 inches

For SI: 1 inch = 25.4 mm.

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