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 721.6 - Wood assemblies
Universal Citation: 780 CMR MA Code of Regs 721.6
Current through Register 1531, September 27, 2024
The provisions of this section contain procedures by which the fire-resistance ratings of wood assemblies are established by calculations.
721.6.1 General. This section contains procedures for calculating the fire-resistance ratings of walls, floor/ceiling and roof/ceiling assemblies based in part on the standard method of testing referenced in Section 703.2.
721.6.1.1
Maximum fire-resistance
rating. Fire resistance ratings calculated for assemblies using the
methods in Section 721.6 shall be limited to a maximum of 1 hour.
721.6.1.2
Dissimilar membranes.
Where dissimilar membranes are used on a wall assembly, the calculation shall
be made from the least fire-resistant (weaker) side.
721.6.2 Walls, floors and roofs. These procedures apply to both load-bearing and nonload-bearing assemblies.
TABLE 721.6.2(1) TIME ASSIGNED TO WALLBOARD MEMBRANESa, b, c, d
| DESCRIPTION OF FINISH | TIMEe(minutes) |
| 3/8-inch wood structural panel bonded with exterior glue | 5 |
| 15/32-inch wood structural panel bonded with exterior glue | 10 |
| 19/32-inch wood structural panel bonded with exterior glue | 15 |
| 3/8-inch gypsum wallboard | 10 |
| 1/2-inch gypsum wallboard | 15 |
| 5/8-inch gypsum wallboard | 30 |
| 1/2-inch Type X gypsum wallboard | 25 |
| 5/8-inch Type X gypsum wallboard | 40 |
| Double 3/8-inch gypsum wallboard | 25 |
| 1/2-inch + 3/8-inch gypsum wallboard | 35 |
| Double 1/2-inch gypsum wallboard | 40 |
| For SI: 1 inch = 25.4 mm. |
| a. These values apply only when membranes are installed on framing members which are spaced 16 inches o.c. |
| b. Gypsum wallboard installed over framing or furring shall be installed so that all edges are supported, except 5/8-inch Type X gypsum wallboard shall be permitted to be installed horizontally with the horizontal joints staggered 24 inches each side and unsupported but finished. |
| c. On wood frame floor/ceiling or roof/ceiling assemblies, gypsum board shall be installed with the long dimension perpendicular to framing members and shall have all joints finished. |
| d. The membrane on the unexposed side shall not be included in determining the fire resistance of the assembly. When dissimilar membranes are used on a wall assembly, the calculation shall be made from the least fire-resistant (weaker) side. |
| e. The time assigned is not a finished rating. |
TABLE 721.6.2(2) TIME ASSIGNED FOR CONTRIBUTION OF WOOD FRAME a, b, c
| DESCRIPTION | TIME ASSIGNED TO FRAME (minutes) |
| Wood studs 16 inches o.c. | 20 |
| Wood floor and roof joists 16 inches o.c. | 10 |
| For SI: 1 inch = 25.4 mm. |
| a. This table does not apply to studs or joists spaced more than 16 inches o.c. |
| b. All studs shall be nominal 2 * 4 and all joists shall have a nominal thickness of at least 2 inches. |
| c. Allowable spans for joists shall be determined in accordance with Sections 2308.8, 2308.10.2 and 2308.10.3. |
TABLE 721.6.2(3) MEMBRANEa ON EXTERIOR FACE OF WOOD STUD WALLS
| SHEATHING | PAPER | EXTERIOR FINISH |
| 5/8-inch T & G lumber | Lumber siding | |
| 5/16-inch exterior glue wood structural panel | Sheathing paper | Wood shingles and shakes |
| 1/2-inch gypsum wallboard | 1/4-inch wood structural panels-exterior type | |
| 5/8-inch gypsum wallboard | 1/4-inch hardboard | |
| 1/2-inch fiberboard | Metal siding | |
| Stucco on metal lath | ||
| Masonry veneer | ||
| Vinyl siding | ||
| None | - | 3/8-inch exterior-grade wood structural panels |
| For SI: 1 inch = 25.4 mm. |
| a. Any combination of sheathing, paper and exterior finish is permitted. |
TABLE 721.6.2(4) FLOORING OR ROOFING OVER WOOD FRAMINGa
| ASSEMBLY | STRUCTURAL MEMBERS | SUBFLOOR OR ROOF DECK | FINISHED FLOORING OR ROOFING |
| Floor | Wood | 15/32-inch wood structural panels or 11/16 inch T & G softwood | Hardwood or softwood flooring on building paper resilient
flooring, parquet floor felted-synthetic fiber floor coverings, carpeting, or
ceramic tile on
3/8-inch-thick panel-type
underlay Ceramic tile on 11/4-inch mortar bed |
| Roof | Wood | 15/32-inch wood structural panels or 11/16 inch T & G softwood | Finished roofing material with or without insulation |
| For SI: 1 inch = 25.4 mm. |
| a. This table applies only to wood joist construction. It is not applicable to wood truss construction. |
TABLE 721.6.2(5) TIME ASSIGNED FOR ADDITIONAL PROTECTION
| DESCRIPTION OF ADDITIONAL PROTECTION | FIRE RESISTANCE (minutes) |
| Add to the fire-resistance rating of wood stud walls if the spaces between the studs are completely filled with glass fiber mineral wool batts weighing not less than 2 pounds per cubic foot (0.6 pound per square foot of wall surface) or rockwool or slag material wool batts weighing not less than 3.3 pounds per cubic foot (1 pound per square foot of wall surface), or cellulose insulation having a nominal density not less than 2.6 pounds per cubic foot. | 15 |
| For SI: 1 pound per cubic foot = 16.02 kg/m3, 1 pound per square foot = 4.882 kg/m2. |
721.6.2.1
Fire-resistance rating of wood frame assemblies. The
fire-resistance rating of a wood frame assembly is equal to the sum of the time
assigned to the membrane on the fire-exposed side, the time assigned to the
framing members and the time assigned for additional contribution by other
protective measures such as insulation. The membrane on the unexposed side
shall not be included in determining the fire resistance of
the assembly.
721.6.2.2
Time
assigned to membranes. Table 721.6.2(1) indicates the time assigned to
membranes on the fire-exposed side.
721.6.2.3
Exterior walls. For an
exterior wall with a fire separation distance
greater than 5 feet (1524 mm), the wall is assigned a rating dependent on the
interior membrane and the framing as described in Tables 721.6.2(1) and
721.6.2(2). The membrane on the outside of the nonfire-exposed side of
exterior walls with a fire separation
distance greater than 5 feet (1524 mm) may consist of sheathing,
sheathing paper and siding as described in Table 721.6.2(3).
721.6.2.4
Floors and roofs. In
the case of a floor or roof, the standard test provides only for testing for
fire exposure from below. Except as noted in Section 703.3, Item 5, floor or
roof assemblies of wood framing shall have an upper membrane consisting of a
subfloor and finished floor conforming to Table 721.6.2(4) or any other
membrane that has a contribution to fire resistance of at
least 15 minutes in Table 721.6.2(1).
721.6.2.5
Additional protection.
Table 721.6.2(5) indicates the time increments to be added to the fire
resistance where glass fiber, rockwool, slag mineral wool or cellulose
insulation is incorporated in the assembly.
721.6.2.6
Fastening. Fastening
of wood frame assemblies and the fastening of membranes to the wood framing
members shall be done in accordance with Chapter 23.
721.6.3 Design of fire-resistant exposed wood members. The fire-resistance rating, in minutes, of timber beams and columns with a minimum nominal dimension of 6 inches (152 mm) is equal to:
| Beams: | 2.54Zb [4 -2(b/d)] for beams which may be exposed to fire on four sides. | (Equation 7-18) |
| 2.54Zb [4 -(b/d)] for beams which may be exposed to fire on three sides. | (Equation 7-19) | |
| Columns: | 2.54Zd [3 -(d/b)] for columns which may be exposed to fire on four sides | (Equation 7-20) |
| 2.54Zd [3 -(d/2b)] for columns which may be exposed to fire on three sides. | (Equation 7-21) |
where:
b = The breadth (width) of a beam or larger side of a column before exposure to fire (inches).
d = The depth of a beam or smaller side of a column before exposure to fire (inches).
Z = Load factor, based on Figure 721.6.3(1).
FIGURE 721.6.3(1) LOAD FIGURE
| Ke = The effective length factor as noted in Figure 721.6.3(2). |
| l = The unsupported length of columns (inches). |
FIGURE 721.6.3(2) EFFECTIVE LENGTH FACTORS
721.6.3.1
Equation
7-21. Equation 7-21 applies only where the unexposed face represents the
smaller side of the column. If a column is recessed into a wall, its full
dimension shall be used for the purpose of these calculations.
721.6.3.2
Allowable loads.
Allowable loads on beams and columns are determined using design values given
in AF & PA NDS.
721.6.3.3
Fastener protection. Where minimum 1-hour fire
resistance is required, connectors and fasteners shall be protected
from fire exposure by 11/2
inches (38 mm) of wood, or other approved covering or coating
for a 1-hour rating. Typical details for commonly used fasteners and connectors
are shown in AITC Technical Note 7.
721.6.3.4
Minimum size. Wood
members are limited to dimensions of 6 inches (152 mm) nominal or greater.
Glued-laminated timber beams utilize standard laminating combinations except
that a core lamination is removed. The tension zone is moved inward and the
equivalent of an extra nominal 2-inch-thick (51 mm) outer tension lamination is
added.
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