Residential Details

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Each portion of a building separated by one or more fire walls that comply with the provisions of this section shall be considered a separate building. The extent and location of such fire walls shall provide a complete separation. Where a fire wall also separates groups that are required to be separated by a fire barrier wall, the most restrictive requirements of each separation shall apply. Fire walls located on property lines shall also comply with Section 503.2. Such fire walls (party walls) shall be constructed without openings.

705.2 — Structural Stability
Fire walls shall have sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall for the duration of time indicated by the required fire-resistance rating.

Fire walls shall be of any approved noncombustible materials. The fire resistance of concrete or masonry firewalls shall be calculated in accordance with Section 720 or determined by test in accordance with ASTM E-119. All other firewalls shall be tested in accordance with ASTM E-119. Firewalls tested in accordance with ASTM E-119 shall be conducted using the alternative hose stream test procedure described in Section 11.3 of ASTM E-119.

Calculated Fire Resistance of Concrete Masonry Units (CMU)

The provisions of this section contain procedures by which the fire resistance of specific materials or combinations of materials is established by calculations. These procedures apply only to the information contained in this section and shall not be otherwise used. The calculated fire resistance of concrete, concrete masonry, and clay masonry assemblies shall be permitted in accordance with ACI 216.1/TMS 0216

720.3Concrete Masonry
The provisions of this section contain procedures by which the fire-resistance ratings of concrete masonry are established by calculations.

720.3.1Equivalent thickness
The equivalent thickness of concrete masonry construction shall be determined in accordance with the provisions of this section.

720.3.1.1Concrete masonry unit plus finishes
The equivalent thickness of concrete masonry assemblies, T ea, shall be computed as the sum of the equivalent thickness of the concrete masonry unit, Te, as determined by Section 720.3.1.2, 720.3.1.3, or 720.3.1.4, plus the equivalent thickness of finishes, Tef, determined in accordance with Section 720.3.1:

Tea = Te + Tef
Te = Vn/LH = Equivalent thickness of concrete masonry unit (inch) (mm).

V = Net volume of masonry unit (inch³) (mm³).
L = Specified length of masonry unit (inch) (mm).
H = Specified height of masonry unit (inch) (mm)

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

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

720.3.1.4  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 C516, respectively.

Calculating The Equivalent Thickness
Note: For Procedures on Calculating the Equivalent Thickness see NCMA TEK 7-1A


If the hollow unit is 50% core voids Its equivalent solid thickness is 4″
The core area is 40-50% gross area.Equivalent thickness is the solid thickness that would be obtained if the same amount of concrete contained in a hollow unit were re-cast without core holes.

Approximate Volume in Standard Concrete Masonry Units

Size Cores Cu ft. to fill Cores % Solid Volume Equivalent Thickness Inches
4x8x16 3 .06 73.6 2.66
4x8x16 2 .06 73.6 2.66
6x8x16 3 .15 57 3.21
6x8x16 2 .16 54 3.06
8x8x16 3 .22 53.83 4.10
8x8x16 2 .25 51.16 3.95
10x8x16 2 .34 49.27 4.70
12x8x16 3 .34 49.4 6.19
12x8x16 2 .40 44.54 5.62

* These Units may vary with each manufacturer depending on the style of the block, mold configuration, and wear etc.
Table 720.3.2 — International Building Code Fire Resistance
Rated Construction Minimum Equivalent Thickness (inches) of Bearing or Nonbearing Concrete Masonry Walls Fire Resistance Rating (hours)

Fire Resistance Rating (hours)

Type of Aggregate 1/2 3/4 1 1 1/4 1 1/2 1 3/4 2 2 1/4 2 1/2 2 3/4 3 3 1/4 3 1/2 3 3/4 4
Pumice of 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
Calcarous of siliccous 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 S1: 1″ = 25.4mm
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 C55, C73 or C90 shall apply.
d. Minimum required equivalent thickness corresponding to the hourly fire-resistance rating for units
a combination of aggregate shall be determined by linear interpolation based on the percent by
volume of each aggregate used in manufacture.