3.2 Fire Resistance Ratings and Materials

Fire Resistance Ratings and Materials

Fire resistance ratings and material properties are critical in confining fires, preventing structural collapse, and limiting the spread of fire throughout a building. These ratings are determined through standardized testing, which measures how long a building component (such as a wall, column, beam, or floor assembly) can withstand fire exposure while maintaining its structural and containment functions.

Standard Fire Testing: ASTM E119 / NFPA 251

The primary test standard for evaluating the fire resistance of structural assemblies is ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials (previously mirrored by NFPA 251). This test subjects an assembly to a standardized fire exposure within a test furnace.

The Time-Temperature Curve

The furnace temperature is controlled to follow the standard time-temperature curve. This curve represents a severe, fully developed compartment fire:

• 5 minutes: 1,000°F (538°C)
• 30 minutes: 1,550°F (843°C)
• 1 hour: 1,700°F (927°C)
• 2 hours: 1,850°F (1,010°C)
• 4 hours: 2,000°F (1,093°C)

Performance Criteria for Success

To receive a fire resistance rating (e.g., a 2-hour rating), the assembly must meet specific criteria during the entire test duration:

1. Load-Bearing Integrity: The assembly must support its design load without collapsing.
2. Contain(ment (Flame Passage): The assembly must prevent the passage of flame or hot gases sufficient to ignite cotton waste placed on the unexposed side.
3. Temperature Rise: The temperature of the unexposed surface must not rise more than 250°F (139°C) on average, or 325°F (181°C) at any single point, above the initial ambient temperature.
4. Hose Stream Test: For walls and partitions, a duplicate assembly is exposed to fire for half the rated duration and then subjected to the impact, erosion, and cooling effects of a standardized water hose stream. This evaluates the assembly’s integrity and resistance to physical shock.

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Flame Spread Ratings and ASTM E84

While fire resistance ratings measure containment and structural stability, surface burning characteristics evaluate how quickly fire will spread along the surface of interior finish materials. This is tested using ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials, commonly known as the Steiner Tunnel Test.

The Steiner Tunnel Test Method

The test uses a horizontal chamber 25 feet long, 17.5 inches wide, and 12 inches deep. A natural gas burner at one end provides a controlled flame, while a draft draws the flame along the ceiling-mounted specimen.

• Calibration Standards: The test is calibrated using two baseline materials:
  • Inorganic Reinforced Cement Board: Flame Spread Index (FSI) of 0, Smoke Developed Index (SDI) of 0.
  • Select Grade Red Oak: Flame Spread Index (FSI) of 100, Smoke Developed Index (SDI) of 100.

Interior Finish Classifications

Based on the results of the Steiner Tunnel Test, building codes categorize interior finishes into three classes:

• Class A: FSI 0–25; SDI 0–450.
• Class B: FSI 26–75; SDI 0–450.
• Class C: FSI 76–200; SDI 0–450.

Note that the Smoke Developed Index (SDI) must always be 450 or less for any interior finish class to limit toxic and obscuring smoke generation.

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Structural Steel Protection

Structural steel is noncombustible, but it is highly vulnerable to heat. At elevated temperatures, steel loses its yield strength rapidly:

• At 1,100°F (593°C), steel retains only about 50% of its room-temperature structural strength.
• At 1,800°F (982°C), steel retains less than 10% of its strength.

To prevent structural failure, steel columns, beams, and trusses must be insulated. The thickness of the protection is calculated based on the member’s weight-to-perimeter ratio ($W/D$ or $A/P$, where $W$ or $A$ is the weight/cross-sectional area and $D$ or $P$ is the heated perimeter). Larger, heavier steel members heat up slower and require less insulation than thin, light steel members.

Methods of Protection

1. Spray-Applied Fire-Resistive Materials (SFRM): Cementitious wet-mix or mineral fiber dry-mix sprays applied directly to the steel.
2. Gypsum Board Enclosures: Layers of Type X gypsum board fastened around the steel member. Gypsum provides excellent protection because it contains chemically combined water (approximately 21% by weight). When exposed to heat, the water is released as steam in a process called calcination, which absorbs heat energy and keeps the steel cool.
3. Intumescent Coatings: Thin-film paints that expand (up to 15 to 30 times their original thickness) when exposed to heat, forming a thick, insulating char layer. These are used when structural steel is left exposed for architectural aesthetics.
4. Concrete Encase: Encasing the steel members in poured concrete. This provides robust physical and thermal protection but adds significant weight to the building.

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Fire-Retardant Treated Wood (FRTW)

Fire-Retardant Treated Wood (FRTW) is wood pressure-impregnated with fire-retardant chemicals. Unlike untreated wood, FRTW reacts chemically under heat to form a protective char layer and release noncombustible gases, slowing down the rate of combustion and flame propagation.

Testing and Requirements

To be classified as FRTW, the wood must undergo testing in accordance with ASTM E84 (or ASTM E2768, which extends the E84 test to 30 minutes). The material must meet the following criteria:

• Flame Spread Index (FSI): 25 or less.
• No Progressive Combustion: No evidence of progressive combustion when the test is continued for a total of 30 minutes.
• Flame Front: The flame front must not progress more than 10.5 feet beyond the centerline of the burners during the 30-minute test.

Practical Application and Limitations

FRTW is used where wood frame construction is permitted but enhanced fire performance is required. However, the chemicals used in FRTW can make the wood hygroscopic (moisture-absorbing), which can accelerate the corrosion of fasteners and cause structural degradation (strength loss) when exposed to high temperatures, such as in attic spaces under hot roofs. Codes require structural design value adjustments for FRTW to account for potential strength reductions over time.