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Fire resistance is defined as the ability of a passive fire protection system or material to withstand a standard fire resistance test.
On the other hand, fire resistance rating (FRR) is the time in minutes or hours for which the construction material or assemblies have withstood a standard fire exposure, under specific test conditions.
This article discusses the FRR of different materials and their significance in the construction industry.
Features of Fire Resistance Ratings (FRR)
FRR is a measure of the fire resistance of a material or assembly. It is determined by performing standardized fire tests and calculations. There is also a fire rating level (FRL), which is defined as the test result obtained after conducting a fire resistance test.
Even though FRR and FRL are used interchangeably, FRR is the desired value of fire resistance required for the design that a fire design engineer sets. While the latter is the actual test value achieved after conducting the test.
FRR of building materials or systems are determined by ASTM E119 or in accordance with Section 703.3. The test sample undergoes rigorous testing under controlled conditions to infer the corresponding FRR. These exposure conditions may not match the real-world environments found in buildings.
The fire-resistance rating of a building element is dependent on the following factors:
- FRL or test results of the product.
- The type of construction material used for making the element.
- The location of the element and its proximity to other building elements during a fire incident. For example, it can be floors, roofs, columns, walls, or beams.
- Presence of non-combustible components near the building element.
How is FRR measured?
In order to establish fire resistance ratings of materials in hours, fire assemblies are installed in a temperature-controlled propane gas-fired furnace, and the temperature is increased over a prescribed period of time as per the criteria established in the American Society for Testing and Materials standard E119 “Test Methods for Fire Tests of Building Construction and Materials.” This time-temperature relationship is called the Standard Time-Temperature Curve.
Fire resistance ratings are calculated based on the time taken by a given material or structure to fail during a fire. It is measured from the time at which the failure of the element occurs. This does not necessarily mean that the element is destroyed at that stage.
The FRR of a material is measured on the basis of three criteria:
- Structural adequacy: This is the stability test where the material continues to bear loads for the duration of the test.
- Integrity: This stage measures the material’s ability to stay intact without cracks or fissures when they catch fire.
- Insulation: This stage measures the ability of the material to slow the spread of heat from one side to the other.
Suppose the design requirement of a material or system is an FRR of 90. In that case, it means the material must resist the fire in three ways- stability, integrity, and insulation, each for 90 minutes. After conducting the test, the FRL rating is represented as three numbers giving time in minutes for each criterion i.e., stability, integrity, and insulation, in the following format _/_/_. For example, an FRL of 90/90/90.
If the FRL value is not obtained, the criteria with a lower value are improved to get a higher fire resistance rating.
Fire resistance rating (FRR) is the time in minutes or hours for which the construction materials or assemblies have withstood a standard fire exposure.
Fire resistance ratings are calculated based on the time taken by a given material or structure to fail during a fire. The FRR of a material is measured on the basis of three criteria: structural adequacy, integrity, and insulation.
FRI 90 for a material implies the material when exposed to a standard fire test, it won't fail for 90 minutes in each of the three criteria-stability, integrity, and insulation, i.e., 90/90/90.