All Things Gypsum: Redux: The Meaning of Fire Resistance Ratings
Fire protection engineers and other fire-testing experts understand that fire resistance ratings are derived from tests run in a laboratory setting. However, based on the number of inquiries we receive at the Association and what we have come to understand to be the perception of the public in general, we believe that a large segment of the population does not understand that fire ratings are actually based on tests conducted under highly controlled laboratory conditions; nor is it generally understood what “hourly” fire resistance ratings mean. The purpose of this article is to explain the meaning of the ratings for fire resistive systems, such as one, two, three, and four hours, that are usually incorporated into model and jurisdictional building codes where appropriate.
TYPE X WALLBOARDMost of the fire resistance rated walls or partitions constructed with gypsum board systems in the U. S. and Canada contain “type X” gypsum wallboard (or perhaps an improved variation with a proprietary trade name). Type X gypsum wallboard is manufactured in accordance with ASTM Standard C 1396. ASTM C 1396 contains a provision that type X gypsum wallboard must meet specific criteria in order to “pass” a test defined in another ASTM Standard, E 119, Standard Test Method for Fire Tests of Building Construction and Materials. It is this widely accepted test method that is used to determine the hourly fire resistance ratings for systems constructed of gypsum board (as well as other materials).
ASTM E 119 was initially published in 1918 (as ASTM C 19) and has been used continuously since then as the principal method to test the fire-resistance of a variety of building materials that are used to construct fire resistance rated systems. (Similar protocols were developed later by other standards groups and laboratories and are used and accepted in some cases.) A variety of refinements have been made to the E 119 standard since 1918; however, several of the test criteria, including the conditions required to “pass” an individual fire test, have remained essentially unchanged.
By the standard’s own definition, ASTM E 119 does not assess individual materials or products for their fire-resistive characteristics. The test method described in the ASTM E 119 standard evaluates “the ability of an assembly (system) to contain a fire or to retain its structural integrity, or both, during the test conditions imposed by the standard;” the endurance time achieved as a result of the performance of the system being tested is converted to an hourly rating. ASTM E 119 only produces laboratory test results to be used to evaluate the general fire resistance of systems such as walls, columns, slabs, and floor- and roof-ceiling systems that contain diverse components.
TEST FURNACE CONSIDERATIONSResults from an E 119 test are not precise predictors of future performance of the system or of the individual products that make up the system. For example, the ASTM E 119 standard does not contain specific details for construction of the test furnace; because test furnaces are subject to variations due to individual characteristics of construction and design, including ventilation, atmospheric conditions, and general thermal tendencies, test results are typically not necessarily repeatable or reproducible from one laboratory to another. Additionally, differences in system components and construction methods, the design and control features of individual furnaces, and other variables regarding the testing regimen, can cause wide fluctuations in ASTM E 119 test results.
It is widely accepted by fire testing experts that a fire test may be properly viewed as a snapshot of a single system at a given time that includes the measurement of the performance of a specific system, composed of specific materials, constructed in a specific test furnace, on a specific day. This simply means that for a gypsum board system to receive a “one-hour fire resistance rating,” all requirements of an ASTM E 119 test were successfully met in a testing laboratory furnace for at least 59 minutes and 30 seconds (test results are rounded off to the nearest minute) for that specific system and with those specific components of the system.
The hourly fire resistance ratings found in commercial and residential building codes that refer to an ASTM E 119 test are not meant to imply that a specific system will remain intact for the prescribed time of the hourly rating in an actual fire situation. Even if type X wallboard is referred to as “one-hour board” or “has a one-hour fire rating,” this does not mean that either a particular system incorporating type X wallboard or any given piece of type X wallboard will necessarily last for an hour in a different laboratory’s fire test or provide “one hour fire protection” in an actual fire.
HOURLY RATING CONTROVERSYNumerical fire resistance ratings created by ASTM E 119 tests are to be considered as points of reference for comparison purposes; the higher the numerical rating (i.e., one, two, three, or four hour(s)), the greater is the system’s comparative endurance. The ratings permit the occurrence of a simple comparison between systems composed of different materials. For example, if a two-hour system is required in a building that is being erected, any system that has been tested to the E119 test and that meets applicable code provisions can be installed. Codes are generally “material blind” and don’t specify specific materials or systems that must be installed for fire-resistive purposes; any system meeting the stated code provisions can be incorporated into the project.
In truth, serious discussion has occurred in testing circles regarding eliminating the use of hourly rating nomenclature to define system ratings. In such a case, systems would be required to have a rating that might be defined by a letter or numerical designation that would have no relationship to time. There is a powerful precedent for doing so in finish ratings conducted in accordance with an ASTM E 84 test. Finish ratings are currently defined on a Class ABC scale with Class A materials displaying the lowest flame spread and smoke developed characteristics.
In further efforts to maintain industry-wide quality assurance standards for type X gypsum wallboard, the Gypsum Association requires that all member companies listed in the generic systems contained in the Gypsum Association’s Fire Resistance Design Manual subscribe to an ongoing, third-party, in-plant product inspection and labeling service. This objective certification and labeling process ensures that manufacturers continue to manufacture the same quality of product as that originally tested. (More information on Underwriters Laboratories testing, certification, follow-up, and labeling procedures is available on its website at http://www.ul.com.)
It is widely recognized and accepted that the ASTM E 119 test method does not incorporate all dynamics essential for fire hazard analysis or fire risk assessment of the systems under conditions in an actual fire situation. The results of an ASTM E 119 test, therefore, should be regarded as one component among a variety of factors used to assess the potential of a system to perform as part of a structure. Fire resistance ratings created through use of the ASTM E 119 test method reflect a relative measure of comparative system’s performance under specific fire test conditions in a laboratory. ASTM E 119 test results should not be construed as having determined performance of a system under different conditions.