STANDARDS AND TESTING
The effort to develop standards for fire resistance ratings brought together the National Bureau of Standards (now the National Institute of Standards and Technology), Underwriters Laboratories Inc. (UL), the National Fire Protection Association, and capital stock and mutual insurance interests. Tests were conducted during 1917 and 19IB, and the standard that exists today is substantially in the same form as it was when developed.
Materials are classified according to their fire resistance (a more accurate term is fire endurance) capabilities under procedures set forth in a standard designated in various ways by different organizations. A committee keeps the texts conformed. The standards are NFPA Standard 251, Standard Methods of l ire Tests of Building Construction and Materials Underwriters Laboratories Inc. ANSI (American National Standards Institute)/UL 263; and the American Society for Testing and Materials El 19 (the test generally is known as ASTM El 19).
The standard essentially requires the following; a standard reproducible test fire, a consistent method for conducting the tests and classifying the results, and specific instruction for selecting and preparing test specimens. The objective is uniformity in testing.
The reproducible standard fire used in the testing procedure follows a time-temperature curve based on specified times and temperatures. Samples of typical structural elements are exposed to the standard fire. A number of thermocouples are peened into the column. When the average temperature reaches 1 ,°F, the test is ended. The minimum size for test specimen columns is nine feet.
Many tests are conducted at UL’s laboratories. Fire protection systems or assemblies that pass the test are rated in units of hours—one-hour, two-hour, and so forth, depending on the time the test unit survived the test fire. Rated units are “listed” in UL’s Fire Resistance Directory, published annually with semiannual supplements. Based on such listings (not approvals), local authorities may permit use of specific materials and assemblies. The authority of any standard or code rests on its adoption by the appropriate legislative body.
SOME MISUNDERSTANDINGS ABOUT THE TESTS
A material or assembly listed as having met test standards does not always mean that the object will perform the same way in a structural fire as it did during the tests. The unit being tested may be seriously damaged or effectively destroyed even though it passed the test. Fire resistance ratings often are misinterpreted in that it is believed that “twohour fire resistance” means that the building will be relatively undamaged for two hours. This is not true; there is no relationship between the hourly rating and real time hours in a fire.
Since there are so many variables, the limit of confidence that one can have is more nearly the concept that a four-hour fire resistive building is more fire resistive than a two-hour fire resistive building, and a twohour fire resistive building is more fire resistive than a one-hour fire resistive building. It is not easy to relate the building ratings to an anticipation of what will occur during a fire either preplanned or actually in progress.
It should be noted that there is no point in a supplier’s achieving a rating for a “nonstandard” time such as two hours and 20 minutes, since the assembly will receive only a twohour rating. In short, the listed assembly is the minimum structure the manufacturer could develop to pass the test. Therefore, all components assembled as specified in the listing are important. Many believe that the standard is developed by engineers who typically include redundant precautions. Nothing could be further from the truth. By the nature of the process, the listed assembly is constructed only to meet (not to exceed) the criteria for passing the test. Anything more could give the supplier a disastrous market disadvantage-competitors could undersell the supplier.
Another point to consider is that building departments sometimes are prone to giving formal or informal approval to variations from the listing. Any departure from the listing means that most probably the assembly installed is not the equivalent of the listed assembly. Components cannot be considered apart front the tested and listed assembly. A very common fallacy is the concept that fire resistance can be achieved just by nailing up some ⅛-inch “firerated” gypsum board.
In rating a membrane fire protection system, UL tests and lists a floorand-ceiling assembly or a roof-andceiling assembly. The assembly—the ceiling, hangers, electrical fixtures, bar joists, left-in-place form work for the concrete floor ( usually corrugated steel), air ducts and diffusers, and the concrete floor—are tested as a unit as it should be built. It, therefore, is incorrect to speak of a “firerated ceiling”; the ceiling is only one part of the total assembly.
The assemblies are tested under the ASTM El 19 Standard Fire Test, whose original designers did not envision an assembly that provides a void—a truss loft (like a cockloft) in every floor. Fire can and has extended through this void. The designers of the test did not anticipate the problem of otherwise unprotected columns passing through the truss loft. However, the UL Building Materials List states. “All ratings are based on the assumption that the stability of members supporting the floor or roof is not impaired by the effect of fire on the supports
