Floors and walls designed as fire separations have been recognized for many years as efficient tools for restricting fires to the area of origin or limiting fire spread. Prior to 1900, relative fire safety was achieved by mandating use of specific materials. However the development of new materials and innovative designs created the need for performance based standards. (It is noteworthy that emphasis on performance based standards has been much in vogue in recent years as well).
Test furnaces in the United States were constructed shortly after 1900 at the Underwriters Laboratories Inc., Columbia University, and the National Bureau of Standards (NBS) which led to the development of ASTM E 119, Standard Test Methods for Fire Tests of Building Construction and Materials, and its counterpart, NFPA 251.
First published in 1918, there have been a number of refinements since that time but the temperature-time curve and the acceptance criteria have remained essentially unchanged.
Fire resistance continues to be based largely on the fire load concept developed by NBS in the 1920s – that the duration of a fire is proportional to the fire loading (the mass of combustible materials per unit floor area). This relationship was established on the basis of burnout tests in structures incorporating materials having heat values equivalent to wood and paper (7000 Btu/lb to 8000 Btu/lb). Fire loads of other materials like flammable liquids were based on the equivalent calorific content of those materials to wood. Simply, this means that 10 lb of wood or paper like materials per sq. ft. of floor area (or 80,000 Btu/sq. ft.) produce a fire of 1-hour duration.
The practical advantage is the ability to quantify materials in a given space and determine the fire duration time. When using barriers to confine fire to a given space, the time the barriers need to withstand the temperatures developed during that time can be determined relatively simply. While this is a bit of over simplification, it provides an ease of application for the users. In actuality fire severity and the temperature-time relationship of a fire depends on several factors, such as the distribution and surface characteristics of the fire load, the size and shape of the fire compartment, relative humidity of the atmosphere, etc.
So fire severity can be simply expressed in terms of temperature and fire duration. It is expressed by equivalence to that defined by the area under the Standard Temperature-Time Curve (The Curve). This standard curve represents a severe building fire. The curve was adopted in 1918 as the result of agreement between a number of organizations interested in fire protection and fire science. It is, and after all these years remains:
In most cases, one does not select the hourly rating for a wall or structural element. These are prescribed by a wide variety of building codes based on occupancy classification, size, number of floors, occupant load, etc. The ratings are also prescribed in fire codes such as the Life Safety Code, the Flammable and Combustible Liquids Code, and many others. One chooses from among the many assemblies that have been pre tested by a recognized authority and assigned a rating in advance.
It is not common to have only a wall to deal with, although there are situations where a blank wall is all that is involved. Most must accommodate openings for the passage of people, equipment, stock, pipes, ducts, and conduit and cable trays. Accommodating such elements without degrading the barrier fire rating has spawned a number of standards establishing the hourly fire rating tests for fire doors, duct dampers and though penetrations. There is a listing at the conclusion of this article that will provide the formal designations for a good variety of such standards should the reader wish to gather more details. There is more involved in the standards than just the fire rating but here we focus on that alone.
The testing is highly standardized. It has to be in order that results can be verifiable and duplicated, which is the scientific way. As it turns out, for a wall, or for the various other elements mentioned previously, the exposure fire for test purposes is always based on duplicating the temperature shown in The Curve with the passage of time.
The conditions of acceptance for each of the elements are given in the standard for conducting the test for the specific element. The conditions are also somewhat summarized in NFPA 5000-2003 “Building Construction and Safety Code.”
Basically the conditions for acceptance of through-penetration fire stops are the same as for walls (paraphrased):
(a) Shall have sustained the applied load during the fire endurance test, without passage of flame or gases hot enough to ignite cotton waste, for a period equal to that required for the hourly rating desired (expressed as the “F” Rating).
(b) Transmission of heat through the element during the fire endurance test shall not be sufficient to raise the temperature on the unexposed surface more than 250°F (140°C) above the assembly’s initial temperature (expressed as the “T” Rating).
The conditions for acceptance of both doors and dampers are the same (paraphrased):
(a) No flaming shall occur on the unexposed surface of the assembly during the first 30 minutes of the fire test. Exception: Intermittent flames not greater than 6 in. shall be permitted to occur for periods not to exceed 10 seconds.
(b) After 30 minutes of the fire test, some intermittent flames not greater than 6 in. shall be permitted to occur along the edges of doors for periods not to exceed 5 minutes.
(c) For a fire test duration of 45 minutes or greater, flames not greater than 6 in. shall be permitted to occur on the unexposed surface area during the last 15 minutes of the fire test, provided that the flames are contained within a distance of 11/2 in. from a vertical edge and within 3 in. from the top edge.
Looking at the acceptance criteria you will notice that walls and through-penetrations are accepted based on limited passage of heat as well as the temperature on the unexposed side. Doors and dampers have no such condition of acceptance and are accepted based on the lack of (or extent of) flaming on the unexposed side. Physical integrity issues such as warping and bending seem to be the governing factors. In addition, dampers and doors are assigned ratings of 20 minutes, 30 minutes, 3/4 hour, 1 hour, 1-1/2 hours, 3 hours, 4 hours, etc. In a 3 hour wall, a 3 hour rated door or damper is required. In a 2 hour wall, a 1-1/2 hour rated door or damper is required. It is possible these differences are based on the supposition that there will likely not be any combustible material near the unexposed side of a damper or door. This might be a good topic for future consideration.
In our professional lives, as well as our private lives, we are constantly relying on things that re certified, rated, listed, guaranteed and approved. It is comforting and reassuring. But we tend to become immune to what the designations really mean. Hopefully you will now have a better understanding of what the fire rating of a wall or barrier means.
ASTM E 119, Standard Test Methods for Fire Tests of Building Construction and Materials, 1997.
ASTM E 814, Standard Test Method for Fire Tests of Through-Penetration Fire Stops, 2000.
NFPA 80, Standard for Fire Doors and Fire Windows, 1999 edition.
NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, 2002 edition.
NFPA 251, Standard Methods of Test of Fire Endurance of Building Construction and Materials, 1999 edition.
NFPA 252, Standard Methods of Fire Tests of Door Assemblies, 1999 edition.
UL 263, Standard for Fire Tests of Building Construction and Materials, 1997.
UL 555, Standard for Fire Dampers, 1999.
UL 10C, Positive Pressure Fire Tests of Door Assemblies, 2001
UL 1479, Standard for Safety for Fire Tests of Through-Penetration Fire Stops