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Fire-resistance ratings have long been used by UL, ASTM and building codes to measure the performance of various constructions for fire containment purposes. As applied to elements of buildings, the fire-resistance rating classifies the ability of an assembly to confine and isolate fire within a zone comprised of fire-resistance rated walls, ceiling and floor assemblies. The ratings relate to fire tests designed to determine how quickly fire can raise the temperature to unacceptable levels. Fire-rated assemblies are tested and certified in their entirety. These designs are identified in the UL Fire Resistance Directory, which is updated yearly and can be referenced at the Underwriters Laboratories website at ul.com.
These ratings are described in terms of fire resistance, surface-burning characteristics and non-combustibility. The following standards are used for fire safety certification:
Fire Resistance: This is the period of time an assembly will serve as a barrier to the spread of fire and how long fire resistance the assembly can function structurally after it is exposed to a fire of standard intensity, as defined by ASTM E119 and UL263. Sometimes this is also called the assembly’s fire endurance.
The test procedure consists of the fire endurance test for all assemblies and, in addition, a hose stream test for partition and wall assemblies. The test specimen must meet all of the following requirements to pass the test. An assembly must resist heat transmission so that temperatures on the side opposite the fire may be maintained below designated values. The temperature of the unexposed surface is measured by thermocouples attached directly to the surface. In the case of walls and partitions, one thermocouple is located at the center of the assembly, one in the center of each quarter section, and the other four at the discretion of the testing authority. In addition, the assembly must support its design load without structural failure or collapse for the duration of the test. Finally, the assembly must not develop gaps or openings that allow it to be breached by flames and hot gases from the fire. The earliest point at which any of these three criteria is violated terminates the test and establishes the maximum fire resistance of the assembly. Walls and partitions must also pass one more criterion, the hose stream test, before a fire resistance rating can be assigned.
The hose stream test consists of subjecting a duplicate test assembly to one-half of the indicated fire exposure (but not more than one hour), followed immediately by exposure to a jet stream of water from a fire nozzle at a prescribed pressure and distance. (See table on page 457 for conditions of hose stream test.) The time-temperature curve used for the furnace is shown below. The temperature is obtained from the average readings of nine thermocouples symmetrically located near all parts of the assembly, and placed 6" from the exposed surface of the walls or 12" from the exposed surface of floors, ceilings or columns.
Additional information on ASTM E119 can be found at this link: http://www.astm.org/Standards/E119.htm
Additional information on UL263 can be found at this link: http://ulstandardsinfonet.ul.com/scopes/scopes.asp?fn=0263.html
Through Penetration: Through-penetration fire stop systems are intended to restore the hourly rating of fire-resistive assemblies that have been breached due to penetration by electrical, plumbing or mechanical items. The ASTM E814 test method was developed in recognition of the special role of through-penetration fire stops. This standard test is applicable to through penetration fire stops of various materials and types of construction. Fire stops are intended for use in openings in fire-resistive walls and floors. They consist of materials that fill the opening around penetrating items such as cables, cable trays, conduits, ducts and pipes.
The test method considers the resistance of fire stops to an external force simulated by a hose stream. Two ratings are established for each fire stop. An F rating is based on flame occurrence on the unexposed surface, while the T rating is based on the temperature rise and flame occurrence on the unexposed side of the fire stop.
More information on ASTM E814 can be found at this link: http://www.astm.org/Standards/E814.htm
Flame spread: This is a measure of a material’s relative burning behavior. Both the flame spread and smoke developed are measured in accordance with ASTM E84.
Materials with a low flame spread prevent a small, localized fire, such as a waste basket ignited by a cigarette butt, from spreading to other combustible materials in the room. Hence, a low flame spread rating indicates a reduced probability of having a small fire develop into a room fire. The production of dense, black smoke when burning creates an additional hazard for building occupants by making it more difficult for them to see and find their way to an exit. Materials that have high flame spread and produce large quantities of smoke are considered undesirable, especially when used in areas where people assemble or are confined. ASTM E84 and UL 723 measure the flame spread and smoke density of building materials when subjected to fire. These indices are collectively known as the surface burning characteristics of the material. The test is often referred to as the Steiner Tunnel test in honor of the originator of the test method.
In the test, a 20" x 25' sample, which is installed as the “roof” of a rectangular furnace, is subjected to a fire of controlled severity. The fire is 12" from one end of the sample. From ignition the distance and time of flaming of the sample material, along with the smoke it produces, are compared against the performance of red oak planks and inorganic reinforced board, which are arbitrarily assigned values of 100 and 0, respectively, for these characteristics.
Interior wall and ceiling finish materials are grouped in classes in accordance with their flame spread and smoke-developed indexes. The classes are:
Class A: Flame spread index 0-25; Smoke developed index 0-450
Class B: Flame spread index 26-75; Smoke developed index 0-450
Class C: Flame spread index 76-200; Smoke developed index 0-450
Fire Class A designation Refers to material that may ignite but will not sustain a flame. Class A products will not generate excessive visibility-obscuring smoke, an important factor in designing safe egress for building occupants. Note that Class A is not a fire-resistance designation.
More information on ASTM E84 can be found at this link: http://www.astm.org/Standards/E84.htm
More information on UL723 can be found at this link: http://ulstandardsinfonet.ul.com/scopes/scopes.asp?fn=0723.html
Non-combustibility: A non-combustible material is one that does not ignite, burn, support combustion, or release flammable vapors when subject to fire or heat, in the form in which it is used and under anticipated conditions, as determined through ASTM E136. In this test, a sample of the material is placed in a ceramic tube furnace operating at 1382 °F. If flaming occurs after the first 30 seconds, the test specimen loses 50% or more of its weight, or the temperature of the test specimen rises by more than 54 °F, the material fails and is deemed to be combustible. If none of those three conditions occur over an exposure period of 30 minutes then the material passes and is classified as noncombustible. More information on ASTM E136 can be found at this link: http://www.astm.org/Standards/E136.htm