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One of the most critical issues for architects is ensuring that building design addresses fire-safety issues. The science of fire safety engineering has advanced greatly over the years to give us an in-depth understanding of the critical stages of fire initiation, growth, containment and suppression.  Such knowledge has enabled today’s building codes and fire protection engineers to provide fire safe structures for our homes and cities. 

Principles

Fire Initiation and Growth

Heat, fuel and an oxidizing agent, usually oxygen, must all be present for ignition to occur. The fire triangle graphic below defines the conditions needed for a fire to start but a fourth condition must be added to sustain the fire and allow it to grow. This is illustrated by the fire tetrahedron shown below, which shows how heat, fuel, oxidizer and an exothermic chain reaction together can trigger a building conflagration.

Fire Safety

There are two types of methods of suppression used in building fire safety design: Passive and Active. Passive suppression uses materials, systems, building elements, and/or building layout to prevent or resist ignition, to limit its spread to other combustible contents in the room and to contain the fire within the room or zone to prevent its spread to other sections of the structure. Active suppression is the employment of mechanical devices such as sprinklers or extinguishers to extinguish the fire in its early stages and thus prevent its spread. Passive suppression uses the natural properties of materials and products that are part of the building design to suppress the migration of the fire.

It is well known in the construction industry that the single most important characteristic of gypsum drywall is its fire resistance. This is provided by the principal raw material used in its manufacture, CaSO₄∙2H₂O (gypsum). Gypsum is noncombustible, which means that it contributes no fuel to a fire. As the chemical formula shows, gypsum contains 21% by weight chemically combined water, also called crystalline water that is part of the gypsum crystal itself. When gypsum drywall panels are exposed to fire, the heat of the fire converts the crystalline water to steam. The heat energy that converts water to steam is thus absorbed, keeping the opposite side of the gypsum panel cool as long as there is water left in the gypsum, or until the gypsum panel is breached.

Performance

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. 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: https://standardscatalog.ul.com/standards/en/standard_263_14

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

Flame spread 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: https://standardscatalog.ul.com/standards/en/standard_723_11

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