Passive fire protection in buildings involves constructing walls, floors, ceilings, beams, columns, and shaft enclosures so they can resist, control, and contain the damaging effects of a fire. It is intended to entail the following:
• Provide structural and thermal integrity of floor, wall, and ceiling assemblies during a fire for a specified time period
• Compartmentalize a room or space to control the fire spread
• Provide exiting systems for occupants to safely and rapidly evacuate the building
If well designed and maintained properly, passive fire protection systems are extremely effective in protecting building occupants and controlling the spread of fire. These systems require periodic inspection and necessary maintenance. Breaches in structural and thermal integrity caused by renovation can lead to lack of proper protection in a fire emergency situation. Examples of passive fire protection measures that are evident in most public buildings are shown in img.s 21.1 and 21.2.
img. 1 Fire separation doors are used to compartmentalize the building.
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img. 2 Exit signage for occupant egress. ( --)
Fire-Resistive Construction
A principal objective of fire-resistive construction is to use materials and construction assemblies that contain the fire in a small area and confine the fire in the room or area for a specific period of time. Fire-resistive construction provides protection for a specific time period so building occupants can be made aware of the fire, the occupants can be evacuated from the building, and firefighters can fight the fire.
A good example of a building that was suitably compartmented against spread of fire is the Empire State Building. In 1945 an aircraft hit the 78th and 79th stories and a severe fire involving large quantities of fuel broke out. Despite the severity of the fire, there were no casualties among the many occupants of the floors both above and below the fire area.
A factor that plays a great role in reducing the overall fire risk in a building is the extent to which fire-resisting construction is used to divide a building into fire-resisting compartments that will contain a fire and prevent its propagation to neighboring compartments. Compartmentalizing means separating a building into compartments so that if there is a fire, the fire damage is confined to certain a room or certain section of the building only. This requires fire separation barriers on walls, floors, and ceilings for each zone of the building that serves as fire compartment.
Fire walls are fire-rated walls that form a required barrier to restrict the spread of fire throughout the building. They serve as a means of dividing a large structure into compartments. Fire walls are normally built of brick, concrete, or masonry. Typically, a firewall must extend from the foundation and intersect a noncombustible roof surface or extend beyond the roof by a specified vertical distance, usually 32 in (813 mm). Openings such as door and window openings are restricted in size and must contain an approved glass (e.g., wire glass) or fire door.
A fire separation is similar to a fire wall except that it does not extend from the foundation to the roof assembly. It is used to divide different occupancies in a building (e.g., a garage from a residence) or enclose exit corridors and stairs. A shaft wall is a protective fire-rated enclosure around an elevator hoist way or mechanical chase. Depending upon construction type (e.g., protected steel, unprotected steel), type of occupancy and size of building, a fire separation or shaft wall must meet a specific fire resistive rating.
A firestop is a specific construction technique consisting of all materials that fill the opening around penetrating items such as cables, cable trays, conduits, ducts, and pipes and their means of support through the wall or floor to prevent the spread of fire. Firestops are needed to compartmentalize a fire. All penetrations through fire separations must be sealed with an approved firestop material that meets the requirements of applicable fire and building codes. The integrity of these barriers must be maintained to provide smoke and flame containment.
Fire-Protective Materials
Several site-applied fire-protective coverings, insulations, and coatings can be used to insulate structural members from the effects of high temperatures generated in fire. Gypsum wall board is a fire-protective covering that consists of approximately 21% water chemically bonded to calcium sulfate. In a fire, a large amount of energy is released to evaporate water in the gypsum material, giving it excellent fire-protective qualities. Insulating materials, include rockwool (a fibrous insulation made from volcanic rock) and vermiculite (a natural insulating material), also functions well. The performance of these insulations is dependent on the thickness: a thicker material provides greater fire resistance. Concrete and masonry also serve well as fire-protective coverings.
An intumescent material swells, enlarges, inflates, and expands when exposed to heat. Fire-protective intumescent coatings are applied like paint to structural steel members at a thickness that ranges from 0.03 to 0.4 in (0.8 to 10 mm). These intumescent coatings expand approximately 15 to 30 times their volume when exposed to high temperatures in a fire, and thus provide a good fire-protective barrier. Most intumescent coatings generate an ash-like char layer during their expansion process. As the fire exposure continues, the char layer erodes, exposing the remaining intumescent coating, which chars again.
Because of their paint-like qualities and the ability to overcoat an intumescent layer with a decorative sealing coat, intumescent coatings have seen increased use in recent years. Intumescent materials perform well as a firestop to sealing penetrations through fire separations.
fgr. 21.1 A typical label indicating the fire rating of a door.
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TABLE 2 FIRE-RESISTANCE RATINGS OF SELECTED CONSTRUCTION ASSEMBLIES.
COMPILED FROM VARIOUS INDUSTRY SOURCES.
Fire-Resistive Rating Basic Description of Construction Assembly* Walls/Partitions 1 hr One layer of 5/8 in, Type X gypsum wallboard appropriately nailed or screwed on each side of wood studs spaced at 16 in O.C.
One layer of 5/8 in, Type X gypsum wallboard appropriately screwed on each side of metal studs spaced at 24 in O.C.
4 in thick, cored brick units and hollow tile units.
4 in thick, solid and hollow concrete masonry units, Type S or N concrete.
4 in thick, solid concrete and concrete panels.
2 hr Two layers of 5/8 in, Type X gypsum wallboard appropriately nailed or screwed on each side of wood studs spaced at 16 in O.C.
Two layers of 5/8 in, Type X gypsum wallboard appropriately screwed on each side of metal studs spaced at 24 in O.C.
6 in thick, solid concrete and concrete panels.
3 hr 8 inch thick, clay or shale, hollow brick (75% or more solid).
6 in thick, solid and hollow concrete masonry units, Type S or N concrete.
8 in thick, solid concrete and concrete panels.
4 hr 8 in thick, clay or shale, solid brick.
12 in thick, clay or shale, hollow brick (64% solid).
8 in thick, hollow brick (60% solid), fully filled with loose insulation.
8 in thick, solid and hollow concrete masonry units, Type S or N concrete.
8 in thick, solid concrete and concrete panels.
Floors/Ceilings
1 hr One layer of 5/8 in, Type X gypsum wallboard appropriately nailed or screwed on bottom side of wood joists spaced at 16 in O.C. A 1 in thick nominal solid wood subfloor and finish floor appropriately nailed or screwed to topside of joists.
One layer of 1/2 in, Type X gypsum wallboard appropriately nailed or screwed furring channels spaced at 24 in O.C. secured to bottom side of wood joists spaced at 16 in O.C. A 1 in thick nominal solid wood subfloor and finish floor appropriately nailed or screwed to topside of joists.
2 hr One layer of 1/2 in, Type X gypsum wallboard appropriately screwed to furring channels wire tied to the bottom chord of steel joists. Topside of steel joist is covered with a 4 in thick concrete slab on 28 gauge steel decking.
One layer of 1/2 in, Type X gypsum wallboard appropriately screwed to furring channels secured to the bottom web of a reinforced concrete joist or double tee web (at least 10 in deep).
Steel Columns 1 hr Two layers of gypsum wallboard: Base layer of 1/2 in gypsum wallboard wire tied to column at 15 in O.C. Face layer of 1/2 in gypsum wallboard adhered to base layer with a laminating adhesive over entire surface.
2 hr Two layers of gypsum wallboard: Base layer of 1/2 in Type X gypsum wallboard screwed to metal studs secured to column outer flange surfaces and side layer attached to ends of flanges. Face layer of 1/2 in Type X gypsum wallboard appropriately screwed to base layer.
3 hr Three layers of gypsum wallboard: Base layer of 5/8 in Type X gypsum wallboard screwed to metal studs secured to column outer flange surfaces and side layer attached to ends of flanges. Two face layers of 5/8 in Type X gypsum wallboard appropriately screwed to base layer.
Doors (includes fire door, frame, hardware, and other accessories) 3 hr (A) Steel doors 1 1/2 hr (B) Rating varies with door type and size, frame type, type and size of lite (glazing), use if intumescent seals, and 3/4 hr (C) hardware.
3/4 hr or 20 min 90 min Wood doors 60 min
Rating varies with door type and size, frame type, type and size of lite (glazing), use if intumescent seals, and 45 min hardware.
20 min
*Some construction specifications are neglected for clarity. Check with the local building code for precise specifications of construction assembly.
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Fire Doors and Windows
Fire doors are typically of steel or solid wood construction and are installed with specially tested components including closers, latching hardware, and fire-rated glass lites (windows).
It is essential that every part of the door and frame assembly contribute the required level of performance. Intumescent seals are fitted to the edge of the door leaf or in the frame reveal. An intumescent seal expands in fire to seal the gap between the edge of the leaf and the frame, as a result preventing the pas sage of smoke and flame. As shown in fgr. 1, fire door assemblies are labeled indicating their fire rating.
Fire-resistant glass can be classified in two categories: insulating and transmitting glass. Fire-resistant heat-transmitting glass contains flames and inflammable gas for a short period of time, but does not prevent the transmission of heat to the other side of the glazing (e.g., wired glass, reinforced laminated glass). Fire-resistant insulating glass contains flames and in flammable gas for a longer period of time and prevents not only the transmission of flames and smoke but also of heat to the other side of glazing.
Fire and Smoke Dampers
Another common way for fire to spread from one compartment to another is through the HVAC ductwork. Fire dampers automatically close to obstruct smoke and fire from a building blaze. Fire dampers are installed in the plane of the firewall to protect these openings. Upon detection of heat, the fusible link (available in 165°F, 212°F, and 285°F) melts, closing the fire damper blades and blocking the flame from penetrating the partition into the adjoining compartment.
Smoke dampers close upon detection of smoke, preventing the circulation of air and smoke through a duct or a ventilation opening. They can be part of an engineered smoke control system designed to control smoke migration using walls and floors as barriers to create pressure differences. Pressurizing the areas surrounding the fire prevents the spread of smoke into other areas. They are controlled by a smoke or heat detector signal that is part of a fire alarm control system.
Fire and Smoke Ratings
Several fire and smoke ratings are used to classify the behavior and performance in a fire. Common ratings include the following.
Fire-Resistance Ratings
A fire-resistance rating, expressed in hours or minutes, is a measure of fire endurance, the elapsed time during which a material or assembly continues to exhibit fire resistance under specified conditions. It is assigned to building assemblies (walls, columns, girders, beams, and composite assemblies for ceilings, floors, and roofs) based on results from laboratory testing that determine their ability to withstand the effects of a fire for a period of time. An assembly meeting the 1 hr expo sure in the standard fire test receives a 1 hr rating; an assembly meeting the 2 hr exposure receives a 2 hr rating, and so forth.
Fire-resistance ratings of selected construction assemblies are summarized in Tbl. 2.
The fire-resistance rating is determined in a standard fire endurance test such as the method specified in ASTM E 119- Standard Methods of Fire Tests of Building Construction and Material ASTME: American Society for Testing and Materials.
This test method evaluates how long a construction assembly will contain a fire and how long it will retain its structural integrity during a predetermined fire exposure. Other tests include ASTM E 152-Fire Tests of Door Assemblies and ASTM E 163-Fire Tests of Window Assemblies.
The ASTM E 119 test of floor and roof assemblies is con ducted using a furnace with horizontal dimensions of approximately 13 ft by 17 ft (4 _ 5). The assembly is installed on top of the furnace and loaded to its design capacity. The furnace temperature is regulated along a standard time-temperature curve. Fire temperatures start at room temperature and then rise to 1000°F (540°C) at 5 min; 1300°F (705°C) at 10 min; 1700°F (925°C) at 1 hr; 1850°F (1010°C) at 2 hr; and 2000°F (1093°C) at 4 hr. The test is terminated and the rating time is established when:
• Hot gases passing through the assembly ignite cotton waste.
• Thermocouples on top of the assembly show a temperature rise averaging 250°F (140°C).
• A single rise of 325°F (180°C) is achieved.
• The assembly collapses.
Horizontal assemblies such as floors, ceilings, and roofs are tested for fire exposure from the underside only. This is because a fire in the compartment below presents the most severe threat. For this reason, the fire-resistance rating is required from the under side of the assembly only. The E-119 test of walls and doors is similar to the floor and roof test. A vertical furnace is used. For load-bearing walls, the test requires the maximum load permit ted by design standards to be superimposed on the assembly.
Loading during the test is critical as it affects the capacity of the wall assembly to remain in place and serve its purpose in pre venting fire spread. The fire-resistance rating of load-bearing wall assemblies is typically lower than that of a similarly de signed nonload-bearing assembly.
Partitions (interior non-load-bearing walls) required to have a fire-resistance rating are rated equally from each side because a fire could develop on either side of the fire separation.
If they are not symmetrical, the fire-resistance rating of the assembly is determined based on testing from the weakest side.
Exterior walls only require a rating for fire exposure from within a building. This is for the reason that fire exposure from the exterior of a building is not likely to be as severe as that from a fire in an interior room or compartment. Because this rating is required from the inside only, exterior wall assemblies do not have to be symmetrical in design.
Another time measured in the test is the finish or membrane rating, which is a measure of ceiling performance. Thermocouples are installed on the lowest face of the structural members and the finish rating time is obtained using the same temperature rise criteria described earlier. A fire-resistive assembly can be negated by poor construction or with a small hole cut through the assembly after the building is occupied (e.g., by a cable, duct, or electrical equipment installer). A small hole, al though it may seem insignificant, breaches the fire resistance of the assembly and causes the space to no longer be confined. In a fire, the hole can allow extreme heat to pass through the assembly to another space, which limits confinement of the fire.
Unfortunately, as building ages, many unprotected openings exist and often are not repaired by the installer; they are not routinely inspected and repaired.
Flame-Spread Ratings
Another common measure used to evaluate the performance of a material in a fire is its flame spread. The flame-spread rating (FSR) describes the surface-burning characteristics of a building material. The most widely accepted flame-spread classification system is specified in the NFPA Life Safety Code, NFPA No. 101. The NFPA Life Safety Code primarily applies this FSR classification to interior wall and ceiling finish materials. Roof coverings must meet a different set of criteria, as discussed later.
The FSR is expressed as a number on a continuous scale where inorganic reinforced cement board is 0 and red oak is 100. The scale is divided into three classes. The NFPA Life Safety Code groups the following classes in accordance with their flame-spread and smoke development based on Classes A, B, and C. Some older model building codes (e.g., International Conference of Building Officials, Inc. [ICBO], Uniform Building Code and Building Officials and Code Administrators International, Inc. [BOCA], National Building Code) refer to the three categories as Class I, II, and III, respectively.
• Class A or I Flame spread 0-25 Good resistance to flame spread
• Class B or II Flame spread 26-75 Fair resistance to flame spread
• Class C or III Flame-spread 76-200 Poor resistance to flame spread
Flame-spread ratings and classifications for common materials are found in Tbl. 21.3. In general, inorganic materials such as brick or tile are Class A or I materials. Reconstituted wood materials such as plywood, particleboard, and hardboard are Class C or III. Although different species of wood differ in their surface-burning (flame-spread) rates, most wood products have a flame-spread rating less than 200 and are considered Class C or III material. A few species have a flame-spread index slightly less than 75 and qualify as Class B or II materials. Flame-spread ratings and classifications for common materials are provided in Tables 21.3 and 21.4.
The FSR informs designers how likely a fire is to move from its point of origin, and how fast. For instance, if the wall coverings and furnishings in a room are all Class A or I surfaces, a wastebasket fire will probably burn itself out without doing much more than scorching the surroundings. In contrast, if one or more of the surfaces has a high flame-spread rating, the fire will spread rapidly, consuming the entire room and its contents. Once the room and the furnishings are totally consumed, their flame-spread ratings no longer affect the fire's progress. Instead, it becomes a matter of the fire resistance of the walls, the floor, the ceiling, and the amount of combustible material in the room.
The FSR is specific to a particular surface and substrate.
Other aspects of the wall or ceiling structure do not affect these surface-burning characteristics. However, painting the surface changes the surface-burning characteristics. For this reason, special care must be exercised in determining the type and thickness of paint applied to Class A or I surfaces because the FSR can be changed.
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TABLE -3 FLAME-SPREAD RATINGS AND CLASSIFICATIONS FOR COMMON MATERIALS. COMPILED FROM VARIOUS INDUSTRY SOURCES.
Flame Spread Material/Species Rating Class Brick 0 A or I Fiber-cement exterior materials 0 A or I Gypsum sheathing 15 to 20 A or I Gypsum wallboard 10 to 15 A or I Inorganic reinforced cement board 0 A or I Plywood, fire-retardant-treated construction 0 to 25 A or I Cedar, Western Red 69 B or II Hemlock, West Coast 73 B or II Spruce, Engelmann 55 B or II Birch, Yellow 80 C or III Douglas fir 90 C or III Fiberboard, medium density 167 C or III Hardboard siding panels <200 C or III Idaho White Pine 82 C or III Maple 104 C or III Masonite <200 C or III Oak, Red or White 100 C or III Oriented strand board (OSB) 150 C or III Particleboard 116 to 178 C or III Pine, Lodge Pole 98 C or III Pine, Ponderosa 115 C or III Plywood, oak 125 to 185 C or III Plywood, pine 120 to 140 C or III Wood structural panels 76 to 200 C or III
TABLE -4 FLAME-SPREAD CLASSIFICATIONS FOR COMMON SIDING AND SHEATHING MATERIALS. COMPILED FROM VARIOUS INDUSTRY SOURCES.
Flame Material Spread Class Typical Use Cement fiber panel A or I Siding Cement fiberboard A or I Siding 1 in gypsum sheetrock A or I Sheathing 5/8 in Type X exterior gypsum wallboard A or I Sheathing T1-11 plywood panel C or III Siding Hardboard board C or III Siding Hardboard plank C or III Siding 1 in log veneer (pine) C or III Siding 1 in oriented strand board (OSB) C or III Sheathing
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Class A-B-C Roof Coverings
Class A, B, or C roof coverings are sometimes confused with the Class A-B-C/I-II-III flame-spread categories mentioned previously. The tendency is to assume that a Class A roof system has a Class A flame-spread rating, and so on, but there is no correlation. Roof coverings must meet a different set of test criteria. The roof-covering classification test does not produce a flame-spread rating. Instead, it is a pass-fail test under which a product either passes the criteria as a Class A, B, or C roof covering system or it does not.
It is an entirely different test from the FSR test (e.g., it includes weather exposure determined in a rain penetration test).
The highest classification for a roof covering is Class A and Class C is the lowest. Note that a Class C roof system is considered fire resistant while an FSR Class C (or III) building material is not. Nonclassified roof systems have no fire rating.
Smoke Developed Rating
The smoke developed rating is a single-number classification of a building material as determined by an ASTM E 84 test of its surface-burning characteristics. It is expressed as a ratio of the smoke emitted by a burning material to the smoke emitted by the red oak standard material.
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