Architectural Record BE - Building Enclosure

Designing for Fire Protection

Expanding the possibilities of wood design
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Sponsored by Think Wood and the American Wood Council
Jeffrey B. Stone, Ph.D. Based on the Code Conforming Wood Design Series by the American Wood Council and International Code Council

Fire Walls

Ideally, fire growth and fire spread will be contained to the building of origin and any adjacent buildings will be protected against fire exposure. A fire wall, as defined in the IBC, is commonly used to divide a structure into separate buildings or to separate a new addition from the existing portion of a structure. Each portion of a building separated by one fire wall or more is considered a separate structure. Generally, fire walls must be of approved noncombustible materials, but may be wood-frame in Type V construction. 

By definition: “A fire wall is a fire-resistance-rated wall having protected openings, which restricts the spread of fire and extends continuously from the foundation to or through the roof, with sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall.” A fire wall is not required to be cantilevered and remain in place if construction on both sides of it collapses. Fire walls must extend to the outer edge of horizontal projecting elements, such as balconies and roof overhangs.

Chapter 7 identifies acceptable techniques and methods by which construction can be evaluated to determine fire performance requirements, including fire walls. In addition, NFPA 221: Standard for High Challenge Fire Walls, Fire Walls, and Fire Barrier Walls specifies requirements for the design and construction of high challenge fire walls, fire walls and fire barrier walls including protection of openings and penetrations. Fire walls are required to be designed and constructed to allow collapse of the structure on either side without collapse of the wall itself. When NFPA 221 is used in the design of a fire wall, it is deemed to have met the structural stability requirement specified in Section 706.2.

Exterior Wall Finish

Wood veneer is permitted on Type I, II, III or IV buildings up to 40 feet above grade—60 feet if FRTW is used, provided the veneer is 1-inch nominal thickness, 7/16-inch exterior hardboard siding or 3/8-inch exterior-type wood structural panels or particleboard. Open or spaced veneers without concealed spaces are not permitted to project more than 24 inches from the building wall (IBC Section 1405.5).

Interior Finish

Wood and wood-based products are widely used in interior wall, ceiling and floor surfaces in all types of buildings. Appearance, acoustical qualities and interior design versatility have made wood surfaces highly desired. In general, wood materials may be used as interior finish within almost all occupancies.

IBC Table 803.11 places minimum finish performance classifications on finish materials based on their location within the building. They are expressed in terms of flame spread index numbers. These values are determined in a standard fire test which evaluates the surface burning characteristics of a material. Different maximum flame spread indices are permitted depending on building occupancy, location of the material in the building, and the presence of sprinklers—Class A (flame spread index 0-25); Class B (26-75); or Class C (76-200).

The standard fire test used to evaluate flame spread characteristics of wood building materials is ASTM E-84, Standard Test Method for Surface Burning Characteristics of Building Materials. Nonsprinklered buildings typically require lower flame-spread materials than sprinklered buildings.

Tested wood products typically have a flame spread index of less than 200, making them acceptable under the IBC for a wide range of interior finish uses. Most wood species qualify as Class C, while some, such as cedar, western hemlock, Idaho white pine, redwood and spruce, can qualify as Class B. Flame spread indices for a range of proprietary wood-based interior finish materials are available from manufacturers. Commercially available fire-retardant treatments for wood and panel products can reduce flame spread performance to an index of 25 or less, meeting Class A requirements.

Traditional wood floor coverings are exempt from floor finish requirements. Wood interior trim, such as baseboards, chair rails and handrails, are required to meet a Class C classification and cannot exceed 10% of the wall or ceiling area to which it is attached.


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Originally published in Engineering News-Record