Multi-Story Wood Construction
Fire Protection Requirements
Fire-resistance rating requirements for building elements are provided in IBC Table 601. Chapter 7 of the IBC covers materials and assemblies used for fire-resistance-rated construction and separation of adjacent spaces.
Wood-frame building elements that require fire resistance are required to have the fire-resistance rating be determined in accordance with the test procedures set forth in ASTM E119, Standard Test Methods for Fire Tests of Building Construction Material. The code also provides alternative methods for determining fire resistance per IBC Section 703.3, which includes prescription tables in 720 or calculations for fire resistance in 721.
Photo: W.G. Clark Construction, Ankrom Moisan Architects
Mercer Court at the University of Washington includes five buildings, each with five stories of wood-frame construction over two or three stories of concrete.
Fire-Resistance Assemblies
One-hour and two-hour fire-resistance requirements are generally provided by fire-rated assemblies that include gypsum sheathing.
Lists of accepted rated wood floor and wall assemblies are available from the American Wood Council (DCA 3: Fire-Rated Wood-Frame Wall and Floor/Ceiling Assemblies, www.awc.org) and APA (Form W305, www.apawood.org).
Other sources are (UL) Fire-resistance-rated Systems and Products, the UL Fire Resistance Directory, and the Gypsum Association's Fire Resistance Design Manual.
Fire-Retardant-Treated Wood (FRT)
The building code (IBC Section 2303.2) permits exterior two-hour rated bearing walls in Type III construction to be constructed of fire-retardant-treated (FRT) wood, an acceptable substitute for a non-combustible material.
MIXED-USE PODIUM BUILDING
Photo: Benson & Bohl Architects
Project: Parkside mixed-use project, a combination of the Parkside affordable housing project (77 apartment units) and East Village Community Church (11,000-square-foot sanctuary and related facilities)
Location: San Diego, California
Architect: Benson & Bohl Architects, Inc.
Code consultant: CHURCHILL ENGINEERING, INC.
Residential (five levels of wood-frame construction): 67,109 square feet
Church (one level plus mezzanine): 12,209 square feet
Parking garage (three levels): 48,213 square feet
Total area of building: 127,531 square feet
Type of construction: Type IIIA platform wood construction over a Type 1A concrete podium
Year of completion: 2010
Parkside, a $20-million Leadership in Energy and Environmental Design (LEED®) Platinum project, demonstrates the use of code provisions to increase height and area. Installing NFPA 13-compliant sprinklers in the wood-frame portion permitted a height increase of one story to a total height of 85 feet with five stories, explains building code and life-safety consultant James E. Churchill, P.E., president of CHURCHILL ENGINEERING, INC. An additional 11,040 square feet in the wood portion of the building was permitted through the frontage provision (IBC 506.2), which allows an increase in building area if the building fronts on a public way or open space. (Unlike the IBC, the California Building Code [CBC] permits an increase in building area based on frontage, but not on the use of sprinklers if that provision has already been employed to gain an additional story.)
The concrete podium provides a three-hour fire-resistant horizontal assembly which separates the wood stories above from the stories below, allowing the wood building and the podium to be treated as separate buildings for the purposes of determining area limitations, continuity of fire walls, number of stories and type of construction.
Two vertical three-hour fire walls extend from the first floor level above the podium to the roof level, separating the residential portion of the building into three separate buildings. Without the special provisions for horizontal assemblies in Section 509.2 (2009 IBC), which permit portions of structures to be treated as separate buildings, the fire walls would have had to run from the foundation level to the roof level, dividing the parking garages, thus making the building program impossible to deliver.
Fire Protection Requirements
Fire-resistance rating requirements for building elements are provided in IBC Table 601. Chapter 7 of the IBC covers materials and assemblies used for fire-resistance-rated construction and separation of adjacent spaces.
Wood-frame building elements that require fire resistance are required to have the fire-resistance rating be determined in accordance with the test procedures set forth in ASTM E119, Standard Test Methods for Fire Tests of Building Construction Material. The code also provides alternative methods for determining fire resistance per IBC Section 703.3, which includes prescription tables in 720 or calculations for fire resistance in 721.
Fire-Resistance Assemblies
One-hour and two-hour fire-resistance requirements are generally provided by fire-rated assemblies that include gypsum sheathing.
Lists of accepted rated wood floor and wall assemblies are available from the American Wood Council (DCA 3: Fire-Rated Wood-Frame Wall and Floor/Ceiling Assemblies, www.awc.org) and APA (Form W305, www.apawood.org).
Other sources are (UL) Fire-resistance-rated Systems and Products, the UL Fire Resistance Directory, and the Gypsum Association's Fire Resistance Design Manual.
Fire-Retardant-Treated Wood (FRT)
The building code (IBC Section 2303.2) permits exterior two-hour rated bearing walls in Type III construction to be constructed of fire-retardant-treated (FRT) wood, an acceptable substitute for a non-combustible material.
Designing for Different Fire Ratings
One framing consideration for Type IIIA is that floors and walls may have different fire ratings. Since load bearing exterior walls are required to have a two-hour rating, the intersection of a one-hour floor requires detailing. This can be accomplished by allowing the wall studs with one layer of gypsum to extend to the underside of the floor sheathing, and supporting the floor framing off the top plates with the use of a hanger designed to span over a layer of gypsum sheathing. Another framing option is to hang the floor framing off a 2x ledger, which provides as much if not more fire resistance as the layer of gypsum. The one-hour wall rating plus the one-hour floor system yields the required two-hour rating.
*Source: IBC Table 601, **IBC Table 706.4
Fire-resistance requirements for Type IIIA and Type VA construction
Fire Walls
Fire walls are not required by the building code; however, they may be utilized to increase the square footage of a building. For R1 (short term residential) and R-2 Type IIIA structures, the IBC requires a non-combustible three-hour fire-resistant wall between buildings. However, for Type V construction, walls are permitted to have a combustible two-hour fire-resistance rating. Under the NFPA 221 Standard for High Challenge Fire Walls, Fire Walls and Fire Barrier Walls, designers may build a two-hour rated fire wall using two contiguous one-hour fire-resistance-rated assemblies.
Image: Togawa Smith Martin
Three-hour fire wall assembly
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