Additional methods of determining fire resistance
The NDS calculation procedures are only applicable to fire-resistance ratings for exposed wood up to two-hours (NDS 16.2, “Design Procedures for Exposed Wood Members,” and IBC 722.1). Therefore, the assemblies using protective membranes and assemblies needing ratings greater than two hours must have alternate methods of substantiating fire resistance. For example, the new Type IV-A construction classification requires a three-hour primary structural frame and cannot contain any exposed wood (2021 IBC 602.4, “Types of Construction”). Therefore, Type IV-A construction elements must be tested to ASTM E119, or have ratings substantiated through an engineering analysis.
A growing number of mass timber assemblies have undergone fire testing to demonstrate fire-resistance ratings. WoodWorks has assembled a database of fire-tested assemblies, including penetration and firestopping systems. Several of the tested assemblies achieve a three-hour fire-resistance rating when coupled with noncombustible membranes such as gypsum. The use of gypsum as a protective membrane can benefit both light-frame and mass timber assemblies. However, where gypsum is required for added protection, wood buildings may be vulnerable prior to installation of the material. As a result, safe construction practices play a pivotal role in protecting wood buildings from fire.
Fire prevention during construction
As discussed in previous sections, mass timber elements are capable of achieving substantial fire resistance. However, higher fire-resistance ratings and more robust construction types require the usage of noncombustible membranes over the wood elements (e.g. Type IV-A and IV-B construction). Similarly, gypsum is used in light-frame construction to provide a protective membrane for the wood studs. Where noncombustible membranes are required, they are often installed some time after the installation of wood elements, leaving the latter exposed for a period of time. As a result, construction site safety becomes a crucial component of the fire-protection strategies for wood buildings.
Several recent construction fires in light-frame buildings have heightened concerns regarding the use of wood construction. The article “Danger: Construction” in the National Fire Protection Association (NFPA) Journal provides details on several of these fires. As a result of the increased fire risk during construction, many jurisdictions and organizations are advocating for more oversight of construction practices. Compliance with NFPA 241, Standard for Safeguarding Construction, Alteration, and Demolition Operations, is often required through a jurisdiction’s fire code, and is referenced by both the International Fire Code (IFC) and NFPA 1, Fire Code (2018 IFC 3301.1, “Scope,” and 2018 NFPA 1 section 16.1, “General Requirements”). However, it is not consistently applied to projects, even though it has existed in some form since 1930.
NFPA 241 requires owners to develop a construction fire safety program and to designate a fire prevention program manager to enforce it. The purpose of the fire safety program is to ensure construction operations are conducted in accordance with the practices established in NFPA 241 (Figure 3).
Additionally, the 2019 edition of NFPA 241 includes provisions specific to wood construction that are intended to mitigate the threat of arson. NFPA 241 now gives the AHJ authority to require guard service for buildings with combustible materials exposed during construction, and that are greater than 12 m (40 ft) in height (NFPA 241, section 7.2.5, “Site Security.”)
NFPA 241 for tall wood buildings
Chapter 12, “Safeguarding Construction Operations for Tall Timber Structures,” of NFPA 241-19 is specific to tall timber buildings (i.e. mass timber). Two significant provisions of the new chapter involve site security and a fire exposure analysis of the building. According to section 12.3, “Fire Exposure Analysis,” of NFPA 241-19, the analysis must:
- be performed by the fire protection program manager;
- be submitted with construction documents;
- detail sequencing of passive and active fire protection installations;
- identify separation provided between adjacent structures; and
- evaluate the adequacy of preventing fire spread
to exposed structures.
