{"id":103370,"date":"2020-01-06T11:04:04","date_gmt":"2020-01-06T16:04:04","guid":{"rendered":"https:\/\/www.constructionspecifier.com\/?p=103370"},"modified":"2020-01-06T11:04:04","modified_gmt":"2020-01-06T16:04:04","slug":"fire-resistance-and-prevention-in-wood-buildings","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/fire-resistance-and-prevention-in-wood-buildings\/","title":{"rendered":"Fire resistance and prevention in wood buildings"},"content":{"rendered":"

by Mariah Seaboldt, PE<\/strong><\/p>\n

\"Photo<\/a>
Photo \u00a9 BigStockPhoto.com<\/span><\/em><\/figcaption><\/figure>\n

It may seem counterintuitive to study the fire resistance of a combustible material, but wood construction has been the focus of such research for decades. The desire to harness the benefits of wood buildings, while still maintaining a safe environment, has fueled studies into both light-frame wood and mass timber assemblies. The results of this research have informed past building codes and are leading to changes in upcoming ones, allowing more projects to capitalize on the cost, schedule, and environmental advantages of wood construction. As the popularity of wood grows, construction safety practices are also receiving greater emphasis in an effort to mitigate fire hazards during one of the most vulnerable periods of a building\u2019s lifetime.<\/p>\n

Overview of mass timber<\/strong><\/p>\n

Light-frame wood construction is common across the United States. However, more projects are now using mass timber in lieu of other building materials. Various types of mass timber products are available, including cross-laminated (CLT), nail-laminated (NLT), glue-laminated (glulam) timbers, and structural composite lumber (SCL). More information regarding typical mass timber elements and common applications can be found in \u201cMass Timber in North America\u201d by Think Wood.<\/p>\n

Mass timber buildings are capable of providing a level of fire-resistance comparable to steel and concrete. As a result, approved changes to the 2021 International Building Code<\/em> (IBC<\/em>) introduce additional timber construction classifications (Type IV-A, Type IV-B, and Type IV-C) and will allow three-hour rated mass timber structures to be built up to 18 stories in height (Type IV-A construction). These recently approved changes can be viewed on the International Code Council<\/a>\u2019s (ICC\u2019s) website.<\/p>\n

\"As<\/a>
As wood is exposed to fire, a char layer forms on the outermost surface. A zone of preheated wood exists directly adjacent to the layer of char.<\/span>
Images courtesy AKF Group<\/span><\/em><\/figcaption><\/figure>\n

While there is a growing desire to utilize mass timber, several stumbling blocks stand in the way of widespread use of these products. Due to the predominant use of steel and concrete, designers, owners, and authorities having jurisdiction (AHJs) may not be familiar with the fire research associated with mass timber. Additionally, while the new provisions of 2021 IBC<\/em> expand on the allowances for mass timber buildings, formal code adoption by individual jurisdictions can be a lengthy process. As a result, the public\u2019s perception of large-scale buildings has been built on a foundation of steel and concrete. Although the concept of tall, mass timber buildings is new, increasing awareness and use of the material can help change this perception.<\/p>\n

Wood behavior in fire conditions<\/strong><\/p>\n

In order to understand the different fire-resistance capabilities of light-frame wood and mass timber, designers should understand the basic fire behavior of the material. When exposed to the heat of fire, all wood undergo a thermal degradation reaction called pyrolysis. As wood is heated, the material decays into volatile gases and leaves behind a layer of char. Flaming ignition of wood is actually the ignition of the volatile gases released during the pyrolysis process. For more details on the science of pyrolysis, read the Society of Fire Protection Engineers\u2019 (SFPE\u2019s) Handbook of Fire Protection Engineering<\/em>.<\/p>\n

The char layer created during pyrolysis insulates the remaining layers of wood and slows the overall degradation of the member. A portion of the wood directly adjacent to the char layer experiences higher temperatures than the rest of the wood (i.e.<\/em> preheated zone). As described in the American Wood Council\u2019s (AWC\u2019s) \u201cTechnical Report No. 10,\u201d the base of the char layer and the start of the preheated zone are accepted to have a temperature of approximately 300 C (550 F). Beyond the preheated zone, the wood has a temperature close to its initial ambient temperature. This area is usually referred to as \u201cambient,\u201d \u201cnormal,\u201d or \u201ccold\u201d wood. In general, wood members with larger cross sections endure the heat of fire for longer durations, since it takes more time to reduce the size of the ambient zone to the point of critical structural failure.<\/p>\n

Basic fire resistance criteria<\/strong><\/p>\n

Whether using mass timber, light-frame wood, or any other material, IBC<\/em> bases fire-resistance ratings on the testing and exposure criteria of ASTM E119, Standard Test Methods for <\/em>Fire Tests of Building Construction and Materials<\/em>, or Underwriters Laboratories (UL) 263, Standard for Fire Tests of Building <\/em>Construction and Materials<\/em>. ASTM E119 is commonly referenced for testing of fire-resistance ratings, though the two standards are effectively the same. IBC<\/em> permits fire-resistance ratings to be substantiated by subjecting assemblies to ASTM E119 or UL 263 testing<\/a>, or through calculations, prescriptive designs, or other engineering analysis (IBC<\/em> 2018, 703.3, \u201cMethods for determining fire resistance\u201d).<\/p>\n

\"Figure<\/a>
Figure 1<\/strong>: The Component Additive Method (CAM) approach sums the fire-resistance times for each component to determine the total fire-resistance rating of the assembly. The table was created by AKF Group based on information from the 2018 edition of the International Building Code (IBC) 722.6, \u201cWood assemblies,\u201d and \u201cAnalytical Methods for Determining Fire Resistance of Timber Members\u201d by Robert H. White.<\/span><\/figcaption><\/figure>\n

ASTM E119 testing procedures involve exposing the assembly to increasing furnace temperatures based on the standard\u2019s specified time-temperature curve. Some assemblies must also withstand a hose stream test to evaluate the assembly\u2019s ability to remain intact during fire conditions. A full list of specified furnace temperatures and acceptance criteria for various types of assemblies can be found in ASTM E119.<\/p>\n

Component Additive Method for calculated fire resistance<\/strong><\/p>\n

ASTM E119 is also used as a reference for comparing the fire resistance of assemblies and materials. As detailed in \u201cAnalytical Methods for Determining Fire Resistance of Timber Members\u201d by Robert H. White in the SFPE handbook, the Component Additive Method (CAM) was developed by the National Research Council Canada (NRC) to determine the fire-resistance ratings of various building elements. CAM was created by correlating data from fire tests of light-frame wood assemblies to ones tested to ASTM E119.<\/p>\n

Through CAM, the protective membranes and framing of an assembly are assigned individual fire-resistance times. The total fire-resistance rating of the assembly is determined by adding the times allotted to each individual component. CAM is only applicable to light-frame wood assemblies and is limited to the specific components tested as part of NRC\u2019s research. Examples of fire-resistance times assigned to various light-frame components are summarized in Figure 1. For additional fire-resistance times and more information on CAM, consult 2018 IBC<\/em> 722.6, \u201cWood assemblies,\u201d and \u201cAnalytical Methods for Determining Fire Resistance of Timber Members\u201d by Robert H. White.<\/p>\n

\"Figure<\/a>
Figure 2<\/strong>: Fire tests of actual exposed wood members have illustrated the accuracy of the National Design Specification (NDS) method for calculating fire-resistance ratings. This chart was created by AKF Group based on fire test data in Part II of the American Wood Council\u2019s (AWC\u2019s) \u201cTechnical Report No. 10.\u201d<\/span><\/figcaption><\/figure>\n

Calculating fire-resistance ratings using CAM is allowed by IBC<\/em> to demonstrate the fire ratings of light-frame wood, and can be used in lieu of specifying an assembly tested to ASTM E119 (2018 IBC<\/em> 703.3 and 722.6). Although CAM can be used to obtain fire-resistance times between 20 and 90 minutes, IBC<\/em> limits the approach for assemblies with a maximum fire-resistance rating of one-hour (2018 IBC<\/em> 722.6.1.1, \u201cMaximum fire-resistance rating\u201d). Additionally, IBC<\/em> 722.6 limits the application of CAM to wood framing spaced a maximum of 406 mm (16 in.) on center (o.c.) and to wood joist construction only.<\/p>\n

Calculated fire resistance using the National Design Specification for Wood Construction<\/em><\/strong><\/p>\n

Since the CAM approach is only applicable to light-frame wood assemblies, alternate methods must be used for mass timber assemblies. IBC<\/em> also allows the use of chapter 16, \u201cFire Design of Wood Members,\u201d of the American National Standards Institute (ANSI)\/AWC, National Design Specification <\/em>(NDS<\/em>) for Wood Construction,<\/em> to calculate the fire resistance of exposed wood members (2018 IBC<\/em> 722.1, \u201cGeneral\u201d). NDS<\/em> procedures are an expansion of the calculation methods originally developed by T.T. Lei at NRC in the 1970s. Lei\u2019s methods were first recognized by model building codes in the United States in 1984 and were adopted by all the three legacy codes organizations\u2014the Building Officials and Code Administrators International, Inc. (BOCA), International Conference of Building Officials (ICBO), and Southern Building Code Congress International (SBCCI). More information on the development and history of the NDS<\/em> calculation procedures can be found in AWC\u2019s \u201cTechnical Report No. 10.\u201d<\/p>\n

Overview of the NDS<\/em> calculation method<\/strong><\/p>\n

The NDS<\/em> method for determining fire resistance is based on a calculation of the char layer depth at the time of required fire-resistance (e.g.<\/em> one-hour). A nominal char rate of 0.6 mm\/min (1.5 in.\/h) is used to represent standard fire exposure (i.e.<\/em> ASTM E119 exposure). Using the nominal or a selected char rate, and the equations provided in chapter 16 of NDS<\/em>, designers can calculate the char depth in a given member.<\/p>\n

While NDS<\/em> calculations use a nominal char rate, the corners of rectangular members experience increased charring due to the impinging heat transfer from two directions. As a result, the corners of wood members become rounded during a fire. Additionally, the preheated zone has reduced structural capacity due to elevated temperatures. As a result of these conditions, NDS<\/em> specifies the use of an effective char depth that is 20 percent deeper than the calculated level. Using the effective char depth to determine the reduced cross section of the member, chapter 16 of NDS<\/em> outlines procedures for determining the remaining average ultimate strength of the member. If the ultimate strength is sufficient for the supported loads, the member is considered to be capable of providing the applicable fire-resistance rating.<\/p>\n

Comparison of NDS<\/em> calculations and fire tests of wood members<\/strong><\/p>\n

Part II of AWC\u2019s \u201cTechnical Report No. 10\u201d compares approximately 145 fire tests of wood members to the calculated fire resistance of the member using chapter 16 of NDS<\/em>. The tests included both large timber members as well as smaller, light-frame options. As shown in Figure 2, the calculated fire-resistance times were within three to 19 percent of the measured fire resistance of the members. The fire test results illustrate the NDS<\/em> calculation procedure is capable of accurately estimating the fire resistance of exposed wood members.<\/p>\n

Additional methods of determining fire resistance<\/strong><\/p>\n

\"Due<\/a>
Due to the rounding of corners and the reduced structural capacity of the preheated zone, the NDS method specifies the use of an effective char depth that is 20 percent deeper than the calculated level.<\/span><\/figcaption><\/figure>\n

The NDS<\/em> calculation procedures are only applicable to fire-resistance ratings for exposed wood up to two-hours (NDS<\/em> 16.2, \u201cDesign Procedures for Exposed Wood Members,\u201d and IBC<\/em> 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<\/em> 602.4, \u201cTypes of Construction\u201d). Therefore, Type IV-A construction elements must be tested to ASTM E119, or have ratings substantiated through an engineering analysis.<\/p>\n

A growing number of mass timber assemblies have undergone fire testing to demonstrate fire-resistance ratings. WoodWorks<\/a> 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.<\/p>\n

\"Figure<\/a>
Figure 3<\/strong>: The National Fire Protection Association (NFPA) 241, Standard for Safeguarding Construction, Alteration, and Demolition Operations, outlines various safety practices for construction, alteration, and demolition operations. Compliance is often required through a jurisdiction\u2019s fire code.<\/span><\/figcaption><\/figure>\n

Fire prevention during construction<\/strong><\/p>\n

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.<\/em> 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.<\/p>\n

Several recent construction fires in light-frame buildings have heightened concerns regarding the use of wood construction. The article \u201cDanger: Construction\u201d in the National Fire Protection Association (NFPA) Journal<\/em> 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<\/em>, is often required through a jurisdiction\u2019s fire code, and is referenced by both the International Fire Code<\/em> (IFC<\/em>) and NFPA 1, Fire Code<\/em> (2018 IFC<\/em> 3301.1, \u201cScope,\u201d and 2018 NFPA 1 section 16.1, \u201cGeneral Requirements\u201d). However, it is not consistently applied to projects, even though it has existed in some form since 1930.<\/p>\n

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).<\/p>\n

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, \u201cSite Security.\u201d)<\/p>\n

NFPA 241 for tall wood buildings<\/strong><\/p>\n

Chapter 12, \u201cSafeguarding Construction Operations for Tall Timber Structures,\u201d of NFPA 241-19 is specific to tall timber buildings (i.e<\/em>. 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, \u201cFire Exposure Analysis,\u201d of NFPA 241-19, the analysis must:<\/p>\n