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WoodWorks “Ask an Expert” This Month: Two Q&As on Mass Timber Building Design

WoodWorks “Ask an Expert” This Month: Two Q&As on Mass Timber Building Design

In addition to offering  education and free project support, WoodWorks – Wood Products Council publishes a monthly “Ask an Expert” Q&A addressing technical questions related to the design and construction of commercial and multifamily wood buildings. The current Q&A can always be found on the home page at woodworks.org, and you can browse or search 40+ Q&As in the archive at www.woodworks.org/ask-an-expert/.

Here are two Q&A examples related to one of the most topical aspects of wood building design: mass timber.

Q: How do you demonstrate the fire resistance of exposed wood members using char methods in lieu of gypsum?

A: IBC Sections 703.2 and 703.3 offer multiple ways to demonstrate fire-resistance ratings of structural members and assemblies. One method noted in Section 703.3 is to calculate the rating in accordance with IBC Section 722.

For exposed wood members and decking, IBC Section 722.1 references Chapter 16 of the American Wood Council’s National Design Specification® (NDS®) for Wood Construction. This chapter gives a nominal char rate of 1.5 inches of wood thickness per hour of fire resistance. Using the calculations and information in this chapter, a fire rating of up to 2 hours can be calculated.

The 2015 NDS allows calculated fire resistance of a number of wood products, including solid sawn lumber (this includes nail-laminated timber), glued-laminated timber (glulam), laminated veneer lumber (LVL), parallel strand lumber (PSL), laminated strand lumber (LSL), and cross-laminated timber (CLT). For structural members using this method of exposed fire-resistance calculations, the effective char rate, which is slightly higher than the nominal char rate and includes a heat-affected zone, is used. For nonstructural members such as blocking, the nominal char rate is used. NDS Chapter 16 includes additional design considerations for certain structural members such as glulam beams, timber decking, and CLT. It also presents adjustment factors applied to the allowable design properties of exposed structural members rated for fire resistance.

The American Wood Council’s Technical Report 10 (TR 10) provides background information for the established methods in the NDS, which can help both the designer and the authority having jurisdiction in applying and evaluating this method. TR 10 also includes design examples for exposed structural wood members using the provisions of NDS Chapter 16.

Photo: Marcus Kauffman, Oregon Department of Forestry

Q: When designing cross-laminated timber (CLT) panels for gravity-load applications such as floor or roof panels, should I use the CLT layups defined in the ANSI/PRG 320 standard as the basis of design?

A: Cross-laminated timber is an engineered wood component manufactured from dimension lumber or structural composite lumber to create large, flat panels of solid wood. It is a member of a new class of massive (or “mass”) timber products—i.e., large-dimension engineered structural wood components that complement the dimension sawn lumber, solid sawn timbers, and structural composite lumber products frequently used in building framing. Other forms of mass timber construction include nail-laminated timber (NLT), dowel-laminated timber (DLT), glued-laminated timber (GLT) panels, and solid panels of structural composite lumber materials. The size and strength of mass timber structural components make them an alternative to concrete, steel, and masonry components for many building applications.

In North America, the availability and acceptance of CLT are relatively new; however, adoption is happening quickly considering the speed at which material design standards and building code modifications typically occur. The ANSI-approved product standard, ANSI/APA PRG 320 Standard for Performance-Rated Cross-Laminated Timber, provides a basis for standardization of CLT quality, manufacturing, and structural properties for structural building applications in North America. Starting with the 2015 International Building Code (IBC), CLT is identified as a structural material, defined in IBC Chapter 23 with reference to the PRG 320 standard. CLT framing is allowed within Construction Types III, IV, and V, and for roof members in Types I and II roof assemblies requiring a 1-hour fire-resistance rating or less.

PRG 320 defines how the structural properties of the CLT panels are to be determined through a qualification process. The PRG 320 standard also defines, as examples, seven stress grades of CLT panels based on commonly available visually graded and machine-rated lumber species groups and grades. CLT grades E1 through E4 use machine stress-rated lumber for layers parallel to the major axis. CLT grades V1 through V3 use visually graded lumber for layers parallel to the major axis. The predefined structural capacities of the CLT example grades are found in the PRG 320 standard and can be useful as a reference; however, not all of the example grades and layups in the standard are being manufactured at this time. Manufacturers also have additional CLT grades and layups with structural properties certified through the PRG 320 qualification process—i.e., there are many more layups available than just those suggested. Because of this, consideration of the CLT products being manufactured is recommended in order to see what specific grades and layups are available.

If you need help finding CLT manufacturers to consider, most of the manufacturers in North American are WoodWorks National Partners and listed here. Or, feel free to ask for assistance by contacting your local WoodWorks regional director (map here) or the WoodWorks help desk at help@woodworks.org.

To read more about CLT floor and roof panel design, check out this article.

About WoodWorks

WoodWorks–Wood Products Council provides free technical support as well as education and resources related to the code-compliant design of commercial and multifamily wood buildings. A nonprofit organization staffed with architects, structural engineers, and construction experts, WoodWorks has the expertise to assist with all aspects of wood building design, including (but not limited to): allowable heights and areas/construction types, structural detailing of wood and hybrid systems, fire resistance and acoustical-rated assemblies, efficient and code-compliant lateral system design, alternate means of code compliance, and energy-efficient detailing.

For assistance with a project, visit www.woodworks.org/project-assistance or email help@woodworks.org.

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