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Mass timber describes wood framing systems that use large solid wood members including large timbers, glued-laminated timbers (glulam), structural composite lumber (SCL), cross-laminated timber (CLT), and nail-laminated timber (NLT). The American Wood Council (AWC) develops the Manual for Engineered Wood Construction for easy-access to design information on these products and recently released the 2015 edition.

The manual provides design information for structural applications of many wood-based products and their connections — including traditional and mass timber products including structural lumber, glulam, poles and piles, I-joists, SCL, structural-use panels, and CLT — in accordance with requirements of the standards referenced. It incorporates dual formats, providing design provisions for both allowable stress design and load and resistance factor design. Informational chapters are organized to parallel those found in the National Design Specification for Wood Construction (NDS).

Primary changes reflected in the 2015 edition of the Manual include:

  • a new chapter on CLT; and
  • new and updated tables on minimum sizes to qualify for heavy timber construction, which now includes SCL and CLT.

Cross-laminated timber

Figure 1: CLT stiffness and strength design capacities are based on flat panel bending according to principles of ANSI/APA PRG 320.

Figure 1: CLT stiffness and strength design capacities are based on flat panel bending according to principles of ANSI/APA PRG 320.

The most recent edition of the manual includes a new chapter (M10) on CLT, which is a prefabricated engineered wood product consisting of at least three layers of solid-sawn lumber or SCL where the adjacent layers are cross-oriented and bonded with structural adhesive to form a solid wood element. Panels are prefabricated based on the project design and arrive at the job site with windows and doors precut. Size varies by manufacturer, but they can include three, five, seven, or more layers.

With the introduction of CLT in the 2015 NDS and 2015 International Building Code (IBC), this manual provides another tool designers and code officials have at their disposal to assist with wood design and construction. The new chapter includes information on reference CLT design values, CLT strength and stiffness properties, and connections to CLT.

Table M10.2-1 (see Table 1) provides allowable design properties for the parallel layers of CLT grades listed, which represent production in accordance with ANSI/APA PRG 320: Standard for Performance Rated Cross-Laminated Timber intended for use by North American CLT manufacturers based on the following layups:

  • E1: 1950f-1.7E Spruce-Pine-Fir MSR lumber in all parallel layers and No. 3 Spruce-Pine-Fir lumber in all perpendicular layers
  • E2: 1650f-1.5E Douglas fir-Larch MSR lumber in all parallel layers and No. 3 Douglas fir-Larch lumber in all perpendicular layers
  • E3: 1200f-1.2E Eastern Softwoods, Northern Species, or Western Woods MSR lumber in all parallel layers and No. 3 Eastern Softwoods, Northern Species, or Western Woods lumber in all perpendicular layers
  • E4: 1950f-1.7E Southern pine MSR lumber in all parallel layers and No. 3 Southern pine lumber in all perpendicular layers
  • V1: No. 2 Douglas fir-Larch lumber in all parallel layers and No. 3 Douglas fir-Larch lumber in all perpendicular layers
  • V2: No. 1/No. 2 Spruce-Pine-Fir lumber in all parallel layers and No. 3 Spruce-Pine-Fir lumber in all perpendicular layers
  • V3: No. 2 Southern pine lumber in all parallel layers and No. 3 Southern pine lumber in all perpendicular layers

Bending strength capacity is the maximum design moment, represented as (FbSeff), where Fb is the reference extreme fiber bending stress and Seff is the effective section modulus of the cross section based on the layup used in the manufacturing process. The units of FbSeff are lbf-in. per foot of panel width.

Figure 2: CLT shear-in-the-plane of the panel (planar or rolling shear) capacities are based on the principles of ANSI/APA PRG 320.

Figure 2: CLT shear-in-the-plane of the panel (planar or rolling shear) capacities are based on the principles of ANSI/APA PRG 320.

CLT apparent bending stiffness is measured as the deflection under a bending load and is represented as (EI)app (see Figure 1). The apparent stiffness is based on the layup used in the manufacturing process and is calculated per the NDS using (EI)eff values. The units of EI are lbf-in.2 per foot of panel width.

Shear strength in the plane of the panel is the capacity to resist horizontal shear breaking loads when loads are applied or developed on opposite faces of the panel (Figure 2), as in flat panel bending. Planar shear capacity is given as Fs(Ib/Q)eff where Fs is the reference material innerlaminar shear stress and (Ib/Q)eff is the effective CLT cross-sectional shear flow at mid-depth. The units of Fs(Ib/Q)eff are lbf per foot of panel width.

Allowable shear through-the-thickness is the capacity to resist horizontal shear breaking loads when loads are applied or developed on opposite edges of the panel (Figure 3), where Fv is the reference shear through-the-thickness stress of the material and tv is the panel thickness for shear. The units of Fv(tv) are lbf per inch of shear resisting panel length. This property can be obtained from the CLT manufacturer or an approved source.

Fire design

Figure 3: CLT through-the-thickness shear capacities are based on the principles of ANSI/APA PRG 320.

Figure 3: CLT through-the-thickness shear capacities are based on the principles of ANSI/APA PRG 320.

The manual also outlines fire considerations, including design requirements and fire-rated assemblies, for traditional and mass timber products. A new table in Chapter 16 of the Manual clearly shows where CLT and SCL can be used in Heavy Timber (Type IV) construction (see Table 2).

Type IV construction is being reinvigorated and has gained momentum in recent years. Building and fire officials have long understood the enhanced fire performance of Type IV structures and now building designers are considering the effects of new technologies and the associated benefits of its use. Because of the structural capabilities of mass timber, wood design is better able to take advantage of the larger building height and area permitted by the IBC. It is well recognized that heavy timber used in Type IV construction has inherent fire resistant properties due to its mass, and with additional protection from gypsum wallboard or other membranes, can achieve high fire resistance ratings while also limiting the effects of the additional fire load created by the massive wood.

Easy-access to design information

Table 2: Table M16.1-1 clearly shows where CLT and SCL can be used in Heavy Timber (Type IV) construction.

Table 2: Table M16.1-1 clearly shows where CLT and SCL can be used in Heavy Timber (Type IV) construction.

Wood construction, particularly with engineered wood products, is getting more attention because wood provides the lowest environmental impact among building materials. The 2015 IBC is the first edition to recognize new mass timber products such as CLT. Because of the structural capabilities of mass timber, wood design is better able to take advantage of the larger building height and area permitted by the IBC.

As a result of this interest, it’s important that the regulatory acceptance of wood construction remain as easy as possible for designers and builders to access. AWC develops the manual for that purpose and encourages designers and builders to review it for the latest design information on structural engineered and traditional wood products.


The 2015 Manual is available as a free download from the AWC website at www.awc.org/codes-standards/publications/nds-2015.


Bradford K. Douglas, P.E., is vice president of engineering at the American Wood Council (AWC), which represents the interests of the North American wood products industry. AWC works to assure the broad regulatory and market acceptance of wood products by developing design tools and guidelines for wood construction. He can be contacted at bdouglas@awc.org.

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