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Cross-laminated timber (CLT) is an engineered wood product that is growing quickly in popularity both in the United States and abroad. It is just one product in a suite of products and, in fact, an entire method of construction that is known as mass timber. A patent for CLT was first issued on August 21, 1923, to Frank Walsh and Robert Watts of Tacoma, Washington (Fig. 1). Like many great innovations, the world was not ready for this game-changing product at the time, and it largely went forgotten for decades. Then, in the 1990s, Austrian engineer Gerhard Schickhofer wrote his PhD thesis on CLT and went on to develop CLT as a commercially viable product with government approvals first granted in 1998. CLT grew in popularity in Europe in the early 2000s and has since exploded into a globally manufactured and utilized product.
Why CLT?
CLT is growing in popularity for a variety of reasons. For one, when CLT lamstock is harvested from sustainably managed forests, it has serious green credentials. Replacing more carbon-intensive materials with CLT can drastically reduce a building’s embodied carbon, especially when taking biogenic carbon into account. Organizations such as the American Society of Civil Engineers, via its SE 2050 Commitment Program, recognize structural engineers’ outsized role in lowering embodied carbon in buildings; low-carbon materials such as CLT have a significant role to play. Building occupants love exposed CLT due to its warmth and its biophilic properties. (Biophilia is our innate desire to be surrounded by nature and natural materials.) Architects love it for the warm and beautiful spaces it can create. Builders love it because panels are prefabricated and get erected quickly and easily on-site. CLT has inherent two-way spanning capabilities that can be used for perimeter cantilevers or even point-supported systems. Cross laminations make CLT dimensionally stable in both in-plane directions and allow it to function as a diaphragm.
CLT Today
Today, many CLT manufacturing facilities are located all over the world. Six facilities in the United States are certified to sell CLT as a structural building product with more planned, and a growing number of international suppliers in Canada, Europe, and South America are gaining U.S. approvals (Fig. 2). CLT factories are busy… according to Woodworks, as of March 2024, 2,115 mass timber projects are built, in construction, or in design across the United States since they began tracking in 2013; 1,197 of these include CLT. Prior to 2013, either of these numbers would have likely been in the single digits. The 2021 International Building Code has helped usher in a new era of tall, mass timber buildings by allowing mass timber buildings up to 18 stories. But mass timber and CLT are equally at home in single-family homes. CLT is also commonly used in office buildings (Fig. 3), multifamily residential buildings of all sizes, schools, fire halls, higher education buildings, and more.
Design of CLT in the United States
Design for Strength
Being an engineered wood product, the design of CLT in the United States is governed by the American Wood Council’s (AWC) National Design Specification for Wood Construction (NDS). CLT first appeared in the NDS in the 2015 Edition in Chapter 10. CLT also made its first appearance in AWC’s Special Design Provisions for Wind and Seismic (SDPWS) standard in its 2021 edition (see STRUCTURE July 2021 article) for use as shear walls and diaphragms.
Reference CLT design values are a bit different than for other wood products—this is due to the unique properties of CLT that are inherent because of the transverse layers. For uniform orthotropic materials like sawn lumber or glulam, an engineer simply divides the bending moment by the section modulus (i.e., M/S) to calculate the bending stress and divides the axial force by the cross-sectional area to find axial stresses (i.e., P/A). CLT, however, is a composite material with layers that are oriented perpendicular to the direction being evaluated, so this simple calculation is not directly applicable. The design of CLT per the NDS takes these unique properties into account in Table 10.3.1. Some key properties from that table include:
- Fb(Seff) for bending. Manufacturers calculate this allowable bending capacity for the CLT layup using the shear analogy method as prescribed in ANSI/APA PRG 320: Standard for Performance-Rated Cross-Laminated Timber, 2019 Appendix X3 for both the minor and major direction of their panels. These values, which are known as Fb(Seff), are published in CLT product code reports.
- (EI)app for out-of-plane stiffness. This is where CLT gets really fun. In most materials and situations, engineers can reasonably ignore the impacts of shear deformation (or take them into account with rough approximations). This is not the case in CLT. Due to the influence of the transverse layers, shear deformation in CLT is significant even at high span-to-depth ratios. A term known as (EI)eff, which considers flexural stiffness only, is calculated using the shear analogy method per PRG 320 Appendix X3. Similarly, (GA)eff, the effective out-of-plane shear stiffness, is calculated. Both of these values are published in manufacturer code reports. (EI)app is an apparent out-of-plane stiffness that takes both the flexural stiffness, (EI)eff, and shear stiffness, (GA)eff, into account. (EI)app can be calculated per NDS 2018 equation 10.4.1 for a given span length, loading type, and end fixity condition.
- Ft(Aparallel) and Fc(Aparallel) for axial loading (tension and compression respectively). Aparallel represents the cross-sectional area of plies parallel to the direction being considered. Transverse plies have a negligible impact on the axial capacity of CLT as wood is far weaker and less stiff perpendicular to the grain, plus most North American CLT is not edge-glued.
- Fs(Ib/Q)eff for rolling shear capacity. If you are not familiar with rolling shear, the name is quite intuitive. Imagine wood as a bundle of straws. For sawn lumber or glulam, parallel-to-grain shear governs the shear capacity of a section. Parallel to grain shear can be imagined by taking some of your straws and sliding them in a direction along their primary axis. Rolling shear is when you take some of the straws and slide them perpendicular to their primary axis. Due to the transverse layers in CLT, rolling shear is developed from out-of-plane loading. Also, since rolling shear allowable stresses are one-third that of allowable parallel-to-grain shear stresses, rolling shear capacity governs the out-of-plane shear capacity of CLT. PRG 320 2019 Appendix X3 provides equations for determining rolling shear capacity, and in practice, manufacturer code reports simply list a value Vs, which can be used directly as an allowable shear strength per one-foot width of the panel.
Fire Design
CLT is often used in building types that require structural elements to have fire-resistance ratings of one or two hours. Due to its inherent beauty and biophilic properties, it is desirable to leave CLT exposed visually, which also leaves it exposed to fire (Figure 4). Design provisions for CLT exposed to fire exist in the 2018 NDS Chapter 16 as well as in the recently published Fire Design Specification for Wood Construction (FDS) by the AWC, 2024. Technical Report No. 10: Calculating the Fire Resistance of Wood Members and Assemblies (TR-10) by the AWC is another good reference since it contains worked example problems.
At a high level, designing timber elements for fire is relatively simple—wood chars in a fire at a predictable non-linear rate. For a given fire duration, an engineer can calculate how much wood is considered ineffective as it heats up and turns to char. The remaining cross-section is evaluated for strength. Only imposed gravity loads are considered, and the reference ASD strengths for bending, shear, etc., are multiplied by an adjustment factor, which effectively makes them ultimate strength values. Things get more complicated at connections. For a brief primer on this, see the May 2023 STRUCTURE Magazine article, “Fire Protection of Mass Timber Connections Based on the 2022 Fire Design Specification.”
Vibrations
When it comes to designing for vibrations, a couple of factors can create challenges for CLT floors. First, CLT has lower stiffness than an equivalent thickness of a concrete slab. Second, CLT has a great strength-to-weight ratio; the primary downside of this is that CLT floors have less modal mass. It also means that a CLT floor has lower overall loads compared to a concrete slab… this is good for strength design, of course, but makes it more likely that vibration considerations can govern the design. As a seasoned mass timber engineer once told me, a concrete slab’s primary job is holding itself up. Consider a typical residential floor; the ratio of self-weight to all superimposed loads is roughly 1.5 for a typical post-tensioned concrete flat plate floor and roughly 0.15-0.25 for a 5-ply CLT floor.
Vibrations due to human activity are a serviceability criterion that is not addressed directly in the building code. The U.S. Mass Timber Floor Vibration Design Guide by Woodworks, published in 2023, is a tremendous resource for the vibration design of CLT floors. It includes some simplified provisions for CLT panels on rigid supports and provides calculation methods for floor plates supported on flexible (e.g., beams) framing as well.
CLT Design Tools Available
Two new CLT design tools enable engineers to design CLT in accordance with U.S. codes quickly and accurately for many common applications. These tools are wrapped up into a single Excel workbook and are available for free at https://eqcanada.com/design-resources/. Within the workbook, one tab performs calculations for floor/roof panels: cold strength, fire strength, deflections, and vibrations are all checked (Fig. 5). The other tab performs calculations for CLT walls loaded axially, including provisions for eccentric loading, out-of-plane wind loading, and P-delta effects. Cold strength and fire strength are checked.
The CLT Design Tools allow users to input any symmetric CLT layup from three to nine plies. While this can be done manually with ease, it is even more efficient to select layups from the built-in database. The layups are called up directly on the calculation sheet from a layup database that exists on a separate tab and includes every commercially available PRG 320 certified layup in the U.S. market at the time of publication. EQUILIBRIUM is committed to maintaining this layup database and making any other critical updates to the tools for the next two years and perhaps beyond pending additional funding.
Less refined versions of these tools have been used by EQUILIBRIUM engineers for years to design CLT on our projects, including some of the projects shown in the images within this article. The CLT design tools were officially launched on August 29, 2023. The first update was made on February 29, 2024. To date, the tools have been downloaded over 1,100 times. In a recent user survey, over 70% of respondents ranked the tool’s Floor/Roof designer as the best tool they have used for design of CLT floors and roofs.
The development, release, and promotion of these new tools was funded by the U.S. Forest Service by way of a Wood Innovation Grant. Additional funding was received from the Softwood Lumber Board. Holmes Structures performed a peer review. Early versions of the tool were originated at Katerra.
Matt Kantner, PE, SE, is an Associate Principal at EQUILIBRIUM, where he leads the company’s U.S.-based team from their office in Atlanta. Like his colleagues, he designs in all materials but is especially passionate about mass timber and sustainability-focused design. Kantner is a member of the SEI’s SE 2050 Committee and the American Wood Council’s Wood Design Standards Committee. (mkantner@eqcanada.com)