What is cross laminated timber (CLT)?

Cross Laminated Timber (CLT) is a prefabricated engineered wood product consisting of at least three layers of solid-sawn lumber or structural composite lumber 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 pre-cut. Size varies by manufacturer, but they can include 3, 5, 7 or more layers.

The 2015 National Design Specification® (NDS®) for Wood Construction has new provisions for CLT. A new CLT chapter, consistent with other wood product chapters has been added. The applicable product standard for CLT is ANSI/APA PRG 320 Standard for Performance-Rated Cross-Laminated Timber and applicable design values are to be obtained from manufacturer’s literature or code evaluation report. Other changes reflected in the 2015 NDS specific to CLT include general connection provisions revised to accommodate CLT in Chapter 12 on Dowel-Type Fasteners; new sections applicable for wood screw and nail withdrawal from end grain of CLT; new sections to address determination of dowel bearing strengths for fasteners installed in CLT; and new placement provisions for fasteners and lag screws.

Common connections for CLT assemblies include wall-to-foundation, wall-to-wall (straight or junction), floor-to-floor, wall-to-floor, and wall-to-roof. Panels may be connected to each other with half-lapped joints or splines made from engineered wood products, while metal brackets, hold-downs and plates are often used to transfer forces. Mechanical fasteners may be dowel-type (e.g., nails, screws, glulam rivets, dowels, bolts) or bearing-type (e.g., split rings, shear plates).

CLT assemblies excel in terms of fire protection because, like heavy timber, they char at a rate that is slow and predictable, maintaining their strength and giving occupants more time to leave the building. CLT structures also tend not to have as many concealed spaces within floor and wall assemblies, which reduces the risk that a fire will spread. The American Wood Council (AWC) conducted a successful ASTM E119 fire resistance test on a CLT wall at NGC Testing Services in Buffalo, NY. The wall, consisting of a 5-ply CLT (approximately 6-7/8 inches thick), was covered on each side with a single layer of 5/8″ Type X gypsum wallboard. The wall was loaded to the maximum load attainable by the NGC Testing Service equipment. The test specimen lasted 3 hours, 5 minutes, and 57 seconds (03:05:57). [NGC-CLT-Report.pdf]

In terms of seismic performance, wood buildings in general perform well because they’re lighter and have more repetition and ductility than structures built with other materials, which make them effective at resisting lateral and uplift forces. However, the Trees and Timber Research Institute of Italy tested a full-scale seven-story CLT building on the world’s largest shake table in Japan with excellent results. Even when subjected to severe earthquake simulation (magnitude of 7.2 and acceleration of 0.8 to 1.2 g), the structure showed no residual deformation after the test. The maximum inter-story drift was 1.5 inches and the maximum lateral deformation at the top of the building was just 11.3 inches.

As with all wood products, the benefits of CLT include the fact that it comes from a renewable and sustainable resource. It also has a low carbon footprint—because the panels continue to store carbon absorbed during the tree’s growing cycle and because of the greenhouse gas emissions avoided by not using products that require large amounts of fossil fuels to manufacture. The architect of the CLT apartment building in the UK estimated that, between the carbon stored in the panels and emissions avoided by not using concrete, he kept about 300 metric tons of carbon out of the atmosphere. The CLT building was also estimated to weigh four times less than its concrete counterpart, which reduced transportation costs, allowed the design team to reduce the foundation by 70 percent, and eliminated the need for a tower crane during construction. It took four carpenters just nine weeks to erect nine stories—and the entire construction process was reduced from 72 weeks to 49.

For technical papers that address building code considerations for CLT:

The 2018 NDS includes provisions for CLT.

CLT Handbook available at ThinkWood.com.