Frequently Asked Questions
What is cross laminated timber?
Cross Laminated Timber (CLT) is a flexible building system suitable for use in all assembly types (e.g., walls, floors and roofs). Made from industrial dried lumber stacked together at right angles and glued over their entire surface, it is an exceptionally strong product that retains its static strength and shape, and allows the transfer of loads on all sides. Panels are prefabricated based on the project design and arrive at the job site with windows and doors pre-cut. Although size varies by manufacturer, they can be as large as 54.1 x 9.7 x 1.6 feet and include 3, 5, 7 or more layers.
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, single or double splines made from engineered wood products, while metal brackets, hold-downs and plates are 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:
To download a Free PDF of the new CLT Handbook, go to www.masstimber.com.
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Section: Wood Building / Structure Types