by Gregory Havel
Cross laminated timber (CLT), a form of manufactured lumber, was developed in Switzerland and Germany in the 1990s. Its use is common in Europe, becoming common in Canada, and may soon be common in the United States. It has been used extensively in commercial and residential buildings of five to eight stories, and its designers proposed that high-rise buildings can be constructed of it in place of steel and concrete. The size and strength of the panels make it useful in prefabricated structures, since it provides nearly the strength of concrete with less weight.
CLT panels (photo 1) are made of several layers of dimensional lumber (2 x 4, 2×6) stacked with adjacent layers at 90 degrees to each other. The boards are glued together on their wide faces and sometimes also on the edges. A CLT panel has at least three layers, and may have seven or more layers.
(1)
The adhesives used to assemble CLT panels are usually thermoset resins like those used today in plywood, oriented strand board (OSB), and other wood products. However, thermoplastic resins have also been used.
For specific applications, CLT panels are manufactured in widths to 11 feet 8 inches (3 m); and in lengths to 59 feet (18 m), depending on transportation regulations and the vehicles that are available. The individual boards running lengthwise in a CLT have finger joints (photo 2) where they butt together.
(2)
The boards in the outer faces of CLT wall panels are usually vertical, parallel to the loads imposed on them from above. For floor and roof panels, the boards usually run parallel to the long span and perpendicular to the beams that support them.
Buildings of CLT panels can be of either platform or balloon construction. The most common connectors between these panels and other structural members are construction adhesives; self-tapping wood screws; bolts; dowels; splines; metal bracket and fastener systems; dowels; and ledger boards.
Although CLT panels may be combined in construction with the familiar I-joists and wood trusses, these are not lightweight structural components. They have about the same weight as a sawn timber of the same dimensions.
CLT panels are promoted as having excellent fire resistance, comparable to that of non-combustible materials and to heavy timber (Type IV) construction, due to the ability of thick wood assemblies to char slowly at a predictable rate while maintaining most of their strength during the exposure. After testing CLT panels to the standard fire resistance test (CAN/ULC S101 (Canada), ASTM E119 (USA), and ISO 834 in other countries), manufacturers propose that a five-layer CLT floor panel can receive a 1.5-hour rating; and a three-layer wall panel can receive a 45-minute rating. This fire resistance is based upon the insulating properties of the char layer that develops during the exposure to the fire, and assumes that a thermoset resin adhesive has been used, rather than a thermoplastic resin.
A 5/8-inch (15.9mm) layer of Type X gypsum board on the exposed side of the panel adds 30 minutes of fire resistance; and two layers add 60 minutes of fire resistance to the assembly.
The use of CLT panels in building construction requires significant changes to building codes, although with the transition to performance-based codes they may already be in use in residential and light commercial buildings.
Cross laminated timbers do not have a long enough history to provide firefighters with information on their behavior during abnormal situations like fires, earthquakes, and hurricanes. We need to watch the construction in our response areas and note on our preincident plans when we see these materials in use. We also need to share information on the behavior of structures using CLTs during abnormal situation to reduce the possibility of firefighter fatalities and injuries due to unexpected structural collapse.
For detailed information on the manufacture and use of cross laminated timber, search the Internet “cross laminated timber.”
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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 30 years of experience in facilities management and building construction; and has presented classes at FDIC.
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