Basics of Cross Laminated Timber (CLT
Developed in Europe in the 1990s, cross-laminated timber, or CLT, is among the latest in a long line of “engineered” wood products that are strong and rigid enough to replace steel and concrete as structural elements in buildings. Already popular in Europe, CLT is only beginning to catch on in North America, where proponents say buildings made with the panels could be a cheaper and environmentally friendly alternative to structures made with other materials (Fountain 2012). The environmental benefits of CLTs include reduced carbon footprint, reduced development footprint, creation of a continuous building envelope and quick erection time. CLT is composed of solid wood laminations of 0.6 to 2.0 inches thick and are adhesively bonded to create a cross-laminated product, similar to plywood. The strength of the wood laminations is distributed across both panel directions, rather than a single direction as in solid-sawn lumber (Figure 1).
Figure 1. Illustrations of CLT construction in Europe (courtesy of KLH Massivholz GmbH, www.klh.at.)
CLT panels can be produced in thicknesses from 4.5 up to 20 inches (APA 2012). The maximum commercial panel dimensions are 10 feet wide and 50 feet long and traditionally use spruce (Picea spp.) lumber (FPInnovations 2011). CLT panels can be used to create walls, floors and roofs of multi-story buildings and can be used similar to pre-cast concrete.
CLT may be desirable to architects and design engineers by providing capabilities for unique and beautiful building design. In Europe, CLT has been used mostly for low structures, like two-story apartment buildings or office complexes and schools, in part because building codes in many countries restrict wooden buildings to four stories. However, building codes in some countries like the U.K. are now permitting building structures in excess of four stories (Figure 2).
Figure 2. Illustrations of CLT construction in Europe (courtesy of KLH Massivholz GmbH, www.klh.at.)
As noted in a New York Times Science (2012) article1 “This is the way we ought to be building,” said Pete McCrone, whose company, Innovative Timber Systems, in Whitefish, Montana (http://www.timberworks.com/), hopes to be the first to produce cross-laminated timber in the United States. So far Mr. McCrone has built one structure, a martial arts studio in Whitefish, with panels imported from Austria. Since building construction from CLT panels is more similar to precast concrete construction than traditional timber frame construction, Borjen Yeh, Technical Services Director of APA - The Engineered Wood Association, notes “Not many engineers in this country understand how to design or construct using CLT” and that education of architects, engineers, and code-enforcement officials will be a key step in the development of CLT as a building construction material.
A USDA funded research and outreach project is currently underway at Virginia Tech and investigates the use of low-quality hardwoods (Yellow Poplar, Liriodendron tulipifera) in CLT manufacture. Virginia Tech is partnering with West Virginia University and University of Tennessee in this important study. To learn more about this exciting CLT project contact,