Mass timber systems present opportunities and challenges
By James Johnson
It is often said that in life everything seems to go in cycles, with what is old soon becoming new again. This has appeared to be the case within the fire service in recent years, with quick water from the exterior prior to entry being relabeled and in many places passed off as a new tactic, a significant resurgence in the use of the beloved smooth bore nozzle, and an overall “back to basics” approach being taken in much of the hands-on training throughout the nation.
The construction industry is no different, and in North America we are experiencing a rebirth in the use of heavy timber for the construction of large buildings. These heavy timber buildings are not the wood structures of our forefathers but a new breed that predominantly uses young wood harvested from premature trees that are milled and laminated together using either adhesives, nails, or hardwood dowels.
These new materials are collectively known as mass timber and, with proposed changes to both the International Building Code and the National Building Code of Canada, it is imperative that, as firefighters, we take the time to become familiar with these unique materials.
Cross Laminated Timber
Of all the mass timber systems, cross laminated timber (CLT) is the one that we most often hear about. The recent completion of the 18-story Brock Commons-Tallwood House building in Vancouver, British Columbia (photo 1), and the approval for the 12-story Framework building in Portland, Oregon, have both created considerable buzz in the construction and fire protection industry circles and brought CLT into the spotlight.
CLT panels are assembled from layers of dimensional lumber that are laid flat and glued together with each layer running perpendicular to the previous one (photo 2). This arrangement of longitudinal and transverse layers, commonly three, five, or seven layers in depth, gives the panel the ability to carry an imposed load equally in all directions (photo 3). The panels can then be installed directly on top of the columns below, and this removes the need for a beam or girder to transfer the load from the floor or roof system down to the columns. CLT panels can be used for floors, roofs, and walls and are commonly paired with columns made from glue laminated timber (Glulam).
Nail Laminated Timber
Nail laminated timber (NLT) is another mass timber panel system that can be used for floors, roofs, or walls (photo 4). NLT is created by stacking dimensional lumber on edge and, as the name suggests, it is then fastened together with nails. NLT is commonly covered on one side with plywood sheathing to provide the structural diaphragm for the floor or roof systems.
Nail laminated timber is not a new concept. Here in Vancouver, British Columbia, where lumber has always been in abundance, many of the early 1900s Type III ordinary constructed buildings in our Chinatown and Gastown areas contain floor systems built in exactly this manner.
Dowel Laminated Timber
Dowel laminated timber (DLT) is the newest edition to the mass timber family and originated in Europe in the 1990s. It is similar to NLT with all the lumber being oriented on edge, but instead of being nailed together the panels have holes drilled through the layers of lumber, and hardwood dowels are then hydraulically pressed into the holes to fasten the panels together. The softwood lumber that is used in the panels has a significantly higher moisture content than the hardwood dowels, about 12 to 19 percent for the softwood vs. approximately five to six percent for the hardwood. Once the dowels have been installed into the panel, the moisture equalizes between the two materials, which causes the hardwood to swell and creates a very tight-fitting connection. One of the advantages of DLT over NLT is the ability to easily modify the profile of the panel, as it is entirely created from wood and there are no nails to get in the way of the milling machinery (photo 5).
Why Build Tall Wood Buildings?
According to many of the organizations advocating for mass timber, one of the main reasons for the increased interest in using wood for tall buildings is the need for a solution to the ever-rising carbon footprint that is associated with the production of the materials traditionally used in these types of buildings. Since wood is a renewable resource and readily available throughout North America, the carbon emissions created from harvesting, milling, fabricating, and transporting the materials and, finally, the construction of the building are said to be considerably less than that of concrete and steel. Some of the other benefits attributed to building with mass timber are the high strength-to-weight ratio of the materials, speed of construction, seismic stability, acoustic and thermal properties, and perhaps the most controversial—its inherent fire-resistant characteristics.
The theory behind the fire-resistant characteristics of mass timber is that, as the surface of the wood begins to burn, a char layer is formed. The creation of this char slows the combustion process and allows the layer to insulate the structural member and help retain a high percentage of its design strength. In the case of Brock Commons-Tallwood House, all the timber surfaces are covered by three layers of 5⁄8-inch Type X gypsum board, but we are already seeing the need for this protection questioned by many of the architects and engineers in the industry.
Those who study and value the incredible lessons learned throughout the history of the fire service always seem to bring up the devastating losses and conflagrations of the past when discussing tall wood buildings. The results of those historic fires have played a significant role in the creation of our building codes and, in turn, have contributed to the practice of using fire-resistive and noncombustible materials in the construction of large-area and high-occupancy structures. These new innovations in mass timber are beginning to challenge many of our current practices for the construction of tall buildings, and if the proposed changes to the International Building Code and National Building Code of Canada are adopted in the next editions, the types of tall buildings that shape our city skylines will forever be changed.
James Johnson is a firefighter for Vancouver Fire and Rescue Services in British Columbia, Canada. He is assigned to Firehouse 1 and is part of the special operations team. Before becoming a career firefighter, Johnson spent a number of years in the construction industry and completed an apprenticeship and the technical training to become a Red Seal journeyman carpenter.