Engineered wood products have changed the way that architects and builders think about and use wood in mid- to large-sized structures. Through engineering and manufacturing advances, basic dimensional lumber can now be transformed into large panels that allow construction of much larger, safer, and more efficient buildings than their stick framed predecessors.
Mass timber is an umbrella term that describes a variety of engineered wood products that involve the lamination and compression of many layers to create solid panels of wood. Those panels are used to frame a building’s walls, floors and roofs.
CLT is one type of mass timber. It is made by gluing 2x dimensional lumber together in perpendicular layers, resulting in a composite product. Once a layer is laid down, glue is applied and the next layer is stacked on top. After layers are set, a powerful press compresses the panel, bonding the layers together. Once pressed, the panel is cut with precision on a CNC machine. CLT panels are typically made of three, five, or seven layers.
These large panels can come in sizes up to 12 feet wide by up to 60 feet long, are pre-manufactured to the exact size and shape needed, and are a direct replacement for concrete and steel structures in many cases.
The lumber used in CLT panels must meet structural requirements. The wood can include beetle kill trees, fire-damaged trees, and small diameter wood as long as it meets the standards of the structural grade. A higher quality wood type can be specified for the bottom (visible) layer if a more consistent look is preferred.
- Learn more about CLT manufacturer, Structurlam
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Learn how panel composition affects structural capacity
More Information: Panel Composition
There are typically an odd number of layers in a CLT panel, meaning there will be more layers with wood oriented in the long direction of the panel and one fewer number of layers in the cross direction. For example, in a 5 ply panel, there will be three layers in the long direction, and two layers in the cross direction. This configuration gives the panel a “strong” direction and a “weak” direction, which is taken into account in engineering. Panels can sit on beams or bearing walls, and can be point supported by just columns at the corners and edges of the panels.
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Learn how 3D modeling translates to extremely precise buildings
More Information: 3D Modeling
When designing a building, architects often use software called Building Information Modeling (BIM) that aggregates many pieces of detailed information into three dimensions. Unlike drawings on paper which represent basic elements in a building, BIM modeling contains detailed information about each element. For example, a wall; BIM provides information about how it is constructed, what layers exist, how much of each material is needed, and exact dimensions for locating and measuring it.
3D modeling allows architects to coordinate any needed penetrations before the panel is manufactured. This way holes can be cut in the factory with precision, rather than be measured and cut by hand on site. It also provides comprehensive information that all parties can reference throughout the project.
Lastly, 3D modeling is an integral aspect of mass timber design because it allows detailed information to be communicated directly to the CNC machines that cut the panels to specification.
Carbon12 used only raw material from timber harvested from certified, sustainably managed timberlands.
— Kris Spickler, Mass Timber Specialist, Structurlam
Manufacturing CLT
Each layer of lumber is oriented perpendicular to adjacent layers and glued on the wide faces of each board. By gluing layers of wood at right angles, CLT panels are able to achieve better structural rigidity in both directions.
Advantages of CLT
Sustainable: A switch from a concrete and steel-based industry to a renewable, wood-based industry can significantly reduce the carbon footprint of our built environment. Not only does CLT require far less fossil fuel during production, it also sequesters carbon in the structure itself, creating a carbon bank. Once trees are harvested, they are replanted and the carbon consumption starts again. Sequestering carbon is one of the most impactful ways to mitigate our impact on the environment.
Biophila: CLT and other mass timber products help to bring the feeling of nature into indoor spaces. Incorporating wood, sunlight, plants, and other natural elements into buildings is called biophilic design. Research has proven that biophilia has a positive impact on our health and well being.
Construction: There are construction advantages to using mass timber as well. Its pre-fabricated nature simplifies on-site work, making it quieter, faster, and safer to build. Because it requires a fraction of the trucks needed to haul concrete, there is significantly less noise and disruption on site and during transportation. CLT panels are lowered into place using a crane requiring just a handful of carpenters. This greatly decreases lead times and increases the speed of construction as well as jobsite safety. Each floor of Carbon12 was assembled in five days.
Wood is the only construction material that we can grow. CLT and other mass timber products are opening up new possibilities to use wood to build larger and taller buildings.
— Iain Macdonald, Director, Tall Wood Design Institute
To make Carbon12’s CLT panels, the manufacturer finger joined the lumber into 30-foot-long pieces, laid them in a press in alternating directions, applied glue between each layer, and pressed the panels under tremendous pressure to bond them together.
Putting CLT to the Test
All building materials in the US are required to meet a strict set of safety standards. Structural materials are put under the most scrutiny because they are weight-bearing and need to weather natural events like earthquakes, tornadoes, hurricanes, wind storms and heavy snowfall.
Certifications: Third party certifications ensure that building materials meet structural requirements. The CLT certification is ANSI/APA PRG 320 which all US manufacturers need to pass to sell their product. Other countries have their own certification systems but Canada, Europe, and the US must meet similarly high standards.
Fire Testing: Fire rating requirements are set by the building code depending on the building type, occupancy, height, jurisdiction and other considerations. CLT has undergone extensive fire testing. This ensures that no matter which manufacturer you buy from, mass timber burns at a predictable rate. The test results enable architects and engineers to design a specific ‘sacrificial char layer’ so that – in the case of fire – the wood will char but the integrity of the panel remains.
If a structural or fire configuration test is not readily available (due to a new configuration that hasn’t been tried before), projects are able to conduct their own tests either in a lab or in the field to prove their design works. These tests, while more expensive for the design team due to project-specific testing, or riskier if done late in the design stages, are instrumental in growing the body of data about CLT.
Testing CLT
Project-specific testing is more expensive and riskier, if done late in design stages, but are instrumental in growing a critical body of data.
Structural Testing — This structural test was conducted by Kaiser+Path for their Canyons project, an example of the tests that project teams are required to do where data does not yet exist.
Fire Testing — Fire tests conducted using CLT in different configurations often outperform expectations and can have fire ratings of up to three hours. (photo credit: Tallwood Design Institute)
Did You Know?
DR Johnson in Riddle, Oregon was the first US manufacturer of certified CLT panels, receiving their APA/ANSI certification in 2015.
A Brief History
Wood has been a primary building material for 10,000 years. In the late 19th century, the rapid adoption of concrete and steel in American cities precipitated the equally rapid decline of wood construction. Later, heightened environmental concerns – specifically the widespread logging of old growth forests – further restricted wood as a viable building material. Mass timber products were developed as an alternative to using old growth timber to get the strength and spans needed for large-scale projects.
Cross laminated timber was developed in Austria in the early 1990s, and standardized in 2002 thanks to the PhD work of researcher Gerhard Schickhofer. As old growth timber became harder to acquire, CLT and other mass timber products started to take hold in Europe in the 2000s. The first commercial CLT project in the US was completed in 2011 in Whitefish, Montana called The Long Hall.