By Maria Fernanda Laguarda Mallo and Omar Espinoza. University of Minnesota
Wood has played an essential role in the development of civilization for its abundance and remarkable properties. Wood’s intrinsic characteristics make it remarkably flexible and versatile, as being demonstrated by the wide variety of successful applications. Particularly, the outstanding structural (exceptionally strong relative to its weight) and environmental performance in comparison to concrete or steel and its pleasuring aesthetics have made the construction industry the main driver for demand of forest products. However, the use of wood as a building material is not free from challenges. Being a natural material, its properties are not homogeneous, thus challenging its users to adopt strategies to cope with this variability. To counter the effects of the inherent variability of the material and to enhance its overall performance, engineered wood products (EWPs) were developed. The strength, reliability and long span capability are attributes that make EWPs attractive alternatives to solid wood in most types of buildings. The variety of EWPs available in the market (Panels, Glulam and I-joist beams, etc.) provides construction professionals with virtually limitless design possibilities.
One of the latest innovations in the area of EWPs has been the development of Cross-Laminated Timber (CLT), also known as “Cross-Lam”, “X-Lam” or “Massive Timber.” CLT technology was developed in the early 1990s in Europe, as a cooperative project between industry and academic partners to find practical uses for usually discarded wood. As the market in Europe began demanding more environmentally friendly products, more people turned to CLT for their construction needs and CLT quickly grew in popularity over the past two decades, with buildings now completed in Europe, Australia and North America, such as the Forté Building, the first 10-story CLT building in the world (Figure 1).
The CLT construction system is based on the use of large multi-layer panels made from lumber boards that are glued together, alternating the direction of their fibers for each layer, which improves rigidity, stability, and mechanical properties (Figure 1). The cross-laminated nature of the panels implies that they can take up forces in all directions, which allows them to be used as walls, slabs or roofs, in a wide range of applications, such as houses, barns, power line towers, churches, bridges, high-rise apartment and office buildings, among others.
Why CLT?
Environmental advantages
Wood offers multiple benefits, especially in regards to environmental performance. When forests are sustainably managed, wood is carbon-neutral, and acts as a repository of carbon, either as growing stock or as a value-added product, by converting carbon dioxide to biomass in the process of carbon sequestration using photosynthesis. Also, in comparison, steel and concrete, wood in general and CLT in particular outperforms both materials. While being high-performing and versatile building materials, steel and concrete show a very different performance in regards to their environmental footprint: for example, five percent of all Green House Gas (GHG) emissions worldwide are caused by concrete manufacturing and use. Moreover, Life Cycle Analysis (LCA) research on wood products has consistently demonstrated that they produce less greenhouse gases, and requires smaller amounts of water, energy, and fossil fuels to transform than concrete and steel [1].
Installation speed and simplicity
CLT elements can be used for roofs, walls and flooring structures, and they can be delivered from the factory in various sizes and shapes (which are usually limited by transportation requirements). Openings for windows and doors are cut with precision using CNC machines. This computerized system improves the efficiency in the use of wood resources, minimizing manufacturing waste and time, and guaranteeing the precision of the final product. in place (Figure 2). The prefabricated nature of CLT allows for high precision and a construction process characterized by faster completion (that may be as short as 3 to 4 days per story, in comparison to more 28 days if concrete is used), increased safety, less demand for skilled workers on site, less disruption to the surroundings, and less waste generation.
Structural capabilities
One of the most impressive advantages of CLT elements is their strength to weight ratio compared to other materials. The reason for this can be found in the cross-laminated configuration of CLT, in which adjacent layers of CLT elements support each other and act as reinforcement, which leads to better mechanical properties. Thus, CLT can be used as load-bearing elements and shear panels, something that distinguishes CLT from other wood-based panels. Some variations of the typical massive panel have also been developed, such as “cassette” floors, where two cross-laminated timber plates are connected with wooden ribs to form the hollow core elements, allowing even greater spans. The idea behind this is to reduce the slab’s weight without compromising its strength. Another variation is the “interlocked” panel which, unlike traditional CLT panels, does not use adhesives during its manufacturing but tongue and groove and dovetail joinery to “lock” each board and layer. This removes the use of volatile organic compound (VOC)-emitting adhesives, allowing the panel to be disassembled at end of life and be reused.
Some variations of the typical massive panel have also been developed, such as “cassette” floors, where two cross-laminated timber plates are connected with wooden ribs to form the hollow core elements, allowing even greater spans. The idea behind this is to reduce the slab’s weight without compromising its strength. Another variation is the “interlocked” panel which, unlike traditional CLT panels, does not use adhesives during its manufacturing but tongue and groove and dovetail joinery to “lock” each board and layer. This removes the use of volatile organic compound (VOC)-emitting adhesives, allowing the panel to be disassembled at end of life and be reused.
Design flexibility
The structural characteristics of CLT allow for shapes and openings of the most diverse sizes and forms, without compromising the structural integrity of the structure. The size of the elements is limited in great part by transportation requirements, and allows designers to have fewer elements to create the same stable construction that could be achieved with more traditional materials, adding to the simplicity of the system.
Fire performance
People wrongly assume that wood buildings perform poorly in fires, since wood is a combustible material. However, wood members with large cross-sections, such as CLT, have the inherent ability to provide fire resistance because of wood’s unique charring properties. Wood burns predictably, forming a char layer that protects non-charred wood, allowing it to maintain its strength and dimensional stability without abruptly collapsing. Results of full-scale fire tests conducted in the Forest Products Lab [2] show that CLT structures have the potential to provide excellent fire resistance, often comparable with non-combustible materials. The tight nature of CLT buildings also improves fire performance by limiting the spread of fire to adjacent spaces.
Seismic Performance
In terms of seismic performance, wood buildings perform well because they are lighter and more ductile than structures built with traditional materials. To confirm this, 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 a severe earthquake simulation (magnitude of 7.2 in the Richter scale), the structure showed no significant deformation after the test [3]. It was shown that CLT walls allow movement of the wall panels, which is an essential condition for structures to resist seismic forces.
Thermal performance
The main advantage of CLT, in regards to thermal performance, is that it offers the possibility of creating a tight construction, due to the large panels and the possibility of having fewer elements and thus fewer joints where air could infiltrate the building. CLT also provides thermal inertia. CLT panels, both in the building enclosure and in interior floors and walls, act as a thermal mass that store heat during the day and releases it at night. Thermal mass can greatly reduce heating and cooling loads, shift the time of peak loads, lower overall building energy use and enhance occupants’ comfort. The insulating capacity of these massive walls may also help reduce the need for insulation, thus reducing construction costs.
Proven success stories around the world show the technical capabilities of CLT mainly regarding its environmental, structural and economic performance. The use of CLT has marked a significant transformation in the way building projects are conceived, shifting the design from a “frame” to “plate” based construction, providing designers with a new and more environmentally friendly and aesthetically appealing building solution alternative to concrete and steel.
Data Sources
[1] Puettmann, M., Bergman, R., Hubbard, S., Johnson, L., Lippke, B., Oneil, E., 2010. Cradle-to-gate life-cycle inventory of U.S Wood products production: CORRIM phase I And phase II products. Wood and Fiber Science, Vol. 42, 15–28.
[2] Dagenais, C., White, R., Sumathipala, S., 2012. Fire Performance of Cross-Laminated Timber. Presentation at the Cross-Laminated Timber Symposium. 28 February. Seattle, United States. Retrieved 2013 from: http://www.slideshare.net/rethinkwood/christian-dagenais
[3] Popovski, M., Schneider, J., Schweinsteiger, M., 2010. Lateral load resistance of cross-laminated wood panels. World Conference on Timber Engineering, Trentino, Italy.
Images Sources
[i] https://en.wikipedia.org/wiki/Brettstapel#/media/File:Brettstapel_Wall_P… CC BY-SA 3.0
[ii] http://www.nrc-cnrc.gc.ca/ci-ic/article/v17n4-4