Cross-laminated timber panels (CLT panels)
Cross-Laminated Timber (CLT) panels offer innovative technology and design solutions for eco-resilient residential buildings. A combination of orthogonal and laminar features result in a higher degree of strength, stability, and flexibility of this material.
Good thermal insulation, sound insulation, and performance under fire are added benefits that come as a result of a massive wood structure.
Definition of Cross-laminated timber
Cross-laminated timber (CLT) panels consist of several layers of lumber boards stacked crosswise (typically at 90 degrees) and glued together on their wide faces and, sometimes, on the narrow faces as well. Besides gluing, nails, screws, or wooden dowels can be used to attach the layers. Innovative CLT products such as Interlocking Cross-Laminated Timber (CLT) are in the process of development in some countries.
A cross-section of a CLT element has at least three glued layers of boards placed in orthogonally alternating orientation to the neighbouring layers. In special configurations, consecutive layers may be placed in the same direction, giving a double layer (e.g., double longitudinal layers at the outer faces and/or additional double layers at the core of the panel) to obtain specific structural capacities.
CLT products are usually fabricated with an odd number of layers; three to seven layers is common, even more in some cases.
The thickness of individual lumber pieces may vary from 16 mm to 51 mm (5/8 in to 2.0 in) and the width may vary from about 60 mm to 240 mm (2.4 in to 9.5 in). Boards are finger-jointed using adhesives meeting severe durability requirements. Lumber is visually graded or machine stressrated and is kiln-dried.
Panel sizes vary by manufacturer; typical widths are 0.6 m (2.0 ft), 1.2 m (4.0 ft), 2.4 m (8.0 ft.), and 3 m (10 ft.), while length can be up to 18 m (60 ft.). In special cases, the thickness can be up to 508 mm (20 in), although typical thicknesses are 105 mm (4-1/8 in), 175 mm (6-7/8 in) and 245 mm (9-5/8 in), when used in buildings. Transportation regulations may impose limitations on CLT panel size.
Lumber in the outer layers of CLT panels used as walls are normally oriented up and down, parallel to gravity loads, to maximize the wall’s vertical load capacity.
Likewise, for floor and roof systems, the outer layers run parallel to the major span direction.
Key advantages of Cross-laminating
CLT used for prefabricated wall and floor assemblies offers many advantages. The crosslaminating process provides improved dimensional stability to the product, which allows for prefabrication of long, wide floor slabs, long single-storey walls, and tall plate height conditions as in multi-storey balloon-framed configurations. Additionally, cross-laminating provides relatively high in-plane and out-of-plane strength and stiffness properties, giving the panel two-way action capabilities like those of a reinforced concrete slab. The ‘reinforcement’ effect provided by the cross-lamination in CLT also considerably increases the splitting resistance of CLT for certain types of connection systems.
Manufacturing Process
A typical manufacturing process for CLT includes the following steps: lumber selection, lumber grouping and planing, adhesive application, panel lay-up and pressing, product cutting, surface machining, marking, and packaging. Stringent in-plant quality control tests are required to ensure that the final CLT product will be fit for the intended application.
Panel dimensions vary by manufacturer. The assembled panels are usually planed and/or sanded for a smooth surface at the end of the process. Panels are cut to size and openings are made for windows, doors, service channels, connections, and ducts, using CNC (Computer Numerical Controlled) routers, which allow for high precision.
Why CLT: Cross-Laminated Timber panels?
Natural materials have low emissions during the lifetime of a building and because wood naturally stores CO2, it also reduces the carbon footprint of the build.
CLT panels are pre-cut to be pre-assembled to a particular project`s dimensions and directly transported to a construction site. The CLT prefabrication period is completed 5-7 times faster than traditional projects. An efficient prefabrication period significantly decreases the erection of structural work, reducing economic costs, labor intensiveness, and construction waste. The manufactured pre-assembly of walls, floors, doors, windows and roof elements is on a smaller scale construction site. Ultimately, smaller - sized cranes, a minimum number of engaged contractors, and better waste performance. The development of CLT structures allows for the usage of higher quality materials than traditional construction projects as labour costs are comparably lower.
According to statistics, developed nations are responsible for “40%” of greenhouse gas emissions, “40%” of energy consumption, and “70%” of electricity (Chen, 2012, p. 8). In Ontario, overall building sector contribution to greenhouse gas emissions progress is “20.5%”; while the average household's energy spending is “$2,165” (Environmental Commissioner of Ontario, 2016, p.54 & Financial Accountability Office of Ontario, 2019, p.1). CLT houses have an efficient energy performance since wood is a renewable construction material and panel joints are tight fit, allowing to observe and store energy. In combination with solar batteries, these houses take advantage of the warmth, and light, reducing electricity bills. CLT solid panels have minimum airflow, resulting in only “⅓” of heat and cooling energy consumption in comparison to the average North American household (Wood Works, 2020). The acoustic system of CLT panels protrudes efficient sound transmission, while maintaining the noise control at a high level.
The panels’ thick mass cross-section features allow the houses to be indubitably fire resistant. CLT`s dimensional stability and rigidity features allow a minimum level of structural deformation. An efficient lateral load resisting system performance of CLT is stable and rigid in seismic events.
The main goal of this experiment was to demonstrate the structural integrity of the CLT building in response to an earthquake. The ultimate result of this simulation model showed a high seismic resistant level with almost no deformation.
The aesthetic presence of the CLT houses can be attributed to the structural simplicity of combining wood in modern contemporary and classic design innovations.
Cross-Laminated Timber (CLT) panels: Questions and Answers
Is CLT panels devastate
a significant number of trees?
The increasing demand of wood as a naturally renewable construction material resulted in a strong market. The CLT current sourcing methods proceed with de-and reformation, allowing responsible manufactures to cut and regrow trees. The strong monitoring by developers over limits on devastation ensured the secured plantation of the same number of trees. The sustainable forest concept was successfully implemented into the environmental management procedures in Europe and North America. The idea surrounds mass timber manufacturers to be responsible to plant trees after cutting and responsible government agencies to monitor their compliance.
This example showed a collective involvement of all interested parties to prove the possibility of mass timber production and management of eco-resilient biodiversity.
Are CLT houses fire resistant?
CLT panels are affected at a lower degree by fire events when compared to traditional buildings. Their mass cross-section feature and fewer concealed spaces allow these panels to gradually char, protecting buildings from further devastation. CLT's structural integrity and fire resistance of projects are compliant with ASTM E119 fire standards.
The major goals of this experiment were to identify the safest solution to use panels in more than two-storied constructions and to demonstrate structural fire resistance capabilities. The result of this fire simulation explicitly displayed the prowess and strength of the CLT building. It took a significantly longer time in comparison to traditional buildings, to cause irreparable damage.
Is the CLT technology adapted and compliant with the North American standards?
The CLT technology originated in the early 1990s in Austria, reaching its European interest in 2000, as a result of increasing demand for sustainably responsible construction. In North America, the CLT idea picked its attention only in 2010, an emerging need for naturally grown and sourced construction material to substitute traditional concrete and steel. The CLT project has to be built within full compliance of the North American recognized standards for Cross-Laminated Timber performance.
The International Building Code 2015 (IBC, Chapter 23)
Updated Canadian CLT Handbook, 2019 Edition
These standards and codes demonstrate the full capabilities of CLT buildings to be exploited at the same degree as concrete or steel. The progressive integration of the CLT in Ontario will soon proceed beyond low and mid-rise structures since its health and safety performance is continuing to be tested.
- References
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• Canada Green Building Council. (2009). LEED Canada Reference Guide for Green Building Design and Construction. Google.
• Chen, Y. (2012). Comparison of environmental performance of a five-storey building built with cross-laminated timber and concrete. Sustainable Building Science Program, University of British Columbia-Department of Wood Science, Vancouver, BC, Canada. Environmental Commissioner of Ontario. (2016, November). Ontario's Carbon
• Footprint-Where Are We Now?
• Estonian Academy of Security Sciences (ENG). ( 4). A CLT Fire Resistance Test [Video].
• Financial Accountability Office of Ontario. (2019).
• The Engineered Wood Association. (2018). ANSI/APA PRG 320: Standard for Performance Rated Cross-Laminated Timber. YouTube.
• International Code Council (ICC). (2015). Ibc2015 - Chapter 23. Digital Codes Library.
• Ministry of Natural Resources and Forestry & Ministry of Municipal Affairs. (2017, October). Ontario’s Tall Wood Building Reference.
• Roberts, D. (2020, January 15). The hottest new thing in sustainable building is, uh, wood. Vox.
• Structurlam. (2013). Environmental Product Declaration. Structurlam | North America's Finest Cross Laminated & Mass Timber Solutions.
• Sustainable Forestry - the Swedish model. (2015, October). YouTube.
• Think Wood. (2020). U.S. CLT Handbook. Google.
• Wood Works. (2020). WoodWorks | Wood Products Council.
• X-Lam Earthquake Test [Video]. (2009, Jan 20). YouTube.
What is the CLT transportation process?
The precise manufacturing technique of CLT panels to a specific project`s parameters produce prefabricated house kits that are ready for an instant assembly process. Following the sequence, after pre-cut and pre-assembly of CLT panels at the factory facility, they are transported to a construction site by flatbed trucks. One of the most unique features of CLT panel is their comparably clear “strength-to-weight ratio (⅙ CLT: concrete; 2:1 CLT: steel)” (FPI Innovations, 2019). As a result of the flexibility, strength, and light-weight of CLT panels are easier and faster transportation service, minimum of contractors engaged, and small-scale construction site with smaller cranes involved. The potential breakdown of costs associated with CLT panels is “1%” for packaging and “2%” for transportation from the overall total manufacturing cost of CLT (FPI Innovations, 2019, p. 4).
