Trees have been used to build structures since prehistory, but especially after many disasters, the wood came to be seen as unsafe and unstable relative to the two materials that have since become staples of the construction industry worldwide: concrete and steel. However, a new way of using wood has put the material back in the spotlight. That is known as ‘Mass Timber’.
Mass Timber is comprised of multiple solid wood panels nailed or glued together, which provide exceptional strength and stability. It’s a strong, low-carbon alternative to concrete and steel. Mass timber building designs are pioneering better places for us to live and work, and new code changes were passed for the 2021 code cycle that allows mass timber buildings up to 18 stories tall.
Mass timber is a generic term that encompasses products of various sizes and functions, like glued-laminated (glulam) beams, laminated veneer lumber (LVL), nail-laminated timber (NLT), and dowel-laminated timber (DLT). But the most common and most familiar form of mass timber is cross-laminated timber (CLT).
Cross Laminated Timber – ‘CLT’
Cross-laminated timber (CLT) is a panel-shaped mass timber product, which is built up from at least three crosswise glued layers. The layers each consist of panels made of the wooden lamella. According to the same principle as for veneer plywood, a high degree of dimensional stability is achieved by the crosswise structure: each layer of wood prevents the dimensional change of the adjacent layer at right angles to it, which occurs with single boards due to changes in wood moisture.
The simple beauty of CLT is its orthotropic quality. Normal wood is strong in the direction of the grain but weak in the cross direction. CLT’s perpendicular layers make it strong in two directions. And because it relies on layers of smaller beams, it can reduce waste by using odd-shaped, knotty timber that lumber mills would otherwise reject.
How is Cross Laminated Timber made?
Cross-laminated timber is manufactured mainly from Spruce, but it may include a percentage of Pine or Larch, and possibly hardwood. In the factory:
- Lumber is smoothed, kiln-dried, and trimmed. The drying reduces moisture to a percentage that does not allow cracking or dimensional variations.
- The treated timber boards are then stacked on top of one another in layers, known as lamellas; each lamella is perpendicular to the one below.
- The lamellas are glued to each other with a non-toxic and environmentally friendly adhesive, then hydraulically pressed to achieve strength.
The size of cross-laminated timber panels is restricted in width to the size of the manufacturing machinery and length to the mean of transportation used. However, mostly, the width of the panel can be 3.5 meters and the length is typically 13.5 meters but can reach up to 22 meters.
Benefits of Cross Laminated Timber
There are many benefits to CLT. Compared with steel and concrete construction it is a low impact material with a much lower embodied carbon footprint. CLT has a quicker construction time over traditional methods with some estimates claiming it is six times faster than a standard build due to panel construction and ease of subsequent fixing and remediation. It is much lighter which allows for reduced slab – a positive impact for project-wide embodied carbon totals. Furthermore, it is clean with much less waste produced on-site, limited wet trades, or brick/blockwork creating dust, while handling is vastly reduced so it is much better in terms of health and safety.
Structural Capability of Cross Laminated Timber
CLT has been called “the concrete of the future,” and in a sense – it’s true. It delivers at minimum the same structural strength as reinforced concrete, but it’s a material with a high degree of flexibility that has to undergo great deformations to break and collapse – unlike concrete. Moreover, 1 m3 of concrete weighs approximately 2.7 tons, while 1 m3 of CLT weighs 400 kg and has the same resistance. The same goes for steel.
Protection of Cross Laminated Timber from Environmental Conditions
Moisture and weather are the most important enemies of wood. Exposed timber suffers, and since CLT is a structural component, we have to protect it to avoid its wear, corrosion, and collapse. While it’s possible to add supplementary layers of coating to wood, such as fiber cement, brick, stone, or other materials, there are also ways to preserve exposed CLT.
Vegetable oils and mineral paints can meet these objectives if applied only once every 5 years, guaranteeing 25 years of protection without detachment or discoloration.