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  • Luis Morgado

CLT – The wood wonder material!

Origins of CLT

As a structural solution, Cross Laminated Timber (CLT) has its origins in the concept of a solid loadbearing wall made with wood. Some systems of the past can be identified as belonging to the same idea: a massive wood shelter made with solid wood walls. Walls made of logs stacked horizontally are part of the mythology of the American log cabin. Vertical logs, for instance, apparently seemed a logical solution but it didn’t work very well because the joints between each log proved to be a very weak point. Fragile was also the solution for anchoring the logs to the ground.

Historically we can notice that a way to fix the lack of weather tightness of the system was to cover the wood elements with additional layers of other materials. Shelters such as the Sami Gammes from Scandinavia and Russia were covered with turf. The structure of the pit houses (from North America, Japan, Finland and Siberia) was covered with sod and earth and in some cases, they were finished with wet earth. Some log houses were cladded with wooden boards and sometimes with stucco.

Lo-huts in the North American West, 1865.

There are also examples of planks nailed in different directions to provide additional resistance to the horizontal forces. This happens for example with the “Quebec plank frame system”. It was also the idea in the elegant and light Marcel Breuer’s Chamberlain cottage. Breuer used a tripled-layer braced wall in which the exterior layer planks are vertical, the interior horizontal and the middle is diagonal. This was an experiment put in place because of the overhanging main floor and the large windows that called for stronger walls in wood built as if they were concrete slabs.

Contemporary CLT is part of a group of “solid timber construction elements” in which different layers are connected by means of nailing, doweling or gluing. From all the variations that we can find in the construction market, CLT has demonstrated to be a winner in terms of performance. It started in the 1990s in Lausanne and Zurich, Switzerland. In 1996 a joint research between industry and University in Austria resulted in the amazing new product that we are going to talk about.

CLT production was initially centred in Austria and Germany and its use was later extended to Central and Northern European Countries. The United Kingdom has here an important role because it was responsible for the important StadtHouse building, with 8 storeys. Waugh Thistleton Architects wrote a book called 100 Projects UK CLT where we can verify how this technology become a success. In a second moment countries like Australia, Canada and the USA welcomed the new material with the setting of new CLT factories.

What is CLT?

Cross Laminated Timber, also known in some regions as X-lam, “massive” or mass-timber, are planar wood construction components. It means that its thickness is substantially inferior to the other two dimensions: length and width. CLT panels are composed of several layers of wood boards (between 3 and 8) glued and finger jointed. Each layer is oriented perpendicularly to the adjacent layer and glued to form a set of at least 3 layers. The boards are usually made of Spruce, Fir, or Scots Pine (Pinus sylvestris) and their dimensions are roughly between 2cm and 4cm thick. The final boards vary between 5cm and 30cm depending on the layers and the producers. CLT can usually be fabricated in eights of 2,95m and lengths of 13,5m or 16m (limited by transportation possibilities).

CLT panels - TISEM-KLH
CLT panels - TISEM-KLH. Photo by Luis Morgado

The great advantage of CLT is its structural resistance in both orthogonal directions of the element’s plane. CLT works as macro plywood, to be used as structural walls, slabs and roofs. Beyond structural and partition functions they can work at the same time as a finishing exposing the wood “as it is”.

Although CLT producers claim that the adhesives present in CLT are not harmful because they use polyurethanes glue with M1 ‘no emission’ certificates, some people want to develop new variations of CLT with wood-based glues ou with no glue at all. The ICLT (Interlocking Cross Laminated Timber), according to Woodsolutions is “similar to CLT in that it comprises layers of smaller pieces of timber, ICLT relies on a complex array of tongue and groove and dovetail connections to lock the individual pieces in the panels together and similar systems to connect the panels themselves”.

Advantages of CLT

In Denmark, for instance, the motivation for CLT development was in part due to the concern of associations related to forest products in making the most of the available Danish wood, which was relatively poor.

As CLT panels can be cut in large dimensions, they can be assembled on site very fast with the support of cranes. The architectural design logic is independent of the usual dimensional grid in plan and the structural performance is better than the major part of structural wood products. The dimensional stability of CLT is excellent and there are no settling problems due to wood shrinkage. So, it is suitable for buildings with multiple storeys. In comparison with other wood elements, such as studs in wood frame and beams in post and beam systems, CLT panels are not subject to the same swelling and shrinkage deformations.

From the point of view of the building envelope, the thermal behaviour of CLT is very good because of the wood’s contribution for the final insulation performance. As a result, external walls can be slender than walls made with other materials like ceramic or cement blocks. A wall with 17cm containing a panel of 94mm with ETICS (6cm insulation) has a U value of 0,38 W/(m2.°C). Planar thermal bridges does not exist in CLT construction because surfaces are homogeneous. Additionally, if CLT panels are left exposed at interior spaces they can work as moisture regulators.

Fire resistance of CLT panels is assured by their mass. The continuous surfaces of wood, combined with the absence of concealed spaces (as in wood frame), is a barrier to the advance of fire because of the slow charring process. When wood is exposed, structural calculations of construction elements consider the first layer as being sacrificial.

A thick cross-section provides valuable and superior fire resistance for a CLT panel. Their mass means they char slowly, slowing and eventually stopping combustion. With fewer concealed spaces, fire cannot spread undetected. Compared to concrete and steel structures in a catastrophic fire event, CLT structures may suffer less degradation. Codes are very demanding about the safety of wood structures, that’s why CLT buildings with more than two storeys usually need a sprinkler system.

Seismic tests on CLT constructions showed that the system presents a very good performance when subject to lateral forces (wind and earthquake). This quality makes it suitable for multi-story buildings or structures in seismic risk areas.

Usually the panels are assembled storey by storey, but it is also possible to have them in continuous facades (with the openings pre cut), with suspended floors between external walls. CLT is also an interesting material when designing and building modular spaces. CLT panels are made with CNC technology, so their dimensions are very precise and ready to assemble in factory or on site.

Construction process with CLT is straightforward, clean fast and easy. A single family house’s structure can be finished in one day. This is a material that is competing with concrete, the standard material for structures and, as we know, concrete produces roughly 5% of global emissions. CLT is made of wood, a renewable resource that comes from forests managed under sustainable processes.

We should think about this, shouldn't we?

Luis Morgado

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