Cross Laminated Timber (CLT) is an engineered wood product used for pre-fabricated structural applications and is making the construction of entire buildings from timber a reality.
Cross Laminated Timber (CLT) is the engineered wood of the future and is making the construction of entire buildings from timber a reality. First developed in Switzerland in the 1970s, CLT is an extension of the technology that began with plywood and may be best described as a 'jumbo plywood'.
Having gained popularity in Europe, CLT - also referred to as 'tilt up timber' or 'pre-cast timber panels' - is slowly being adopted more widely as a viable alternative to more traditional building practices.
Layers of timber, known as lamellas, are glued together with the grain alternating at 90 degree angles for each layer. The exterior layers' grains run lengthways, giving optimum strength. CLT is manufactured in a similar way to glulam, except that glulam is layered with the grain.
Cross-laminating layers of wood veneer improves the structural properties of wood by distributing the along-the-grain strength of wood in both directions, and this means that CLT panels can be used to form complete floors, walls and roofs.
While CLT is a timber product, it should not be thought of as a timber frame product - it is a timber panel product that actually has similar characteristics to that of a pre-cast concrete panel. The advantages this offers new buildings is quite exciting - timber panels are much lighter than concrete, more easily worked and easier to erect.
The potential of CLT as a sustainable building material is only just being realised around the globe.
CLT is available in Europe and has only recently entered the North American market. It is not currently manufactured in Australia, however increasing local interest has led to a number of imports entering the Australian market. You'll find some of these in the Supplier section of WoodSolutions.
CLT durability is dependent on the base material that it is made from. As most CLT is made from low durable softwood, it should be protected from the weather.
CLT has minimal shrinkage and swelling in part due to dried lamellas, precision cut dried pre-fabricated panels. Moisture content is generally around 12%.
Each CLT manufacturer will have their own sizes, however lengths of up to 18 m and widths of up to 2.95 m are possible with variable thicknesses based on application. Lengths are generally dependent on the ability to transport the finished panel.
Individual thicknesses of layers can start at 19 mm and range up to 43 mm. Lamellas are grouped into 3, 5, 7, or even 9 layers with over thicknesses of CLT panels reaching up to 500 mm.
Wall panels can range from 57 mm to 161 mm (overall thickness based on 3 - 5 layers) and ceiling panels can range from 57 mm to 400 mm (overall thickness based on 3, 5, 7 layers).
Panels are usually manufactured to 3, 5, 7 layers however some manufacturers will provide more layers upon request.
For the European market, manufacturers subject the lamellas to a visual and mechanical quality sorting procedure that produces a range of visually graded standards. This can vary between manufacturer, however they generally will relate to the following categories:
No Regulations, Standards or Codes exist specifically for CLT in the Australian construction industry.
Codes are in place for European Standards, but not for other markets. An implication of this is that local building surveyors can't properly cost CLT at this point in time for Australian conditions.
*Disclaimer : The species mentioned here are just a guide and for specific information refer to Suppliers Section
As CLT is not currently available in Australia there is no data available for Australian conditions or Standards. CLT consists of large timber sections, so in theory it would have good fire rating ability.
European testing has shown that CLT offers advantages in fire resistance when compared to concrete or steel. As with other timber materials, during a fire a charred layer will form around the material core which helps it to retain its load baring capacity and delay the charring rate.
The charring rate of CLT will be dependent on the base timber lamella used, the higher the density of the timber species used, the lower the char rate.
One Austrian manufacturer of CLT, KLH, quotes the burning rate for CLT to be 0.76 mm* per minute.
*from KLH website www.klh.at. Information correct as at October 2010.
CLT currently manufactured in Europe uses a formaldehyde-free polyurethane structural adhesive which helps to make CLT more environmentally friendly.
CLT is an industrially prefabricated timber panel of superior strength. CLT's structural performance is comparable with pre-cast concrete. In fact, CLT has already been used in two residential structures in the UK, both reaching eight storeys.
CLT offers very good load distribution properties in both directions. Each panel is made up of layers of timber; 3-5 board layers are used for walls while 5-board layers are used for floors. Panels can be pre-cut for windows and doors. All panels are cut using CNC technology to ensure millimetre-perfect accuracy.
Due to its prefabricated nature it is easy to assembly onsite and because it is a dry building system, ie uses no concrete or masonry, it also minimises shrinkage and swelling.
CLT is most suitable for earthquake zones due to the ductile behaviour of timber buildings. It also provides better thermal mass than wooden frame structures and there is no break in the insulation layer. CLT also possesses inherent noise reducing properties.
CLT is also compatible with steel, glass, aluminium for architectural projects.
Refer to manufacturer guidelines.
Refer to manufacturer guidelines.
CLT requires little or no maintenance, however individual manufacturers will have their own guidelines.
CLT is easy to work, easy to cut and easy to assemble onsite. It's pre-fabricated for dry construction and panels are lighter and easier to work than concrete.
Using prefabricated structural timber components minimises onsite work, reduces total construction time and there is also a greater accuracy achieved with structural components.
Panels can be installed by crane and lightweight power tools. CLT is also a bonus for follow on trades, where mechanical and electrical services can be easily screwed or fixed to the panel, requiring no drilling.
At present CLT is only available from select European and Canadian producers who use sustainable forestry certified sources. Being a timber manufactured product, it means CLT has a very low embodied energy.
In Europe the manufacturing trend is geared towards absolute sustainability so that the CLT manufacturing processes tries to ensure nothing is wasted.
As this product is not currently manufactured in Australia there are no guidelines for Australian conditions. In Europe, spacings of screws/nails/bolts should be in accordance with European Technical Approval ETA-06/0138.
Manufacturers will provide their own guidelines on finishes, however CLT can be clad in other materials including brick and stone, zinc, aluminium, terracotta and steel.
Lateral loads such as wind or earthquake on framed timber buildings - either post and beam or stud and joist - need to be resisted and shear walls and diaphragms offer an effective and economical solution.
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