Knock-On Wood: Benefits of Cross-Laminated Timber (CLT)

8/15/2014

CLT is a “New” Energy-Efficient Building System

Energy use in buildings is getting more attention, as energy costs continue to rise. This not only includes the amount of energy necessary to operate buildings, but also the amount of energy required to build them. New systems are being developed to make the operation and construction of buildings much more energy efficient. One such building system is cross-laminated timber (CLT).

While relatively new to North America, CLT has already seen popularity elsewhere in the world, particularly in Europe and Australia. Now interest in CLT is increasing in North America – particularly in Canada and the Pacific Northwest, where there is a strong timber industry.

CLT is an engineered wood system consisting of multiple layers of lumber boards, stacked at 90-degree angles and adhered with structural adhesive in a hydraulic press or vacuum press. The results are sturdy wood panels that can range in size up to 60′ long and 10′ wide, with panel thicknesses ranging from 2″ to 20″.

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Cross-laminated timber panel

Many Benefits of CLT Construction

Cross-laminated timber construction provides numerous potential benefits, which make it a strong contender as an alternative to traditional building systems.

Sustainability

One of the biggest benefits of CLT construction is increased sustainability. CLT panels are manufactured in a controlled environment, resulting in substantially less construction waste and on-site pollution (air, water and noise). CLT buildings are lighter in weight than other building systems, resulting in fewer deliveries to the construction site, with attendant reductions in fuel consumption and pollution.

Another advantage to using a timber-intensive building system like CLT is carbon sequestration. The Graphite Apartments in London, a 9-story residential building using CLT, employed 950 cubic meters of wood in its construction. Between the amount of carbon sequestered in the wood, plus the amount of carbon dioxide that would have been released during the production of an equivalent steel or concrete structure, this building prevented the release of 1,080 metric tons of carbon dioxide into the atmosphere.

CLT structural elements also have a high thermal mass and insulation properties, making the buildings more energy-efficient to heat and cool.

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Diagram of CLT structure in London’s Graphite Apartments

Construction Time & Cost

Since CLT elements are prefabricated off-site, the construction time is significantly reduced. Most of the construction activities can be completed by a relatively small number of carpenters using only hand tools. An experienced crew can assemble up to 8,000 square feet per day – a significant improvement over other types of construction. Typically, a CLT building can be erected in less than half the time of an equivalent building using precast concrete. This results in substantial labor cost savings.

Additionally, fewer trades are required to finish the building, and they are able to get on and off site very quickly. Further cost reductions are seen in the reduced insurance premiums resulting from the shortened construction schedule.

Performance

CLT building systems have a number of performance benefits in comparison to traditional building systems. CLT panels have extremely robust load-bearing properties that make them suitable for walls, floors or roofs. CLT floor panels are capable of spanning up to 50′, and are highly dimensionally stable owing to their assembly method. Also, the dense wood construction of  CLT panels results in superior acoustic and fire performance characteristics.

CLT and Building Information Modelling

CLT construction also lends itself to optimization through the use of BIM design tools. In a previous post, I discussed how Building Information Modelling (BIM) could be directly applied to 3D printing of buildings and building components. In a similar manner, CLT building components can make direct use of the information in a BIM model during the fabrication process. One of the few U.S.-based CLT suppliers, Innovative Timber Systems, calls CLT production “3D printing with wood.”

There are some significant differences between 3D printing and the production of CLT panels, but the core idea of using a computer model to drive the production of the final product is the same. After the boards in a CLT panel have been adhered, computer-controlled CNC machine tools are used to cut the panels to a specific size. Additionally, openings for doors, windows and mechanical services can be created during the fabrication process. Even small channels for plumbing and electrical conduit can be routed out of the panels. It is this level of precision during the fabrication process that allows for the significant reduction in construction time, as the panels can be numbered, then shipped and erected in numerical order in a very short time.

In order to produce these highly specific building elements, a detailed computer model is needed to run the fabrication machines. If the building and its constituent parts are properly modeled using a BIM tool such as Revit, the CNC files can be much more quickly generated. Manual translation of building panel design from 2D files such as AutoCAD can be time-consuming. Creating these files from a Revit model can make the process considerably easier. As CLT construction becomes more widely accepted in the U.S., there will be an additional incentive to develop tools and plug-ins that will allow for the fabrication of the CLT panels directly from the BIM model.

The Future of CLT in North America

While CLT construction has been in use in Europe and Australia for over two decades, it is only recently making inroads in North America. The performance, sustainability and financial advantages of CLT projects abroad is attracting favorable attention in the U.S. and Canada. Projects such as the Forte building in Melbourne, Australia are demonstrating the advantages of a CLT building system. The 10-story building is the tallest timber building in the world, took only 8 months to construct, and sequestered 1,400 metric tons of carbon dioxide.

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Forte building, Melbourne, Australia

One of the main obstacles of the adoption of CLT construction in the U.S. is how the system is covered in the structural building codes. Currently, North American building codes do not specifically recognize mass timber systems such as CLT, but it can be used under alternative method provisions. Unfortunately, this can mean a lot of extra effort in structural calculations in order to demonstrate the structural stability of the design. However, changes to the 2015 International Building Code (IBC) will streamline the process, which should lead to wider acceptance in the North American design community.

CLT has numerous distinct advantages to traditional building systems. While not widely used in North America yet, its track record in Europe and Australia, coupled with changes in the IBC should result in a new “timber boom” in this sustainable and cost-effective building system.

Tesseract Design can provide great architectural design in CLT or any other building system. Please see www.tesseract-design.com/architectural-design for more information.