Cross laminated timber (CLT) is the ‘it’ material of recent years, the rising star of the global mass timber construction industry. Often described as ‘pre-cast timber panels’, CLT is created by stacking kiln-dried timber boards with their lengths laid at 90 degree angles to each other, and gluing their surfaces together with non-toxic adhesives.
These boards are then hydraulically pressed to make solid structural wood panels for floors, walls and roofs, which are typically available in a range of sizes and can be customised to suit specific needs.
The key features and benefits of CLT are founded on the way it is produced. Cross laminating, for instance, improves the structural properties of the product by distributing the along-the-grain strength of the wood in both directions, while the perpendicular cross lamination negates any significant shrinkage or deformation of the panels.
Moreover CLT’s high precision design and construction process, whereby all design must be completed upfront before it progresses to the manufacturing phase, minimises waste during production. However, this requires a new way of thinking about the building process.
“Using CLT requires proper planning, detailing and advanced structural design ability. More time must be spent in the technical offices than on site in order to fully document the project,” explains Bernhard Waschl of GCE Consulting Engineers, who has worked with CLT structures since its early introduction in Europe.
“The very common way of designing in Australia while already in construction cannot be adopted for this product. Building tolerances on site need to be adjusted need to be adjusted to fit in with the tight tolerances used to produce the CLT panels.”
The upside of this considered and holistic approach is that it allows for complete control of a project from concept to completion. Installation sequencing, crane calculations, all logistics and even delivery can be pre-planned, and the risk of errors on site is significantly reduced. Safety is another major bonus, as high risk activities are limited to a controlled factory environment.
The nature of the durable and high quality prefabricated panels also simplifies construction, allowing buildings to be realised and built exactly as they were envisaged. They are also lightweight – a mere 20 per cent the weight of concrete – which results in reduced foundation loads and distribution requirements.
Additionally, the advantages of CLT extend beyond the structural and practical into the realm of environmentally sustainable design. Wood is already a good insulator of heat, but the precise nature of the manufacturing process gives CLT buildings an additional thermal benefit by preventing air leakage within the building envelope. This generates stabilised temperatures within the building, which in turn lowers heating and cooling usage and costs.
Forte is the world's tallest high-rise apartment building constructed from CLT. Image: Lend Lease
Despite the various benefits of CLT construction, the bulk of Australian buildings, especially larger commercial and public structures, are still predominantly constructed with concrete and steel. However, that is not to say we have not gained any ground at all.
From homes to the library
The nation’s key success story is, of course, the now widely publicised Forte in Melbourne, which was the first building in Australia to utilise CLT panels. Delivered by Lend Lease, the project made global headlines after it became the tallest timber high-rise apartment building in the world at 10 storeys.
But, another more recent project by Lend Lease worth taking note of is The Library at The Dock, a three storey community hub and library in Melbourne designed by Clare Design and Hayball.
The Library on The Dock
As the first public building in Australia to be constructed with CLT, the $23 million project features a combination of engineered timber and reclaimed hardwood applied to the upper floor slabs, roof, columns, beams and core wall construction. Glulam beams were also used during construction.
The 3,000sqm building, which includes heritage exhibitions, quiet study areas, meeting rooms and reading lounges, was completed in ten weeks – an achievement possible with the prefabricated CLT, which is approximately 30 per cent faster and more efficient than conventional construction.
The initial drivers for the Forte and Dockland library projects were the weight and efficiency of CLT, says Simon Dorries from Engineered Wood Products Association of Australasia ( EWPAA).
“For the same structural performance, the weight of the structure is only a fraction of what it would be with steel and concrete, meaning there is huge savings on foundations and savings. And because CLT can be prefabricated and erected very quickly, there can also be large savings on the cost of erection,” Dorries explains.
For The Library on The Dock, these benefits have translated into the minimisation of remediation works on the existing wharf structure, therefore allowing the building to be constructed right on the waterfront – just eight metres from the edge.
The Library on The Dock. Photography by Diana Snape
When asked why Lend Lease chose CLT over other types of materials and systems, the developer’s Head of Timber Solutions Andrew Nieland says they were originally exploring lightweight construction solutions for a site to be built on silt. Lend Lease had assessed a variety of pre-fabricated systems, including some other timber systems, but found that they were either overly complex in their construction on site, or unsuitable for buildings above two to three levels.
In addition to lightweight and prefabrication options, they were looking for a highly sustainable outcome which precluded a number of systems that were carbon intensive in their manufacture. This search for a sustainable solution seems to have come to fruition, with the library awarded Australia’s first 6 Star Green Star rating for a public building.
“Lend Lease spent over three years undertaking detailed due diligence on CLT from both a technical and commercial perspective. This work drew on the strength of the Lend Lease integrated model and found there were significant benefits beyond the lighter weight nature of CLT,” says Nieland.
“CLT uses a more efficient construction process that is environmentally sustainable, durable, better quality and safer.”
Nieland adds that the team is currently reviewing their forward pipeline of work to identify appropriate sites for developments in engineered timber. This comes as the Australian Financial Review reports that the next step for the developer would be a wooden “100-plus” apartment building with more than the 23 units featured in Forte.