Though the pace of adoption remains slow compared to conventional materials, engineered lumber is starting to make make headway for use in taller wooden buildings, sometimes called "plyscrapers."
Recent developments have helped to shine a light on mass timber use in taller buildings. The US Department of Agriculture (USDA) has launched the US Tall Wood Building Prize Competition, with a $2 million (U.S.) prize for “US developers, institutions, organizations and design teams willing to undertake an alternative solution approach to designing and building taller wood structures.”
Though the competition is open only to American teams, Daryl Patterson, head of operational excellence for Lend Lease, is a part of the jury composed of nine international experts. Lend Lease’s Forte project is currently the world’s tallest residential wood structure at 32 metres, though it will soon be surpassed by a project in Norway.
According the the USDA, the competition will “identify proponents with building project(s) in the concept-, schematic- or design-development stage in the US that can safely and successfully demonstrate the use of wood as a viable structural material in tall buildings.”
Besides the $2 million prize, the winner “will receive support for incremental costs of pioneering wood construction techniques to address the engineering design and code variances needed to incorporate wood technologies into new building sites.”
The competition is also intended to inspire builders, architects, designers, and manufacturers to continue use and development of engineered lumber.
Engineered lumber is typically made from layers of small pieces of wood joined with adhesive to produce cross-laminated timber (CLT) panels, glulam beams, laminated veneer lumber and the like. Sometimes called “mass-timber,” the products are not intended to replace typical lightweight stick-frame construction. Instead, architects have been starting to use mass timber to replace concrete and steel in non-residential construction.
CLT and other mass timber products offer attractive design qualities for taller buildings. Cross-laminating the boards results in improved dimensional stability and enables the manufacture of long and wide floor slabs, as well as long wall sections. In addition, CLT offers good thermal insulation and good sound insulation.
According to civil engineer and CLT expert Kris Spickler of Structurlam Products in Penticton, Canada, several myths abound regarding CLT due to builders and architects knowing little about it.
For instance, it is often assumed that cost must be higher than concrete and steel. On the contrary, thanks to faster installation than concrete, a CLT system may cut installation cost by half. Speedy installation could lead to shorter project build times. Its lighter weight compared to a concrete and steel structure means the entire foundation can be smaller and cheaper, and crews accustomed to concrete panel systems adapt quickly to CLT panel systems, as their installation processes are similar.
Naturally, it’s assumed that fire is a serious threat, but solid wood CLT panels will be part of the 2015 International Building Code, after having passed the fire endurance test. A five-ply CLT wall approximately 17.75 centimetres thick was covered with 1.6-centimetre thick gypsum board and subjected to a fire that reached nearly 1000 degrees Celsius. The panel endured just over three hours, though the test required only two hours.
Concerns about the amount of wood used in engineered lumber have also surfaced. In Europe, where CLT was developed in the 1990s, a culture of sustainable forest management yields plenty of timber for production of engineered wood at a sustainable rate.
Furthermore, the report Introduction to Cross Laminated Timber by M. Mohammad, Sylvain Gagnon, Eng., Bradford K. Douglas, P.E., and Lisa Podesto P.E., concluded that CLT offers a smaller environmental footprint than concrete. When they compared the life cycle analysis (LCA) of CLT to concrete, the authors wrote, they estimated “that the CLT will substantially outperform concrete in every environmental metric addressed by LCA.”
In addition to using renewable resources, waste resources are also suitable for CLT production. Western forests in North America have been devastated by the mountain pine beetle, leaving millions of acres of standing dead trees. Those trees can be used for building materials such as CLT, locking up their carbon for the life of the building. If these materials are not used, they will eventually rot and release their carbon.
In fact, according the the USDA, increasing the use of engineered wood products in buildings of seven to 15 storeys “could have the same emissions control effect as taking more than 2 million cars off the road for one year.”
USDA Secretary Tom Vilsack has said wood “may be one of the world’s oldest building materials, but it is now also one of the most advanced.”
Worldwide, multi-storey wooden buildings have been completed in Sweden, Austria, Germany, Great Britain, Italy, the US and Australia. The Forte Building is currently the world’s tallest wooden residential structure, though the 14-storey, 49-metre Treet (“tree” in Norwegian) project in Norway will eclipse it when complete in 2015.
These developments have helped to raise awareness of the benefits of mass timber, though adoption by the construction industry can be expected to be slow until they develop their own expertise with the materials.