Cross-laminated timber can fast-track construction

Matthias Oberholzer, a project manager with Montreal-based Nordic Engineered Wood, described CLT as an “excellent alternative” to concrete.

“With the Canadian winter, it’s nice for playing ice hockey, but for curing concrete, it’s not exactly ideal.”

Oberholzer was one speaker at the Cross-Laminated Timber symposium hosted by the Canadian Wood Council, FPInnovations and Ontario Wood WORKS, a Canadian Wood Council initiative to promote the use of wood in construction.

Larry McFarland, principal at McFarland Marceau Architects Ltd. of Vancouver., talked about the use of CLT and glulam in Vancouver. For the complete story, please see Cross laminated timber used for biomass plant at the University of British Columbia in the Daily Commercial News .

CLT, used in walls and floors, consists of wood strips stacked crosswise on top of each other. CLT is not the same thing as glulam.

During his presentation, Oberholzer showed slides of buildings constructed using glulam. Nordic Engineered Wood is currently using glulam for columns and beams in a four-storey apartment complex in Chibougamou, Que., about 700 kilometres north of Montreal, where Nordic’s plant is located.

He said engineers can use glulam and CLT in the same building, but the main challenges are the building codes.

Richard Desjardins, manager for building systems at FPInnovations, said the industry needs to demonstrate to governments that CLT complies with building codes.

Peggy Lepper, director of technical services at the Canadian Wood Council, said the Canadian Standards Association has a document that deals with CLT, called CSA 086, or the Code of Recommended Practice for Engineering Design in Timber.

But the industry is “tight on time” for getting CLT into the next National Building Code of Canada. Although the next NBCC is not scheduled for publication until 2015, the research data must be ready by November, 2012 in order to be considered for inclusion in the next NBCC, Lepper said.

Lepper said a North American CLT product standard is under development by the APA-The Engineered Wood Association in the U.S. (formerly known as the American Plywood Association). The draft covers factors including dimension tolerances and material quality.

CLT is used mainly as a building element with one pronounced direction for load transfer, said Gerhard Schickhofer, an Austrian professor from the Graz university institute for timber engineering and wood technology, who also spoke at the CLT symposium.

“We are beginning to use CLT instead of reinforced concrete for ceiling elements,” he said, but added civil engineers need to know more about the load carrying behaviour of mounting parts.

But connecting CLT parts is not as complicated as some other structural materials, said Nordic’s Oberholzer.

CLT is a “very simple system,” Oberholzer said. “Just using screws, nails and some steel plates to connect everything is very simple compared to, let’s say, glulam.”

Fire safety is a major concern with wood structures, but another speaker said using CLT in buildings can reduce or eliminate concealed spaces in floors and walls, which in turn reduces fire risk.

“Large wood members have the inherent ability to provide significant fire resistance due to their unique charring abilities,” said Steve Craft, a research scientist with FPInnovations. “CLT can provide excellent fire resistance, particularly when we design for it.”

Craft showed photos of experiments in which furnaces were used to expose wood floors to high temperatures.

Although solid wood is a good insulator, he said, it is important to ensure that the assembly is sealed up and does not allow movement of air through the assembly. Otherwise, hot gasses can get through.

CLT can reduce the time required for construction and the carbon footprint, said Tristan Wallwork, a structural engineer and associate director at Ramboll U.K. Ltd., an international consulting firm with offices mainly in northern Europe.

Wallwork said CLT typically has about 50 to 60 per cent less embodied carbon than traditional building materials such as concrete and steel.

One of Ramboll’s buildings was Open Academy, a Norwich, England school with a three-storey accommodation building that was erected using glulam and CLT.