The dependency on concrete and steel to build everything from homes to sports stadiums comes at a severe environmental cost. Concrete is responsible for 4 – 8% of the world’s CO₂ emissions.
Some architects are therefore arguing for – and pressing ahead with – a return to wood as our primary building material. Wood from managed forestry actually stores carbon as opposed to emitting it: as trees grow, they absorb CO2 from the atmosphere. As a rule of thumb, 35 cubic ft. (1 cubic meter) of wood contain around a tonne of CO² more or less, depending on the species of tree.
Cross-laminated timber, or CLT, has become the primary material on the construction site. It is an “engineered wood”, the planks of which are made stronger by gluing them in layers of three, with each layer perpendicular to the other. This means that the CLT does not bow or bend, it has integral strength in two directions allowing the manufacture of plates or surfaces – or walls.
It is a plywood made of boards that can reach enormous dimensions: between 7.8 ft. (2.40 m) and 13 ft. (4.00 m) high, and up to 40 ft. (12 m) long. CLT is a renewable, green and sustainable material, since it is made out of wood and does not require the burning of fossil fuels during production. CLT, however, is below 1% adhesive, and typically uses a bio-based polyurethane. The planks are bonded together under heat and pressure to fuse that small amount of adhesive using the moisture of the wood.
CLT was first developed and used in Germany and Austria in 1994 after Austrian-born researcher Gerhard Schickhofer at Graz University of Technology presented his PhD thesis research on CLT, “Starrer und nachgiebiger Verbund bei geschichteten, flächenhaften Holzstrukturen” (“Rigid and resilient composite in layered, flat wood structures”).
This was partly in response to the death of the furniture and paper industries. 60 % of Austria is forest and they needed to find a new sales outlet.
Indeed it was Austria which published “Holzmassivbauweise”, the first national CLT guidelines in 2002, based on Schickhofer’s extensive research. These national guidelines are credited with paving a path for the acceptance of engineered elements in multi-story buildings.
Many CLT factories in Austria are even powered by renewable biomass using the offcuts, branches and twigs. Some factories produce enough electricity to power the surrounding communities. (tugraz.at)
Nail-Laminated Timber (NLT) and Dowel-Laminated Timber (DLT) have been revived, while stick-framing started looking good again because it is so efficient in its use of wood.
An increasing number of architects now build tall with CLT, allowing the construction of buildings with up to 30 floors for the 180 ft. (53m) Brock Commons Tallwood House, in Vancouver, in Canada, up to 18 floors in Finland and in Sshickhofer’s native country, the 276 ft (84m), 24-storey ‘HoHo Tower’ nearing completion in the Seestadt Aspern area of Vienna, Austria.
76 % of the latter structure will be constructed from CLT, which will save a 2,800 tonnes of CO₂ emissions over similar structures built out of steel and concrete. Moreover, 1 m³ of concrete weighs approximately 2.7 tons (2.5 tonnes), while 35 cubic ft. (1 m³) of CLT weighs 882 lbs (400 kg) and has the same resistance. The same goes for steel.
Completed in March 2019 after two years of construction, the 280 ft (85.4 m) “Mjøstårnet” 18-storey skyscraper, located in Brumunddal, some 60 mi (100 km) north of Oslo is built in CLT. It takes its name from Lake Mjøsa, on the edge of which it was built.
Designed by Voll Arkitekter its timber was located and prepared within a radius of 10 mi (15 km) around the tower. Containing apartments, hotel, a 10,760 ft² (4,700 m²) swimming hall. office space and a restaurant, it has been declared “The Tallest Timber Building in the World.” by the Council on Tall Buildings and Urban Habitat.
In 2019, Gerhard Schickhofer, Head of the Institute of Timber Engineering and Wood Technology at Graz University of Technology, was awarded the Marcus Wallenberg of SEK 2 million (US$ 209,000).
What you can do: Live and work in buildings constructed using CLT
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